Bus Switches as Analogue Switches
These points, plus detailed figures and derivations, are presented
below, together with my recommended 'cure' based on interpolating the
device parameters given in the manufacturers' data sheets.
- Bus switches are not designed as analogue switches
- The design of N-channel switches does not give rail-to-rail
- On-resistance varies with applied signal, rising to infinity at
1.5V below the supply.
- The on resistance and threshold voltage of TI devices is poor.
Fairchild parts are better.
- The mid-rail bias point used throughout STAR is inappropriate,
since it provokes high on-resitance and consequent non-linearity and
- Use of these devices within 50R paths needs much lower bias
In the fullness of time, you may find a Step By Step Guide
to modifying your PICASTAR on this page, or at least a list of
'chain-bottom' resistor references for Glenn's ComboBoard and what
value to shunt them with to obtain correct biassing for low
on-resistance and best dynamic range. They all need changing. But see my disclaimer.
As a late note, the first manifestation of improper biassing has
surfaced in the 2nd IF Transmit Buffer,
where positive peak clipping occurs in the Bus Switch IC604. Similar
effects might occur throughout the PICASTAR, since all the bus switches are biassed
incorrectly at mid-rail.
If you decide to use any of the information here, you will make
your PICASTAR non-standard. Even if you wish to do the changes on a
'new-build' STAR, please build it first using all the standard build
information, with no changes from the published design. Also you need
to calibrate it using all the excellent BASIC programmes, to verify
that it works correctly, before you modify it. This is what I did
If you do not 'RTFM' (read and use all
the PICASTAR documentation in the Yahoo! Groups Pic-a-Project
group before you modify
your build in any way, then no-one will be able to help you to get your
own build working. You must
join this group in order to obtain (and be permitted to use) the design
information that you need. If you then need help (with an un-modified
STAR) you will get it by posting your problem on the picastar-users
If your STAR is modified in any way, then the very helpful people on
the picastar-users group will probably not be able to help you.
This is very much a Work In Progress and will be changed and added to
in a wholly personal way. If you are viewing this it is either because
you have stumbled upon it or have been invited to see it. Please do not
share it with others; when the time is right I will do that
I will happily receive comments or criticisms
in the usual way.
The work below arose from my study of the usage of bus switches in
(© Peter, G3XJP) but is not
limited to that use.
PICASTAR (STAR) uses many Fairchild
FST3125 and FST3126 devices for all kinds of
routing and switching purposes. These are sold as Bus Switches,
intended to be used for interconnecting or isolating logic buses. By
their nature they are bidirectional, and they have excellent
performance in the intended application.
Texas Instruments also has 'equivalents', the SN74CBT3125 and SN74CBT3126. Do not
use 'C' suffix parts, which have 'undershoot protection' circuitry that
adds to the 'off' switch feedthrough capacitance.
Also there are some 3V parts from TI, which you should avoid since they
have built-in voltage generating oscillators to give adequate
enhancement voltage - not a good idea in the same device that is
switching nV radio frequencies.
The TI parts look similar and I intend to check them later.
There are probably other manufacturers of similar parts, please let me know what you are using.
STAR uses the devices in a switching mode in the first & second
mixers. They are rapidly switched on or off at local-oscillator speed
and modulate the input waveform, acting as cheap and excellent mixers.
The devices are also used as general route-steering switches, mainly in
paths, for enabling and disabling specific signal paths. An example is
the use in the Bandpass Filter area, for switching into circuit the
desired filter and isolating the others.
In all such uses, the switch pins are a.c. coupled and one side is
biassed by a potential divider to be at half the 5V supply rail. This
has been said to provide the maximum dynamic range. But it doesn't.
Study the datasheets
If we look at the Fairchild information, we find that the single
N-channel MOSFET used for the switching element has a gate threshold of
about 1V (Fairchild AN-5007 "An Introduction to Fairchild Switch
Products"). Provided that either the source or drain is at less than
this threshold (the FET is symmetrical), the FET is enhanced and a
conduction path exists between source and drain. If the gate voltage is
below this threshold, the channel is off and isolation between source
and drain results.
However, as the source/drain voltage approaches the gate threshold, the
channel resistance increases, as is normal for a single FET. It does
not suddenly change from very low to very high. Fairchild
indicates that the point at which the FET is 'off' is with a Vgs of 1V,
but at levels approaching this, the channel will have high resistance.
We are given a feel for this by the specification for the FST3125 and
FST3126 devices, which are idential except that one has an inverting
logic buffer on the gate and the other a non-inverting logic buffer.
In both cases, this buffer ensures that the main pass FET gate voltage
is either at one of the device supply voltages or the other,
disallowing intermediate gate voltages.
If the FET gate is at 0V, then the FET is 'off' and will remain off so
long as neither switch port is taken below -1V.
If the FET gate is at Vdd (probably +5V), then the FET will be 'on' for
as long as either of the switch ports is below Vgs, i.e. less than
about +4V for this supply voltage.
But as the switch ports vary in voltage between 0V and Vgs, the
on-resistance of the channel will vary, being least at 0V and most at
the Vgs threshold, where it can be considered to switch off. The actual
resistance value is a function of the design of the FET.
For many signal-gating devices, such as CMOS bilateral switches, the
manufacturer shows how the switch on-resistance varies according to the
voltage being switched. With CMOS switches the highest resistance
occurs at nominally mid-rail, when neither the P- nor N-channel element
is fully enhanced; either side of this the resistance decreases as one
or the other FET takes control.
But for our Bus Switch, there is only an N-channel device, so the
on-resistance increases continuously as the switch voltage increases,
until eventually the switch turns itself off. This is not at all a
problem in the anticipated digital use of the device, but when used as
analogue switch it is very important indeed. If our switch ports
are at 0V then the FET is fully controlled by the logic input, but as
the ports approach the threshold voltage Vgs so the resistance
increases, affecting the switched circuit.
Varying the supply voltage
Some designers who wish to use Bus Switches as analogue switches have
employed higher than usual supply voltage on the device, to maintain a
high threshold point and increase the range of low on-resistance, often
up to the manufacterer's 'Absolute Maximum' rating of 7.0V.
This, it is
pointed out by Fairchild, should not be done. They say:
" Stresses exceeding the absolute
maximum ratings may damage the
device. The device may not function or be
operable above the recommended
operating conditions and stressing the
parts to these levels is not recommended. In addition, extended
exposure to stresses above the recommended operating conditions
may affect device reliability. The
absolute maximum ratings are stress ratings only."
In other words, the absolute maximum is not intended to be a normal
operating condition, but the limit of a safety zone.
Proper operating levels
Fairchild, like other manufacturers of ICs, does provide a statement of
the conditions under which their devices may safely be operated; the
"Recommended Operating Conditions". For the FST3125/FST3126 this states:
"The Recommended Operating Conditions
table defines the
conditions for actual device operation. Recommended
operating conditions are specified to
ensure optimal performance to the
datasheet specifications. Fairchild does not recommend exceeding them
or designing to Absolute Maximum Ratings."
The maximum recommended Operating Power Supply is 5.5V.
Interpret the data for our usage
The Fairchild data gives us in insight into how the FET on-resistance
varies with channel voltage.
The 'Switch On Resistance' at 0V with a Vcc supply of 4.5V is typically
4R, maximum of 7R .
The 'Switch On Resistance' at 2.4V with a supply of 4.5V is typically
8R, maximum of 15R.
The 'Switch On Resistance' at 2.4V with a supply of 4.0V is typically
11R, maximum 20R.
From this we can see the enhanced channel resistance.
These figures really relate to
how the Fairchild FET channel resistance varies with the enhancement
|Vgs = Vcc - Vin/out
|Ron (Typical) Ohms
|Ron (Maximum) Ohms
The transfer characteristic can be likened to most NMOS FETs, and will
show a similar shape. If we plot the values above and fit a curve to
them, it looks like this, using Average values. It's a shame we don't
have more points:
This shows clearly the way that the FET will be fully off by the time
the Vgs is 1V, as stated by National.
In most PICASTAR usage, the switch is
used in a 50-Ohm circuit, so if the on-resistance is significant
compared with 50R then it will neither be efficient nor will it permit
a reasonable match to be maintained. In the Bandpass Filter
section, there are two switches used per signal path, one at each end
band's filter, but two of the FETs in a package are paralleled for each
switch, so it is better than at first sight (the other 2 FETS serve
That's Fairchild; what about TI?
These figures can be compared
with the TI part SN74CBT3126; typical values are used:
|Vgs = Vcc - Vin/out
|Ron (Typical) Ohms
From this table it is clear
that TI devices have higher on-resistance. Since they also are
specified to have higher 'off' capacitance, they do not represent a
good choice and TI parts are best
This data makes more sense to us if it is transposed to be the
resistance against the actual applied voltage on the FET. I have only
done this for a supply voltage of 5.0V, since this is a
the 'typical' values. If the supply is at the low limit of a 78L05
regulator, the X axis must be offset by -0.25V.
Although I didn't fit a curve, the trends are obvious. The graph shows
clearly the superiority of the Fairchild device (from published
'typical' specification). It also gives us a feel for the operating
voltage (bias) we might wish to use - it won't be anywhere near
3V or we have no dynamic range!
Note that the value at 0V on the pin is probably virtually
the same as at 0.5V; using the data-sheet figures this value does not
Best Operating Condition
To obtain a low 'on' resistance for the switch, we need the lowest
possible 'bias', or steady-state voltage that we can tolerate whilst
still allowing a reasonable symmetry of signal swing on either side.
Clearly, from the graph above, voltages above 3.5V are no use at all,
since the FET is
switching 'off' when we need it 'on'. To get a reasonably symmetrical
characteristic, we might choose the bias point to be, say 1.5V,
although even this would result in a large signal resistance variation
for large signals. This would modulate the path resistance and is
undesirable since it represents voltage-dependent non-linearity.
Let us consider the normal habit within PICASTAR.
In all cases that I recall, where signals are switched using Bus
Switches, the source or
drain is biassed to half the supply voltage of 5.0V. This is done in
the hope that it provides the maximum voltage swing.
would be the case for a CMOS switch which has minimum on-resistance for
each supply rail due to the paralleled PMOS and NMOS switches, but
these bus switches have only the NMOS element as explained before. As
can be seen from the graph of typical characteristics above, the switch
resistance is low for an input of 0V (around 4 Ohms) but rises
considerably as the switched signal increases in voltage until, at an
input voltage of 2.75V, the channel resistance is 10 Ohms, rising
rapidly thereafter as the FET turns off, which is not very useful for
passing signals in a 50-Ohm circuit.
Remember that the action of 'turning off' is not due to the switch
control input pin, but is entirely
due to the voltage level actually being switched by the nominally 'ON'
What about Worst Case figures?
in mind that we are still dealing with typical figures; as designers we
should take note of worst-case
figures. This gives a different result:
The Typical plot uses the same data as before, for comparison. Both
sets are derived from the Fairchild data-sheet figures. It is easy to
worst-case figures are, indeed, a lot worse than typical figures! If we
look closely, there is a footnote on the data-sheet that further
typical figures, saying "Note 4:
Typical values are at VCC = 5.0V and
TA = +25°C."
This means that
the 'typical' figures
don't actually apply if our supply voltage is not the 'nominal'
regulator output of 5.0V, nor at temperatures other than 25°C.
Which figures should we use?
In a commercial design environment, for this type of device, we must
consider the worst
case, whether we like it or not, unless it is clearly demonstrated and
recorded in the design specification that
the conditions of use are limited to those of the typical figures.
we only use the typical figures,
our production department could end up manufacturing items which do not
work properly in hot or cold conditions, or if the supply voltage is
less than 5.0V, although the chip manufacturer says his devices are
within specification; they are. Who will tell The Boss that the
day's manufacturing is all faulty, that Fairchild won't accept any
liability, nor will they take back the unused chips?
We also have to
Boss that, in future, we must purchase specially tested chips from the
vendor, all characterised to ensure that, in our usage, they have
are no worse than typical!
In the end, the chip manufacturer will
select out all the 'better' chips for sale to our Company at premium
and everyone else will get the rest, still within specification, but
all worse than the 'typical' figures. This used to happen with
resistors; all those close to the nominal were selected out and sold as
2% or 5%; if we bought 10% for cheapness and tried to select those
within 2% we would find a gap in the value distribution - someone had
been there first (this is not so prevalent with resistors these days).
So what does our design department do? I have seen various
some assuming that the worst case is the 3-sigma points of a normal
pattern (it isn't necessarily so) and some requiring a design point
typical and worst-case, but the only satisfactory way for our
production designer is to assume that every device of one type can be
or to seek a better device to use.
Apply statistics over the whole
design, but not over single component types. If you conclude that 1% of
production will statistically be rejected because of tolerance effects,
so be it. Just be sure that this is what the boss is
comfortable with. I have seen warehouses full of such rejects awaiting
'rectification' - even 0.01% of a million units a day is an awful lot!
After all we might say to justify using 'typical' values, the device
manufacturer provides worst-case specifications
so that we can use them collectively to calculate the limits of our
design, not just for fun. If 'typical' figures were sufficient, his
daunting worst-case figures would simply not be published. They are for
and our protection against an unsound design.
For us, as amateurs, it is highly tempting to use the 'Typical' figures
(and most amateur designers do just this), but what if one of our ICs
is at worst case, as it may well be? When we build a PICASTAR, do we
test each circuit block to ensure that the performance is what we would
like? Do we then change each and every component that we deem to be
faulty because it is outside its Typical specification? Do we even know
which part is causing the problem?
We have to use the device, because it has been designed in and there
may be no better alternative part. It may not even be apparent that
there could be a problem. After all, you are probably not the original
Operating bias point
Suppose I have an AC signal of 0.5Vpp to switch with a bus switch, in a
50R circuit. I believe that I can tolerate an on-resistance of no more
than 5R for the switch, 10% of the circuit impedance. That sounds
For a 0.5Vpp AC-coupled signal, the peaks are at -0.25V and
+0.25V. We need to bias the switch to meet our requirement for 5R
on-resistance with minimal change for our signal, or we end up with
signal distortion due to peak compession as the on-resistance rises.
Let's see how our worst-case bus switch fares:
We would like the bias point to be as far from 0V as reasonably
possible, since at
0V we run into clipping (clipping probably won't occur in practice
clamp diode conducts, however).
To fulfill item 2, the worst-case device shown in the graph must
have a lower on-resistance than it can achieve (it extrapolates
about 6R with the whole 5V as Vgs). We
- The positive peak must not see a switch resistance significantly
greater than the negative peak, or the circuit will be non-linear.
- At no time must the on-resistance exceed our target of 5R
- We must allow that our supply regulator is specified to provide
5.0V +- 0.25V, so the supply may be 4.75V
- There may be additional power voltage drop due to circuit
resistance; let us provisionally allow 0.1V for this.
In our favour, the 'worst case' applies to the full temperature and
supply-voltage range. It is
the worst case. We are not expecting to operate our devices at
extremes of temperature, but they may still be above the 25deg C at
which 'typical' applies. But we have no figures available except
'typical' and 'worst case'... Should we guess at something?
Now let us consider a 'typical' device, using the above criteria to
determine a bias point.
To fulfill item 2, on-resistance < 5R, our Vgs must be greater than
3.5V, which we can
achieve by restricting the whole signal swing. With our (4.75V - 0.1V)
supply voltage, to obtain a Vgs greater than 3.5V, no part of our
switched signal must exceed (Vsupply - 3.5V), or our Vgs will be
inadequate and on-resistance too high.
The signal must therefore not exceed +1.15V on the positive peak. We
can achieve this for our 0.5Vpp signal by biassing the switch channel
so that the peak remains below +1.15V. Our bias point must not exceed
(1.15V - 0.25V), 0.9V to achieve this.
As a check, we do not want the -ve peak to clip either, so at our
chosen bias point the -ve peak must not drop below 0V. It will be 0.25
below the bias point, so will not go below 0.65V, which is OK.
I have assumed that our signal is symmetrical in amplitude above and
below the a.c. mean value. This is true for a sinewave, but is
obviously not true
for an asymmetrical signal, so we must be very careful with our
conclusions. For a small duty-cycle signal, practically the whole
peak-to-peak voltage may appear either above or below the mean value!
But let us not worry about that yet.
We have fulfilled item 2 by setting the bias point at 0.9V, and could
reduce this without clipping the negative peaks in the chip protection
diode (if it has one).
Let us check on item 1. We want an insignificant variation in
resistance across the range of our input signal. If we are biassed at
0.9V, the resistance on the positive peak is 5R, because that was our
criterion in item 2.
At the negative peak, the device pin will be 0.5V lower than on the
positive peak (it is a 0.5Vpp signal). This will put (+1.15V - 0.5V) =
0.65V on the switch input. Our Vgs will be (Vsupply - 0.65V) = (4.65V -
0.65V) = 4.0V. From the (typical) graph, this Vgs gives a switch
resistance of around 4.3R. Our switch has changed resistance from 5.0R
on the positive peak to 4.3R on the negative peak. We must decide
whether this is excessive. If so, we could try moving the bias point
lower, since the on-resistance curve is non-linear and there is range
Best signal range method
If we wish to allow the largest possible signal to be applied to the
switch without it either dropping below 0V or causing the switch
resistance to exceed out 5R target, we clearly have a range span of the
signal from 0V to the 5R on-resistance point at 1.15V (the figure we
above). to achive this, we will want our bias point to be in the
centre of this range, +0.575V. This will also fulfill the
requirement for our 0.5Vpp signal, and since it is lower than the
previous figure we would expect less on-resistance variation with
signal. This is a good choice for bias point.
The calculations above have used the manufacturer's specified figures
for the device. Nothing is invented or guessed at, except our own
particular criteria for on-resistance and signal amplitude.
We have seen that a worst-case device is no use to us at all unless,
maybe, we can change our criteria for on-resistance. This is unlikely,
given the extent of change we would need!
Also we have seen that, within our criteria, a bias point of 0.575V is
If we could tolerate a maximum on-resistance higher than 5R, and didn't
mind non-linearity of resistance, the bias point could vary upwards -
maybe enough to allow a worst-case device to be used. The calculation
is easy, but the acceptable criteria must be defined first.
Can we go -ve of ground?
Up to now we have steadfastly refused to allow our signal to go below
0V on the basis that it may 'touch' a protection diode, with disatrous
consequences for signal integrity! Normally, protection diodes
are built into the substrate of a chip and are often unisolated
parasitic diodes. Not always, though. Most of us have encountered the
type of level converter that permits an input (or output) to rise
beyond the +ve supply; they are not uncommon. There are also plenty of
devices that have no diode clamp to the -ve supply.
If the bus switches fall into that category, it would enable us to take
the signal negative without any conduction until the channel dropped
far enough below the supply to cause conduction of a nominally 'off'
This would happen
because the NMOS FET gate clearly cannot be taken below the rail by the
driver buffer; so when the source or drain becomes negative enough the
will turn 'on' in spite of the control input selecting the 'off'
condition. This is not the conduction of a clamp diode, but the
unwanted conduction through the FET - it turns itself on. This will
happen by the time we reach the gate threshold, just as before. For our
typical device, we have established that this is at around 1.3V. A
silicon clamp diode would conduct at around 0.5 to 0.6V, of course.
The Fairchild datasheet for FST3126 tells us that the Clamp Diode
Voltage for Vcc of 4.5V is max -1.2V at Iin=-18mA. That current is
quite large, so it may represent a very 'soft' substrate diode,
but the figure is actually very close to our expectation for the
threshold of the FET. We will have to use a meter to check it.
What do we find? There is a
diode conduction path present. It seems to
have lower than expected conduction voltage (0.5V; a 1N4148 has 0.6V)
it might be a FET substrate diode. We had best avoid significant
negative excursions (down to -0.2V or -0.3V is probably acceptable).
We do not need to worry about spurious conduction through the 'off'
FET, because clamp-diode conduction occurs first.
It is OK to let our signal become a scrap negative before we
need worry about the clamp conducting. This is still a better bet than
trying to operate at a positive level at which the FET on-resistance is
Anyway, we started off by
considering the best bias point for the Bus Switch and wondering if 50%
of the 5V supply, as originally used, is a good level. It is not,
as we have seen.
The graph shows us that
the TI device already has over 9 Ohms of on-resistance when biassed to
mid-rail, and that a small increase due to a signal can send the
resistance soaring. Also
remember that this is a 'typical' condition that may not be correct for
an individual part.
For our 'typical' device, a bias point of 0.575V allows us the maximum
swing in our 50R switched circuit before we exceed 10% of the 50R as
switch resistance (using the better, Fairchild part), but the swing
0.575Vpk and should ideally be considerably less in order to avoid
voltage nonlinearity of resistance. We may consider a bias of 1V or
even more to be acceptable for small signals, but 2.5V is not at all
good. If we were to superimpose a 0.575Vpk waveform on the present 2.5V
bias level, the resulting +ve peak of 3.075V (for a symmetrical
waveform) would take us close to cutoff, especially for the TI
device. The on-resistance would be too high (10R or more) and very
device and voltage dependent. All for a (probably mythical) 'typical'
device with 5.0V supply and at 25degC.
Half-rail was nice & easy to implement. Equal value resistors do
the job, maybe 10k each. In a 50R circuit the resulting 5k in shunt is
to be a problem.
I would aim at 0.5V bias point, since it feels as if it should be
easier; the signal
can swing -ve by 0.075V without touching the clamp diode and we can end
up with good on-resistance symmetry even for a 1Vpp signal.
we only wish to change one of the existing resistors; the 'bottom' one.
0.5V from a nominally 4.75V supply with a 10k 'top' resistor, we must
drop (4.75-0.5V) in the 10k i.e. 4.25V in 10k. By ratio, to drop 0.5V
'bottom' resistor must be (10 * 0.5 / 4.25 k) or 1.176k. If we use the
next preferred value of 1.2k, will the voltage be OK? It will be (4.75
1.2 / (10 + 1.2))V, 0.50892V. This is even closer to our desired 0.575V
and sounds acceptable. In our 50R circuit, the parallel load of 1.07k
will not be likely to pose a problem; it represents a drop of 5% of the
50R and we already tolerated a rise of 10% due to switch resistance,
but we need to be happy with this.
Next we should perform a tolerancing exercise. Maybe we are using 5%
resistors, maybe 1%. If they are 5%, the two cases are with (1.14k and
10.5k) and (1.26k and 9.5k) which give 0.465V and 0.556V respectively
for our bias, which sounds OK. 1% resistors would give a closer control.
Up to now we have considered a minimum chip supply of 4.75V. We may be
regulate it much better than this. This is the lowest we should expect.
What if it is higher?
The voltage on the FET will rise with the supply. For a 5.25V supply,
the FET will see 0.5V more on the gate for 'on' and will conduct more
enthusiastically. At the same time, the bias divider will have moved
the bias point up by only about a fifth of the supply change; it goes
the right way but by less, so we have more margin and things are even
How about no change?
If we leave the equal value (half-rail) bias resistors in place, what
will be the consequence? It's a very easy 'modifcation'!
Consider our 'typical' device, and let's make it 'typical' for the
supply also, +5.0V.
We have a Vgs of half rail, 2.5V, as the bias point of our FET. This,
from the graph, corresponds to an 'on' resistance of 7 Ohms (9R for TI
parts), not bad
but higher than our R target. Our +-0.25V signal will modulate the
on-resistance between roughly 6.5 and 7.5 Ohms (for a Fairchild
device). This may be totally
acceptable... But remember this is for a 'typical' device at 5.0V
supply and 25°C chip temperature... And the signal may not be
What about switch Capacitance?
In the past there has been discussion about the 'off' capacitance of
these switches. Because there are two sections in parallel used to
switch each end of each bandpass filter onto the input and output
'buses', this is probably the most affected area.
Fairchild's latest datasheet (March 2008) from their website specifies
this as 2pF per switch, but the older datasheet dated August
1997 (from the Farnell website) gives it as 5pF.
This is a big
difference, but I suppose that if they have bothered to change it then
observe the more recent figure of 2pF. This gives a feedthrough
capacitance of 4pF for the National devices in the bandpass filter,
which is perhaps more than we would like. Certainly if it is 5pF (TI is
currently specified at 4pF) then two parallel switches at 10pF is
high. It is worth noting that Glenn's boards have provision for
adding an inductor to null the effect on the highest frequency; this
has been used by some
builders. But how much do we need to null? Is the switch capacitance
4pF or 10pF? Are they Fairchild or TI parts? What about other stray
capacitances (PCB etc)?
What should we do?
- First, measure the supply voltage to the device. If it is 5.0V or
more then things are not so bad. If it is 4.75V then change the
regulator to get more voltage; every little helps a lot, but please
remain within the manufacturers' recommended operating conditions,
remember their warning and don't push your luck. Recall that the
'typical' figures that we used actually only apply for a 5.00V supply.
- Second, see how large the signal might be and consider whether
the modulation of resistance is likely to be troublesome. For a few
millivolts it will probably not be a problem; for half a volt it
probably will. 'Signal' means anything that will appear at the switch;
for a radio it could be anything received by the antenna (at the switch
on the input of the BPF).
- Third, decide whether the value of 'on' resistance is really
significant. Will there be untoward signal loss in a 50R path? If so,
is it so early in the signal path that amplifying back up to the
previous level will increase
the noise level excessively? Or is it switching a high-impedance
circuit where on-resistance has small effect?
- Fourth, trust to good fortune that the devices are all at or
below 'typical' threshold voltage. But recall what we found about
- Fifth, if you can and want to play more safely, reduce the bias
voltage to as low as you dare. Every little
helps. Don't feel that doing so will give less dynamic range - it
will most likely give more. Bias at mid-rail is inviting switch
more positive on signal peaks and the switch will probably open
completely, but there is (nominally) 2.5V of range in the negative
direction. You will end up with more
dynamic range rather than less
by reducing the bias!
When using Bus Switches as analogue switches, great care must be taken
to avoid setting an operating condition that approaches the cutoff
voltage of the single NMOS switching FET.
If we assume that our devices conform to the 'typical' published
parameters, which are only valid at 25degC and for a 5.0V supply, then
any signal voltage within 1.5V below the +ve supply is in great danger
of causing the FET to turn off; it will at least increase the switch
resistance depending on the voltage.
Do not treat these devices as having rail-to-rail signal switching
capability; they are not CMOS switches and do not have the benefits
that CMOS switches offer. As with CMOS switches, the on-resistance is
not linear with applied switch voltage, but unlike CMOS switches they
actually turn 'off' as the signal exceeds a certain level. Due
to the gate threshold of the N-Channel switch, mid-rail is not a good
bias point if lowest on-resistance and maximum signal range are desired.
For switching in a 50R environment, the FET on-resistance should
be a small part of the 50R, or unwanted attenuation and even voltage
non-linearity will occur. This is worse with TI devices, SN74CBT3126,
in all respects and should
always be the choice. Some TI bus switches are intended for 3.3V
supplies, but these contain gate voltage-boosters and must never be
used for RF switching.
In addition, to maintain a switch on-resistance which is less than 5R
(10% of the 50R path), the switched voltage must never exceed +1.5V
even with Fairchild devices, even if our supply is 5.0V. If we
use TI devices, the voltage must not exceed +0.5V under the same
conditions; not very useful!
It may be that an on-resistance greater than 5R is deemed accetable,
but remember that it becomes increasingly affected by small variations
of amplitude (i.e. is non-linear) and may accentuate non-linear
distortion products of the signals being passed (intermodulation
The normally-used 'mid-rail' biassing and a.c. coupling of signals does
not meet the above criteria, even statically. The static on-resistance
is 7.5R for Fairchild, 9R for TI, rising rapidly as the signal becomes
more positive. This can be overcome, giving a reasonable dynamic range,
by reducing the bias voltage.
To maximise the linear dynamic range when using Bus Switches as
analogue switches, the bias point for an a.c. coupled circuit should be
no greater than 0.575V if we wish to maintain an on-resistance of
<5R for maximum signal-voltage range with Fairchild parts, or 0.25V
with TI devices. The voltage can then swing between 0V and (2 x bias)
without the on-resistance exceeding 5R and without dramatic
non-linearity of on-resistance. With TI devices this limits the
switched a.c. signal to 0.25Vpk, making the Fairchild device a far
better choice at 0.575Vpk.
Even with a larger signal, it is possible to swing maybe 0.3V below the
0V supply before the protection diode begins to conduct. The positive
increase of 0.3V to 0.875V (for a symmetrical signal) will give an
increased on-resistance, but this is still far better than if a
mid-rail bias were used, when even a typical
Fairchild part has a rapidly rising on-resistance of 11R and a TI part
After writing this, I encountered two very real examples of this: bus
switch elements actually running close enough to the cutoff point to
clip the positive peaks (by turning 'off' as the signal reaches the Vgs
One of these is also shown in the 2nd IF Tx
page; it seriously clips the high-impedance signal from the CODEC
Shunting R631 with 5k6 cures it, as predicted.
This is what it looks like on Glenn's latest STAR using TI bus switches:
Once again, the 5k6 shunt over R631 cures it.
The other example is in the MR, in a 50R path. The transmit IF signal
into the diplexer is switched in IC505, but the amplitude is high
enough (untrimmed) that the positive peak is clipped (view at IC505 pin
12 where it feeds the diplexer):
I have left the full 'scope face visible. The 'Y' origin is at the
lowest graticule line. Y scale is 0.5V/div, so clipping occurs at
+3.25V, no surprises. Of course, at levels near this there will be
compression as the FET on-resistance rises; some of this is actually
visible. The usual cure applies, shunt R520, this time with 1k2
since R520 is 2k2 rather than 10k.
My STAR uses Fairchild bus switches, which have better characteristics
(lower Vgs and Ron) than TI.
What will M0RJD do?
Good question! Since the old fool has taken the time and trouble to
look at these things, it would be nice to know his intentions!
I will shunt all the 'bottom' resistors in
the bias networks throughout
order to improve the dynamic range available and to prevent substantial
non-linearity. A drop in bias from half-rail to say a third of
would give better conditions regardless of level and impedance. At 1.4V
bias the signal can swing 1.4V negative before it has any danger
whatsoever of clipping, and 1.35V of positive swing before the
on-resistance exceeds 8R, which might still be deemed acceptable.
on-resistance increases too rapidly to be useful. Using a 10k 'top'
resistor, I need a 3.9k
bottom resistor. Maybe shunting the bottom 10k with 5k6 would be
simplest, giving 3.6k.
In the RF sections, a 2k2+2k2 network is used rather than 10k+10k. For
most of these, a 1k2 shunt on the lower resistor is appropriate. Beware
R510, used for biassing the H-mode mixer switches. Because of the
higher (6V2) supply a larger shunt value is appropriate, around 3.9k
will be appropriate to place the bias centrally within the switch range.
Most of the switching in STAR is at very low level, so it is possible
that no improvement will be noticeable; it depends. But it cannot
possibly make it worse.
I have fortuitously already built my own STAR with National
place of the more normal (and specified) 78L05 devices, since I had
them and have found them to be life-savers many times in the past.
National's LP2950C5V0 has a tested
limit at 25deg
C of 4.975V to 5.025V, which is 5.0V±25mV.
78L05 is variously specified as ±5% or ±0.2V, at best
5.0V±200mV, depending on manufacturer (there are many).
I will expect that, in the absence of any fault, my supplies will be OK
at least, if they come from an LP2950C5V0.
Why should we believe it?
I have been asked by [anonymous] why he should change all his switch
bias points when the original design works so well as it is. He asks
how he can
be sure that changing it will not break something - (setup | IP3 |
sensitivity | MDS | calibration | whatever)?
It is, of course,
wise to be cautious. You, as an individual, must decide for yourself.
Sorry, but I can only present the information as I have done, and leave
you to draw whatever conclusion you wish.
If you think that what I have said is balderdash, that is entirely your
priviledge. Please let me know if this
is what you decide, and why you believe it to be so.
I have presented information and calculations based on manufacturers'
published data, and I believe that my calculations and conclusions are
sound. If I were designing equipment using Bus Switches as analogue
switches, I would go through exactly the same steps and, unless I have
made an error, end up with exactly the same figures and conclusions.
The main thing that I would question is whether Bus Switches would be
the right device to use, but that is an entirely different scenario.
They work in practice, but 'could do better if they tried harder', as
one of my School reports said many years ago.
I am very aware that every design I have seen using bus-switches in
this way, not just STAR, sets the bias at mid-rail. This doesn't
make it right.
If you prefer disbelief or scepticism on the basis that someone,
sometime in the past, used different values, without seeing or asking
to see their calculations, then so be it. It really doesn't bother
You can check and double-check every step of my reasoning and
calculations - can you do that with the original design, or is it just
based on trust? If the latter, I suggest that you also look at some of my other pages, then come here
Feel free to show me the original calculations upon which the bias
level was based - I have not seen these so cannot comment on them.
Will your unit break if you change the bias? Why should it - you
are improving the linearity and dynamic range; if this breaks it would
you not be surprised? As long as you take care not to upset the working
point of any other devices then nothing else will notice (I believe
that the Bus Switches are all a.c. coupled except for my own modification, where the d.c.
coupling actually benefits several switches all at once). You can
always revert to the original design if you wish, it is easy enough, as
with all the modifications I have suggested on these pages. Of
course, when modifying things, you may accidentally break something. I
can't guarantee that your hand will be steady and your soldering
perfect. Mine certainly isn't!
Will the calibration change? Well, you may see more signal as a result
of reducing the on-resistance of switches in the 50R signal paths, so
yes, it might require re-calibration. Would you rather have a better
signal path or not? You decide.
Will the sensitivity improve? It might, if the bus switches in the Band
Pass Filter (BPF) are losing your valuable signal before it gets into
those noisy amplifiers.
Will it get worse? Tell me why you think
it would. Seriously; I would like to know.
What about distortion and intermodulation? OK, perhaps you can
explain why having a smaller dynamic range with the original half-rail
bias (before the switch becomes non-linear) will give better distortion
and intermodulation performance? You may not notice any difference after
changing the bias as I suggest, but why would better dynamic range and
reduced non-linearity make it worse? Let me
know why, please.
I really can't be bothered to try to measure the 'before' and 'after'
performance for you, since I have no doubt that sound design is, in
itself, worth while. I'm not trying to sell you anything; please don't
suggest I should prove that there is a tangible improvement. However,
if you perform any
measurements, and send me the results, I will happily add them here and
credit you with them if you permit.
guide to modifying PICASTAR
This is a tabular list of pulldown resistors on PICASTAR ComboBOARD bus
switches, showing both new values and recommended shunt resistors to
use over existing components.
The chosen values maximise dynamic range whilst avoiding the cutoff
region of the FETs.
I have allowed for the signal to swing slightly below 0V which is OK
until it hits clamp diodes.
For 1.35V bias on 5V supply, symmetrical swing is +-1.6V, 3.2Vpp.
Above 2.8V the FET on-resistance is becoming too high in 50R path,
especially with TI devices (10R and rising rapidly).
THESE VALUES ARE RECOMMENDATIONS ONLY. Use at your own risk. E&OE
NOTE: Sheets 1, 2, 3, 4, 5, 6, 7, 9, 13 and 16 do not include bus
Note: for R510 on IC504, Vcc is 6v2, so bias is 2v0 for reasonable
symmetry of on-resistance. Maximum signal before clipping is now 4.6Vpp
approximately, was originally approximately 3Vpp before +ve peak
|1k8 (see note below)
This is also available as a spreadsheet in
May I remind you that any change from the published PICASTAR design
will make your unit non-standard. You may not receive support from the
STAR team if something goes wrong. I am unable to offer any support or
guarantee whatsoever; I'm too old for that sort of thing <g>. I
have presented all the design information accurately, to the best of my
knowledge and limited ability. If you find an error please let me know.
Maybe you feel that the changes I have described should become an
'official modification'. New builders could then incorporate the
changes from day one. If this is your opinion, then why not say so on
the Picastar-users group mailing list on yahoogroups.com?
© 2010 M0RJD