* The Original AD9850 design has no
transformer; 10mA source into 200Rload //200 filter gave 0-1V output,
1Vpp into the filter, no transformer losses. The 200R source load is
necessary for d.c. conditions.
* AD9851 with transformer had initially 3+3:12t, later 3+3:8t,
but with no resistive loads on the DDS pins; the reflected ultimate
load is the only load.
* AD9851 has greater overall compliance range +1.5/-0.5. This means
the original total load of 100R to ground kept well within compliance
range (0 to +1v from this circuit), tolerating impedance ripple in the
* Taking, for now, only the 3+3:8t
transformer eventually used
with AD9851, there is no source load (no resistor, just the Hi-Z DDS
output) so each output can be said to see the Zload according to its
own turns ratio; there is no primary terminator to share by transformer
action. The 200R final load reflects to the outputs as 28R per output
(3^2 / 8^2) * 200R. Because there is no source terminating resistor,
there is no effective paralleling of resistances as there is in the
AD9951 design; each half-primary sees the other half in inverse
due to transformer action, but each presents an 'infinite' load since
it is a current source. Thus 10mA drive gives 280mV on an output at
each +ve peak; this reverses polarity due to the transformer inversion
when the other o/p drives 10mA and gives 560mVpp at each output, or
twice this between them; this transforms up by 8:3 to become 1.5Vpp
into the filter load of 200R.
This is 50% more drive than the 9951
It is well within the AD9851 -0.5V compliance limit as long as the load is present
If the load is absent, the current has 'nowhere to go' and the device
will conduct it to wherever it can (probably to parasitic substrate
diodes on the -ve swing).
* Using the same 'simple' calculation
as above; with the original AD9851
transformer of 3+3:12t, the corresponding per-output DDS load reflected
from the 200R filter would be 12.5R, so 10mApp gives 125mVpp, the
transformer primary sees +-125mV per half-primary, which is transformed
up by the transformer ratio of 12:3 to +-500mV or 1Vpp into 200R
filter. So the transformer ratio change from 3+3:12 to 3+3:8 actually
produces 50% more load voltage! The output pin swing is small, not
making use of the compliance range.