I'm going to use the AD1939 in a design where the differential to single ended output stage
is supplied by an assymetrical 0-8V power supply.
I would like to couple the DAC outputs in DC to the diff-to-SE stage, for which I
have an half-supply common mode voltage at 4V.
I start with the same schematic as the one presented in your evaluation kit for
- I replace the OP275 by a low noise opamp that works with my supply and input
- I change the connection to ground of one resistor by a connection to a
low impedance 4V common mode. (see attached figure).
When I simulate this with my spice, where the DAC is replaced by two
symmetrical voltage sources having a DC common mode at 1.5V, I get
the functionality I am looking for. But...
... doing this, I ask the DAC outputs to absorb about 400uA DC ( (4V - 1.5V) / 2*3k).
Is this going to work ? Is this going to degrade performance ?
To further understand, can we consider the DAC outputs like OPAMP outputs ?
If yes how much current can we sink/source on these ?
Thank you for your support,
I do not know the exact load that this part is tested with. The evaluation board that we use to do bench testing with has around a 6K-ohm load per pin. I know that on a similar product I worked on recently the tested impedance was 3100 ohms per pin. We do not recommend using a lower impedance than 3100 ohms (higher load) but the part can drive it. You will find performance will degrade slowly as the load impedance goes lower. At some point it will become non-linear but for the most part it is a gradual decrease in performance.
Basically, the DAC output ports are not designed to drive low impedance loads so keeping it in the 3k1- 10K range should provide a good tradeoff between good performance of the DAC and noise immunity.
Hope this helps.