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,
400uA is quite small but it is not recommended to sink or source any current from the DAC outputs. It will degrade performance if it is high enough so it concerns me. Also, any noise on the pseudo ground point can be coupled into the common mode voltage source and cause other performance problems. You will not damage the part with this small of a current.
In thinking about your circuit, It is a tricky problem. If you allow the DAC outputs to remain at the CM voltage and just use a single ended op-amp with an 8V power rail you will end up clipping the signal on the low side. I think the simplest and lowest cost solution would be to use a passive differential filter circuit on the DAC outputs then reference the circuit to your 4V reference and amplify the signal to your required level.
I have attached a schematic of the filter. Instead of referencing the two 49.9K resistors to ground you would simply reference them to your 4V pseudo ground. You will have to either reverse the polarity of the 10uf caps or use bi-polar versions.