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.
I realized I put the file up as a TIFF. Here it is as a PNG file.
Thank you Dave for your recommendations. I am going to make provision for
two coupling caps on my PCB as drawn in your pictures. About AD1939 DAC
outputs, on which load is the performance specified ?
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.
Thank you Dave, all this was helpful,