Hi John,
Thanks for using ADI products for your application. With regards to your questions, please see my answer below.
1. To what is the output DC-voltage (OUT-pin) referenced?
- The OUT pin is referenced to OGND
a. Can I supply what ever voltage I want at OGND as the voltage I want it referenced to (for instance 0V) ?
- I tried to supply the OGND with different voltage level and it offsets the output. I guess it can work that way but I still need to verify it to our product expert if there will be any trade off on the performance of the device.
b. Or must it be "real" ground = 0V or half supply = Vcc/2?
- Any levels as long as your output will not saturate on the rails.
2. When using single supply and want to use a gain >1 with the input buffer, I believe there is something fishy with the data sheet. According to the explanations for the eval board, page 19, it seems that the amplification resistors connected to IBUFIN- would terminate in ground (=0V in this case). I guess the input opamp would rail-out because of this?
-The output will not go to rail as long as your input voltage x gain will not exceed to the supply rails. For single supply operation, your ac input voltage should have an offset voltage ideally on the the VCC/2 to make sure that it is within the IVR of the opamp. Make sure also that the inputs are DC coupled.
a. Should the resistors terminate in IGND instead?
- Termination should be on ground. If you terminate it on IGND, the input buffer output will saturate on the negative rail.
b. ..and would it even work? - the IGND is not a very stiff voltage source as it is supplied with a two 100kOhm voltage divider.
- It will not work.
c. Can I supply IGND myself at ~Vcc/2, or would it introduce other problems, even if it is a good source?
- IGND is an output pin with VCC/2 voltage level. You should not put any voltage on that pin.
3. Capacitor choice for the RMS pin - would an electrolyte be fine here? Even if very small signals < 1mV is used.
-If you are referring to the capacitor for ac coupling, I suggest to use non polarized capacitor like ceramic caps.
Best regards,
Emman