THS3217EVM: Output DC bias

Javier Serrano

Part Number: THS3217EVM
Other Parts Discussed in Thread: OPA134

Hello

We are new to this board and are not sure of having read the manual properly. The issue is we get 4 mV or 40 mV when we use the OPS amplification even if the differential signal we want to amplify is not connected. In our application sending 0V when the system is in an idle state is very important.

Our setup is

Both switches are at GND (we switch PD_SW for non-amplification)
V+ is at 6V, V- is at -6V and the GND is connected
Use J7 as output (J5 for non-amplification)
We have not removed or added any resistance to the board

So, how can we get 0V at the output?

Thank you

Javi

11 months ago

0 Michael Steffes 11 months ago

Guru 47545 points

LIttle hard to follow (a schematic is easier), but if you have disabled the OPS you should not expect 0V. Also, even with everything set up correctly, there will be an output offset voltage. The OPS is not a precision VFA part, but a very high slew rate CFA design - usually, the inverting current bias current will dominant the output offset for low gain settings in the OPS

0 Michael Steffes 11 months ago

Guru 47545 points

Also, now that I am back off airplanes, I had put this section in the datasheet for OPS output DC error calculations,

+1 Javier Serrano 11 months ago in reply to Michael Steffes

Prodigy 10 points

Thank you, Michael

0 Michael Steffes 11 months ago in reply to Javier Serrano

Guru 47545 points

You bet Javier, incidentally there are a couple of composite circuits that work pretty well to improve CFA DC precision. We devoloped those in the early days at Comlinear as designers whined about the poor DC precision, the inverting one is my preference,

https://www.ti.com/lit/an/snoa365c/snoa365c.pdf

I think this was the one I had in mind, somewhere I have a detailed development for the AC response here as there has to be a region of passing control which gives a response ripple and long settling tails if not matched well.

0 Michael Steffes 11 months ago in reply to Michael Steffes

Guru 47545 points

So actually, I found my earlier work and the one I like better was this one, quite involved analysis wise, but starts and ends with the same gain over F with some ripple along the way,

I had modified it later, the attached is a few steps to a pretty good result (25uV output offset) and flat through 200Mhz,

here is the modified OPS stage running it inverting now with feedback to the V+ input for DC offset servoing, The probe is on the OPA134 output.

THS3217 OPS composite with OPA134.docx

And this last file, remember I am only exercisin the OPS here, but the whole device is available.

THS3217 with OPA134 composite.TSC

0 Javier Serrano 11 months ago in reply to Michael Steffes

Prodigy 10 points

Thank you again for this additional information. Looks like a good solution to reduce the offset.

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THS3217EVM: Output DC bias