The input CM limit is I think tested and as reported in the datasheet, but that voltage is a combination of source signal and CM loop effects. 

Hello Clement,

  Were you able to solve your issue? 

Thank you,
Sima 

Hello Sima,

Yes but Vicm is 2.075V, close to the limit with 3V3 VCC, I am not very familiar with fully differential op amp. I have V input signal common mode of 2.5V and output common mode of 1.65V with Rf=Rg, so Vicm= 2.075V.

I would like to have more margin, if I understand correctly I need to reduce  common mode voltage in Vin or Vout, or reduce de gain? I supply the op amp with 3V3, I can also supply it in 5V to have more margin in Vicm but I am concern at power up to break the ADC fed by the op amp since it's a 3V3 input max.

Regards,

Hello Clement,

  For THS4531, the headroom is slightly larger by 100mV. Can you share your Tina-TI schematic file? Would you want to AC couple the input, and rely on setting the overall common-mode to the ADC? What is the ADC's input common mode/reference voltage? 

• www.ti.com/.../sbaa406.pdf

  Correct you would need to reduce either parameter. You may supply the amplifier with a 5V supply, and to protect the inputs of the ADC, you can add Schottky diodes at the output of the amplifier to the rails of the ADC.

• https://training.ti.com/protecting-low-voltage-adc-from-high-voltage-amp?context=1139747-1140267-1128375-1139109-1137815

  Here 

Hello Sima,

I already added a Schottky diode on the ADC input like in the video. Max voltage ADC is 3V3.

I think it will be ok. Also it's a very low frequency(<50Hz), maybe this op amp is slightly too expensive for what I do ( I need an auto one so I use

THS4541-Q1)

The simu doesn't have the diode and output filter:

5432.THS4531.TSC

Hello Clement,

  I would try something like below if you are using the THS4531 (Tina-TI file attached). I would also suggest moving to the THS4531A for better DC performance. 

  If you need an automotive qualified device, then that is correct, THS4541-Q1 would be the next choice of device.

  Also, here is a precision video series on Fully Differential Amplifiers (FDAs). It walks through in detail on input/output limitations, common-mode, and stability. 

3426.THS4531A.TSC

Thank you,

Sima 

Hello Sima,

Thanks, I am surprised you can use capacitor in series for such low frequency? it adds a huge impedance in the path and the gain  is modified?

Also something not clear, on all the apps note, I see Rg and Rf values of a few hundred ohms, why not to use a few kilo ohms?

Clement

Hello Clement,

  True, these are very low frequencies. As set in the circuit above, yes, you will see a very high impedance but at DC and a bit above (10-15ish HZ). Capacitor will start looking like a short gradually above these frequencies. In general, it is a tight cutoff. 

  Correct, general trend in high speed area is to use lower resistor values. This is because of interacting with input internal capacitance of the amplifier could cause stability issues and because of high input bias currents creating an undesirable noticeable offset for DC-coupled outputs. Another factor is the increase in thermal noise due to larger valued resistors, but you do get the advantage of lower power usage. 

Thank you,

Sima