INA128: about INA128 in single supply

Hi Vera,

To determine if this configuration is valid there are a few things missing:

1) What is the common-mode input voltage and differential input voltage range?
2) What is the gain?
3) What load is the INA128 driving?

Additionally, the reference pin of an instrumentation amplifier cannot be driven with a voltage divider. The reference pin must be driven with a low impedance source (such as the output of an op amp) to preserve common-mode rejection. You could use an op amp to buffer the voltage divider to accomplish this.

You may find the following tool useful for understanding the limitations of our instrumentation amplifiers in a given configuration: www.ti.com/.../ina-cmv-calc

hi Zak,

thank you for your support!

for your questions, pls help comment besides the ref, anything need be changed, tks a lot! 

1) What is the common-mode input voltage and differential input voltage range?

    common-mode input voltage---48V，differential  input voltage range----several 10s mV.  

but as you can see from the circuit, the input is AC coupled, why should we concern about the VCM?

2) What is the gain?

    26db.

 3) What load is the INA128 driving?

  phantom MIC.

Hi Vera,

In the configuration you have described, you cannot operate the INA128 with a 0V common-mode input (see plot below). 

If you want to maximize your output swing and maintain linear operation then you will need to lift your input common-mode up to at least 4.3V. In this case the easiest solution would be to tie the same 6V reference to each of the 330kOhm resistors on the input.

Hi Zak,

thank you very much!  

because if add 330 Ohm, it will cause the input impedance be lower,  customer revise it into dual supply, as below,  is it ok now?

Vera,

No problem! The schematic looks good from a functional standpoint now. One last thing I would recommend: If you're going to have differential and common-mode input filters, then it is best to make the differential capacitor (C1573) larger than your common-mode capacitors (C707 and C1715). 10x larger is a good rule of thumb. This places the differential cutoff frequency approximately 20x lower than the common-mode cutoff frequency, which prevents common-mode noise from being converted into differential noise due to component tolerances.