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INA129: Using Two Amplifiers in Parallel with Different Gains — Possible Issues?

Gleb Korenzvit1
Prodigy 50 points
Part Number: INA129
Other Parts Discussed in Thread: PGA848

Tool/software:

Hi everyone,

I'm working on a design where I need to measure the voltage drop across a current-sense resistor.

The sensor output is a differential signal with a typical range up to 0-2V, but in certain conditions (cutoff state), the differential voltage can go as high as 5V.

The instrumentation amplifier (INA129) is powered by a dual ±5V supply.

To support two measurement ranges, I’m planning to use two INA129 amplifiers:

  • One configured with gain = 1

  • The other with gain = 50

The idea is to connect both amplifiers to the same resistor (i.e., parallel inputs), and then select between their outputs downstream ( Different ADC channel).

However, I'm concerned that connecting both INA129 inputs in parallel might cause interference or loading effects between the two amplifiers, especially given their different gain configurations.

Would one amplifier load or otherwise affect the other due to the parallel input configuration?
Is there a better way to implement this dual-range measurement approach?

Any advice or experience with similar designs would be appreciated!

Thanks in advance!

  • 0
    TI__Mastermind 27550 points

    Hi Gleb,

    You might have already considered this, but what about using a programmable gain amplifier like the PGA848? It has the two gains that you are needing and integrates how you select between the two. 

    For the INA129, I do have a few concerns with having differential voltages as high as 5V. With ±5V supply and a gain of 1, you can't have a differential voltage larger than 4.1V. I'm using the analog engineer's calculator as shown below. It's a super helpful tool to figure out if your able to support different signals. 

    If you are able to increase the supply voltages that would greatly help. Another option would be to change the reference voltage to Vref = -1V then your system can handle differential voltages from 5.1V to -3.2V. 

    Best Regards,

    Robert Clifton 

  • 0 in reply to Robert Clifton56
    TI__Mastermind 27550 points

    Hi Gleb,

    You might have already considered this, but what about using a programmable gain amplifier like the PGA848? It has the two gains that you are needing and integrates how you select between the two. 

    For the INA129, I do have a few concerns with having differential voltages as high as 5V. With ±5V supply and a gain of 1, you can't have a differential voltage larger than 4.1V. I'm using the analog engineer's calculator as shown below. It's a super helpful tool to figure out if your able to support different signals. 

    If you are able to increase the supply voltages that would greatly help. Another option would be to change the reference voltage to Vref = -1V then your system can handle differential voltages from 5.1V to -3.2V. 

    Best Regards,

    Robert Clifton