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ADS122C04: ADS122C04 PGA GAIN ERROR

Gustavo Ventura
Prodigy 10 points
Part Number: ADS122C04

Hello everyone!

I am using ADS122C04 on a new project, and I have had problems with the PGA.
I put 3.3V voltage both digital and analog, both using the same GND.
I use an external 2.5V VREF. But I also tried using the internal VREF.
My problem is:
If I use the PGA with more than 4 (8, 16, 32 ...) the value that is converted is wrong, not linear.
A very small input signal, for example 150mV, if I set PGA = 4, the AD informs me 600mV. But if I put PGA = 8, I always see below, something like 975mV in the conversion.
What will it be? Is it a lack of bipolar voltage to excite the second stage of the PGA?

Thanks!

  • 0
    TI__Mastermind 20835 points

    Hi Gustavo

    welcome to our e2e forum and thank you for your interest in our ADS122C04.

    I think what you are seeing is just the limitation of the device to measure single-ended input signals when using gain >= 8 together with a unipolar analog supply.
    In case the negative input of the ADC is connected to GND (same as your signal source I would guess) and you select a gain >= 8 then you are violating the absolute input voltage range as specified in the Recommended Operating Conditions table in the datasheet. Both the negative AND the positive ADC inputs need to meet those requirements. That means you would somehow have to level shift the input signal.

    If your maximum differential input signal is 150mV then both the positive and negative inputs always need to be larger than:
    AVSS + 0.2V + |VIN_MAX| x (Gain - 4) / 8 = 0V + 0.2V + 0.15V x (8 - 4) / 8 = 0.275V.

    What is happening in your case is that the negative output of the PGA tries to drive below GND. However as you are using a unipolar supply, the negative output of the PGA clamps at 0V. The differential output of the PGA feeding to the ADC is therefore smaller than it needs to be.

    Regards,