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Original question:

ADS131M08: Overall accuracy 4-20mA loop

ADS131M08: Input impedance

Theo Keinarth
Intellectual 315 points
Part Number: ADS131M08
Other Parts Discussed in Thread: OPA2325, ADS127L18

Tool/software:

Hi TI-Team

I want to use this ADC in my safety plc project to acquire 4-20mA signals. One requirement is to filter 50/60Hz noise if needed. To achieve it I will sample with 800SPS for 50Hz and 960SPS for 60Hz and will then use a CIC (SINC1 or SINC3) software filter to get to 50/60SPS. I will use the ADS131M08 in combination with the REF7012 so I will get as much accuracy as possible especially over time. The goal is to have +/-0.1% over 0-60°C.

The only thing I am concerned about is the input impedance. Since the ADC is designed to measure high voltages with really high resistor values, I assume the impedance to be something that can be ignored after calibration? Or do I need to add an impedance converter (e.g. OPA2325) to compensate for this error?

Thank you in advance!

Regards Theo

  • 0
    TI__Mastermind 45285 points

    Hi Regards Theo,

    If you only do the calibration once after the device is powered up, you have to consider the input impedance change over temperature, you may need to do the calibration periodically. Adding an amplifier is a solution to minimize the impact of the input impedance of the ADC, but it will also introduce the noise and offset error etc., I'm not an expert on amplifiers, you can post your query on the amplifier forum, they may do a simulation for you to evaluate it.

    BR,

    Dale

  • 0 in reply to Dale Li
    Intellectual 315 points

    Hi Dale

    Thanks for clarification. So there is a drift there as well. Is there a official number how bad it can get. I want to let the system run for 20 years without any calibration. If I need to use an OPV I would need to perform a groundshift to compensate for the amplifier to not be able tog below 0V. I think the OPV isn't adding mentionable error after calibration because the ADC seems to be the problem in this case. 

    Are there any plans on releasing a more accurate version of the ADS131M08 series with an external REF input. Maybe an ADS131B08? I need a simultainious sampling ADC with SINC3 filter for 50/60Hz and a minimum of 6 channles. And the ADS127L18 is not an option because our MCU can't handle the data interface and its far to expensive.

    Regards Theo

  • +1 in reply to Theo Keinarth
    TI__Mastermind 45285 points

    Hi Regards Theo,

    The datasheet has stated all information we have characterized. You could use 6 sigma statistics method as an estimation for min/max value .

    Amplifier has drift spec that can add an error after one time calibration, for example, offset drift.

    ADS131E08 can be a good option for you since it has a high input impedance.

    BR,

    Dale