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ADS112C04: External buffer topology for high-impedance (3kΩ to 1.6MΩ) thermistor measurement with ADS112C04. Schematic review request.

Nick Alexeev
Intellectual 645 points
Part Number: ADS112C04
Other Parts Discussed in Thread: OPA2333P,

Tool/software:

Colleagues,

I’d like to measure a thermistor with an ADS112C04.  The resistance of the thermistor will be between 3kΩ and 1.6MΩ .  It’s a high impedance signal source, which means that external buffers are called for.  OPA2333P would do the buffering.  Is this an appropriate good topology for external buffers?

VDD = +3.3V

What should be the value of CDIF for ADS112C04 (the C1 in the schematic)?  The data rate sample rate will be 20sps.  Normal mode, probably not turbo.  Would 10000pF be sufficient?  It would be difficult to fit more than 10000pF of C0G due to size constraints.

What’s the value of the sample&hold capacitor in the ADS112C04?  I looked for either of those in the datasheet, but was unable to figure it out.

Best regards,
- Nick

  • 0
    TI__Prodigy 580 points

    Hello Nick, 

    From what I can see your setup using the OPA2333P looks good. The feedback loop with the 10kOhm resistors(R15 and R117) and the 10000pF(C16 and C18) capacitors will cause a small increase in settling time but shouldn't be a problem at 20SPS. 

    The 10000pF CDIF should be sufficient for your design you, and will have around a 8kHz 3db cutoff frequency. 

    For the sample and hold capacitor this device has an input buffer scheme that will leak a small amount of current that shouldn't effect the measurement. The datasheet specifies the absolute input current +/-5nA when bypassing the PGA, however one of the benefits of the input buffer you don't directly charge the capacitors. 

    Regards, 

    Andrew