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Original question:

ADS1261: Is there a maximum limit on what my transducer bridge frequency can be for implementing AC bridge chopping using...

ADS1261: Use of attenuating op-amp when using AC excitation

Peks1
Prodigy 35 points
Part Number: ADS1261
Other Parts Discussed in Thread: VCA810, INA849, ADS125H02

Hi,

I need to measure a variable resistance over a wide range, the top of the range being 2Mega ohms. The intention is to reap all the benefits of differential measurement with AC excitation however, if the resistance is measured at the minimum current setting of 50uA of the ADS1261, the output would be clipped i.e. would need need attenuating. Am I correct in saying that the attenuating amplifier would also need to be a differential one, in order for the AC excitation (drive) function to work properly? That is, e.g. VCA810 wouldn't be an option in this case as it has only single-ended output. Correct?

Have you got (op-amp) suggestions on how to implement the attenuation?

Many thanks!

  • 0
    TI__Guru 54176 points

    Hi Pekka,

    The IDAC from the ADS1261 cannot drive a voltage greater than the supply. So if you try to drive 50uA through a 2Mohm resistor, the current will collapse such that the voltage across the resistor will be a max of 5V (or whatever AVDD is)

    I would typically expect voltage excitation to be used with such large resistances, is there any reason you are considering using current excitation for this application? Voltage excitation would also allow you to keep the input signals within the range of the ADC.

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 35 points

    Hi Bryan,

    Thank you for your quick reply.

    Sorry, my mistake with my numbers. For sure ADS1261 cannot drive a voltage greater than its supply.

    My original question still stands though: an amplifier in front of the ADS1261 would also need to be a differential one, in order for the AC excitation (drive) function to work properly, wouldn't it? I suppose I could potentially use the internal PGA...

    The measurement range is wide which is why it seems sensible to use the flexibility of the IDAC in covering the measurement range, instead of having a separate adjustable voltage source for voltage excitation. A single voltage excitation isn't viable. Whether the +-4% accuracy of the IDAC is enough is a different question....

    Kind regards

    Pekka

  • +1 in reply to Peks1
    TI__Guru 54176 points

    Hi Pekka,

    Again, I am not sure how well the IDACs would work in this case, especially with such large resistances. Putting 50uA across 2Mohm results in a voltage of 100V, whereas the IDAC can only support 5V. That means that current coming out of the IDAC will have to drop dramatically (1-2uA), and could shut down all together since this is not an intended use-case for the IDACs. Unknown behavior can occur, and therefore this would not be a reliable system. You could create an external, adjustable current source, but I am not sure this would be more or less challenging that an adjustable voltage source.

    To that end, I know the max resistance to be measured is 2Mohm, what is the minimum? And what resolution are you targeting? With this information we can determine if large excitation voltages are actually required.

    Since I assume the sensor output is differential, and the ADC input is certainly differential, any signal conditioning circuitry in between the sensor and the ADC should also be differential. It would be possible to use an INA configuration similar to the INA849 shown below, where the OUT pin is connected to AINP on the ADC and the REF pin is connected to AINN. This is called a pseudo-differential measurement (learn more about input types in module 4.1 of our Precision Labs content: https://training.ti.com/ti-precision-labs-adcs)

    However, in your case, it is probably not necessary to use an external gain stage, as the ADS1261 already has an internal, very low noise PGA. As long as your input voltages are within the ADC recommended operating conditions, this amplifier should be sufficient.

    If you want to excite the sensor with voltage >5V, such that the output signals will also be >5V, you could consider using our ADS125H02. This is basically a high voltage (+/-15V) version of the ADS1261, albeit with fewer channels. In your case though it seems like one differential channel would be sufficient. The ADS125H02 does not natively support AC excitation like the ADS1261 does, but does have 4x GPIOs that could be used in a similar manner. You would have to manually swap the reference voltage however. I will also point out that the IDACs on the ADS125H02 are still limited by the lower voltage, 5-V supply.

    -Bryan

  • 0
    Prodigy 35 points

    Hi Bryan,

    The intention is not to use high-voltage excitation but to operate within the limits of the device. Instead and like you said, the excitation current would need to be somewhere around in uA scale, for 2Mohm resistance.
    The other end of the scale is such that it would be problematic for voltage excitation (conversely to current excitation challenges at the high resistance measurement).

    I think I have all the answers for now so, thank you for your assistance.

    Kind regards
    Pekka