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ADS1278: Using ADS1278 for wheatstone bridge

Lucy Fanelli76
Prodigy 110 points
Part Number: ADS1278
Other Parts Discussed in Thread: INA851, ADS1261, THS4551, ADS1261EVM

I am interested in using the ADS1278 to measure the output of a wheatstone bridge with four variable resistors.  I see the example circuit in figures 89 and 90 of the datasheet for the ADS1278, using fully differental amplifiers. I do not want to hook up a wheatstone bridge directly to one of these circuits because the output resistance of the bridge can vary by a lot, and that would affect the gain of these amplifiers by combining with Rg.  I'm not sure whether to do something like this with voltage followers on the two ends of the bridge:

Or whether to use an instrumentation amp that produces a single-ended signal going to the ADC pseudo-differentially, where the N side of the ADC is driven either by VCOM or a DAC to correct the bridge's offset (drawing shows a DAC)

Ideally, I would use an instrumentation amp with a differential output... does such a product exist?

If driving the ADC with a single-ended instrumentation amp output, do you still recommend the 2.2nF capacitor across the ADC input?

Is there some other way to drive a differential ADC so that the source resistance does not factor into the gain that I am not thinking about?

Thanks,

Lucy

  • 0
    TI__Mastermind 46760 points

    Hello Lucy,

    Since you will likely need some gain in the amplifier stage, option A will require a precision resistor network for the input amplifiers, but should work well if best AC performance (lowest distortion) is the goal.

    In order to avoid precision resistor networks, option B will also work well for fast, high speed measurements, but the distortion will be higher than option A.

    Take a look at the INA851; differential output instrumentation amplifier with programmable gain.

    If you do not need high data rates (and high bandwidth), then you may want to consider a device similar to ADS1261, which includes a PGA and can directly interface to bridge sensors.  The ADS126x supports data rates up to 40ksps.

    We have a very good application note that discusses bridge measurements.

    A Basic Guide to Bridge Measurements (Rev. A)

    Regards,
    Keith Nicholas
    Precision ADC Applications

  • 0 in reply to Keith Nicholas
    Prodigy 110 points

    Hi Keith,

    Thanks for the info. I am working my way through the app note. The 40ksps of the ADS126x might be sufficient.

    Thanks for sending the instrumentation amp datasheet. I would likely use this instead of having the input amplifiers and fully differential amplifier being discrete parts.

    One thing I was curious about-my plan when I didn't know about these other options was to use straight buffers as the input amps going to a fully differential amp. I think the FDA would have all the gain I required. But you said I'd likely want gain on the input amps. Is there some reason that it would be better to have gain on the input amps, rather than all the gain on the FDA?

    Thanks,

    Lucy

  • 0 in reply to Lucy Fanelli76
    TI__Mastermind 46760 points

    Hi Lucy,

    If building your own instrumentation amplifier, for example 2 buffers and the THS4551 FDA, it will be easier to do all of the gain in the first stage due to the high accuracy requirements of the resistors.  Most INA's and PGA's have matched resistors better than 0.001%, and it will be easier to do this in the first stage (single pair of matched resistors) verses the second stage FDA (2 pairs of matched resistors).  

    Assuming you need a gain of 50 to 100 for your application, option B above is probably the better choice.  The INA will include the precision resistors, and the FDA can easily convert the INA output to a differential signal suitable for driving the ADC inputs.

    If the speed of the ADS126x is sufficient, this will be the easier approach since all of the details for the PGA have been integrated into this ADC.

    Regards,
    Keith

  • 0 in reply to Keith Nicholas
    Prodigy 110 points

    Thanks Keith,

    For the AD1261, is that 40kSPs for each channel or all channels? Each channel would be fast enough, all channels combined would not. I looked at the datasheet but couldn't find an obvious answer.

    Do you sell an eval board for that part?

    -Lucy

  • +1 in reply to Lucy Fanelli76
    TI__Mastermind 46760 points

    Hello Lucy,

    The ADS1261 has a single ADC with an integrated multiplexer.  As soon as you scan through channels, the effective update rate per channel will be reduced.

    In addition, changing channels requires additional time for the internal digital filter to fully settle.  As an example, reading a single channel, you can achieve 40ksps.  If you use the multiplexer to read 2 channels, then the conversion rate per channel will be reduced to about 2.8ksps.  In this case, you would need an ADS1261 per channel to maintain 40ksps per channel.

    Yes, there is an ADS1261EVM available:

    https://www.ti.com/tool/ADS1261EVM

    Regards,
    Keith