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ADS1256: Voltage reference for ratiometric strain gauge measurement

Johan Parr
Prodigy 20 points
Part Number: ADS1256
Other Parts Discussed in Thread: OPA335, OPA192, ADS1261, ADS1263, ADS125H02

Tool/software:

Hello,

I'm designing system which measures strain gauge with ADS1256. Ratiometric design as described in TI Application Note "A Basic Guide to Bridge Measurements"  (https://www.ti.com/lit/an/sbaa532a/sbaa532a.pdf) sounds like a good option.

I would however have separate REFP and AVDD. AVDD would be +5V and REFP done with 2V5 voltage reference.

But voltage referencences typically output only 10mA or 20mA and strain gauge with Wheatsone bridge with 120ohm resistors will need 20.83mA.

Is there a "standard way" to boost voltage reference output for more current? Or how is this typically done?

I have seen in some application notes that OPA335 is used to buffer voltage reference but that buffering is not done to boost current output.

Thanks,

Johan

  • 0
    TI__Mastermind 43011 points

    Hi Johan Parr,

    What is the reason for having such a low-Ohm bridge? I am just curious, as I was under the impression that most bridges / strain gauges were moving from like 350 Ohm to 1kOhm or higher to reduce the current through the bridge (and therefore power / heat dissipation, thermal issues, etc).

    For a higher resistance bridge, I would expect that you would use the 5V excitation source as the ADC AVDD supply, the ADC reference, and the bridge excitation source. This would just be fed from an LDO that would not have any issues sourcing the current your system requires. I assume because of the low resistance of the bridge you want to use a lower excitation voltage to reduce the current through the bridge, correct?

    If you stick with this approach, using a buffer would be possible to boost the current output. There are so many options to choose from I am not sure which to select to be honest. Maybe the OPA192 is a good device to start with?

    Also, you might consider the ADS1261 instead of the ADS1256. The ADS1261 has an integrated voltage reference that also has a REFOUT feature, where the internal reference output voltage is available at the REFOUT pin. You can use this voltage to bias the bridge, and then use the internal reference for the measurements to get a ratiometric reference configuration. This device would also need a buffer to drive >10mA of current, so this would make the system slightly less ratiometric. But something to consider

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 20 points

    Hi Bryan,

    And thanks for your answer.

    I wasn't clear. +5V excitation voltage would be fine otherwise but +5V can't be connected to Vref. Max Vref for ADS1256 is 2.6V. So, to have ratiometric measurement, excitation voltage has to be <2.6V. From there came the idea to use 2.5V voltage reference and somehow boost the output to use it for excitation also.

    I would otherwise use just LDO to make 2.5V (for VREF and excitation) from AVDD (+5V), but in one of the ADC inputs is optionally connected laser displacement sensor which outputs 0-10VDC. So to support that in the same design, I can't use inaccurate LDO but need a real voltage reference.

    Some customers may want to use or need to use 120R strain gauges so I try accomodate that.

    I'm also looking at ADS1261 and ADS1263. Lot of features in both...

    Thanks,

    Johan

  • 0 in reply to Johan Parr
    TI__Mastermind 43011 points

    Hi Johan Parr,

    Thanks for clarifying what you are trying to do

    I think the ADS1261 would actually be the best device for you then, because it can support a 5V VREF directly. Then you basically just need an LDO to generate your 5V supply, which can be used for AVDD, VREF, and the bridge excitation. And, if the internal VREF specs are sufficient for your application, you could use the ADC's internal 2.5V VREF to bias your laser sensor, which I understand would use the same 2.5V VREF you were going to use for the ADS1256

    For higher voltage sensors, you could consider the ADS125H02. This device can accept up to +/-15V voltages directly, without the need for an attenuator circuit. However, the ADS125H02 requires high voltage bipolar supplies e.g. +/-18V, to support high voltage input signals. But the ADC core is the same as the ADS1261, just the PGA design is different to accommodate voltages >5V

    Let me know if you have any questions about these devices, or if I missed something in what you are trying to do with all of the different sensors / inputs you need to measure

    -Bryan