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PGA308: Strain gauge amplification of positive and negative VDiff signals

NickB
Prodigy 100 points
Part Number: PGA308

Hello,

I'm looking at the PGA308 to amplify a strain gauge on a force sensor. I need to be able to measure both tension and compression forces so my strain gauge will produce both +ve and -ve output signals to the PGA308 inputs.

The gauge is powered from a 3V reference so it's output will be 1.5V with no load (assuming an ideal zero balance). It has a 1mV/V rated output so I will only get a few mV swing around this mid rail voltage.

Before discovering the PGA308 I was designing a traditional load cell amplifier and manual variable gain stage. I was calculating gains of about half what the PGA formulas are suggesting which to me indicates that the PGA308 will only work +ve VDiff voltages (i.e. compression forces only, not tension).

Is this a true limitation of the amplifier or can it work with a strain gauge that can bend in tension and compression and therefore provide +ve and -ve VDiff voltages?

Any help is appreciated.

  • 0
    TI__Expert 5830 points
    Hi Nick,

    I wanted to let you know that our PGA expert is out of the office, he will be back in the office tomorrow.

    Best regards,

    Errol Leon
    Texas Instruments
    Precision Amplifier Applications
  • 0
    TI__Expert 6250 points
    Hello Nick,
    PGA308 should work with inputs of either polarity. This will require appropriate offset settings to be chosen so that the PGA308 output can swing negative from its zero input value. For example, selecting an offset that gives 1.5V out with no bridge deflection would allow the PGA to swing towards ground when the bridge goes into tension, while still allowing the PGA to swing towards the positive rail when the bridge compresses.
  • 0 in reply to Alexander Davis
    Prodigy 100 points

    Hi. Thank you for the advice. My plan is now to set the low voltage (i.e. no load output voltage) to be a half Vref (1.5V) which as you suggest will now allow me to measure force in both directions.

    When I calibrate this product I won't accurately know the strain gauge zero offset voltage or output.

    i.e. The spec is rated output of 0.9 to 1.1 mV/V and zero offset of +/-0.05 mV/V, so they could be anywhere in this range.

    Is it best to first set the front end gain, offset DAC,2nd stage fain and the divider to a nominal value and the fine tune the coarse offset until the beam is balanced. I would then plan on setting the front end gain to a value and adjusting the offset DAC ad 2nd stage gain until I achieve the desired results.

    Is this the best way to set this device up?

    My application will have the force beam soldered onto the amplifier board so I will set up once in a production environment where I have the ability to measure voltages and send any gain setting parameters to the PGA device.

    Thank you for your help.

  • 0 in reply to NickB
    TI__Expert 6250 points
    Hi Nick,
    It should be okay to do a coarse calibration first to get the PGA output roughly into range, then do a fine calibration once the sensor is assembled. It may be best to just calculate the coarse calibration, but not program it into OTP when you do that first calibration, otherwise you'll use up one of the OTP slots at coarse calibration, then another at fine calibration. If you calculate a typical coarse value, you can start from there for fine calibration, then write the OTP when you have your desired calibration complete.
  • 0 in reply to Alexander Davis
    Prodigy 100 points

    Thanks Alexander,

    I will give it a try and see. I was planning on havingy micro set the pga308 up at every power up. This will allow my to configure multiple gains and ranges for the dingle strain gauge.  Am I correct in thinking I don't have to programme the OTP locations but instead can just programme a volatile location for each setting?

  • 0 in reply to NickB
    TI__Expert 6250 points
    Hi Nick,
    There's no issue with having a microcontroller do your configuration at power-up. The OTP option is provided for cases where the PGA308 is operated as a standalone solution, with no ability to program it at power up.
  • 0 in reply to Alexander Davis
    Prodigy 100 points

    Hi Alex,

    Thank you for all your help. I'm prototyping now so will have a play with my final design shortly.

    Thanks,

    Nick