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PGA308: as signal conditioner for sigma-delta ADC of MSP430

Uwe Maschke59
Prodigy 50 points
Part Number: PGA308

I want to use PGA308 as signal conditioner for the sigma-delta ADC of an MSP430F(G)4250. The sensor shall be a measuring bridge for pressure and it shall work in both directions. Excitation voltage of the measring bridge shall be 5V as well as the power supply for PGA308. I asssume it will be best to set Vref of PGA308 also to 5V for achieving a proper coarse offset compensation. A/D conversion of the PGA308 output signal will be done by the 16-bit sigma delta converter of the MSP430 which has a power supply of 3.6V. For measuring positive and negative pressure values, I have to use a virtual ground of about half the supply voltage of MSP430 (1.8V) and I have to shift zero voltage of PGA308 to this voltage. Unfortunately, the PGA308 has no input pin for such a virtual ground and it seems that its output amplifier is also related to GND instead to Vref/2 or to the Zero DAC voltage. Has anybody an idea how to measure positive and negative signals with this bridge amplifier?

  • 0
    TI__Expert 6250 points
    Hi Uwe,
    It should be possible to use the coarse/fine offset registers in the PGA308 to set up this 1.8V offset when the PGA308 is calibrated. You'd adjust the PGA gain to accommodate the expected voltage swing of the sensor, then use the offset registers to set up the 1.8V offset.
  • 0 in reply to Alexander Davis
    Prodigy 50 points
    Hi Alex,

    thanks for your quick answer. Thats also what I suspected, that every change of gain behind the Front-End-Amplifier will shift my 1.8V offset far away. Nevertheless, it should be possible to change gain at the Front-End-PGA without an offset shift. Only the remaining sensor offset of coarse offset calibration will cause a corresponding shift. Am I right?
    Changing gain during operation could be a requirement of our application for better resolution in low pressure range.

    Regards,
    Uwe
  • 0 in reply to Uwe Maschke59
    TI__Expert 6250 points
    Hi Uwe,

    You're correct that changing gain would change your programmed offset as well, as both the coarse and fine offset DACs are summed in ahead of the PGA.

    If you only need a handful of ranges (<=7), and anticipate calibrating one time at the factory, you could store the required offset + gain settings for each range in a different bank of the PGA308's OTP memory. That is only programmable once, unfortunately, so would be baked in at the factory. Another option would be to include a small I2C EEPROM to store offset+gain data for the configured ranges, which would let you adjust these if recalibration were necessary.
  • 0 in reply to Alexander Davis
    Prodigy 50 points
    Hi Alex,

    good idea to use several OTP banks for different gain settings!
    I think we can handle it. Thanks!

    Regards,
    Uwe