• State TI Thinks Resolved
  • Locked Locked
  • Replies 5 replies
  • Answers 1 answer
  • Subscribers 30 subscribers
  • Views 630 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

PGA305: PGA Gain and offset

Allen12
Intellectual 435 points
Part Number: PGA305
Other Parts Discussed in Thread: THP210, , PGA302

Hello, The sensor output is 1mv/V with an offset of 7mV. Setting the PGA gain set to 200, results in an offset of 1.4V. Can the PGA 305 compensate an offset of 1.4V?

What is the maximum offset value the PGA can correct for?

thanks      

  • 0
    TI__Mastermind 31655 points

    Hello,

    There is no analog offset compensation, so the effective full scale range of the input will be decreased as the offset increases.

    The digital offset correction technically has no limit, meaning it can be up to 24 bits and encompass the full range of the ADC output. Of course offsetting the entire ADC range does not leave much or any resolution for the signal, so it is best to minimize the offset externally as much as possible.

    Regards,

    Scott

  • 0 in reply to Scott Cummins
    Intellectual 435 points

    hello Scott, The  THP210  Vocm pin sets the output common mode voltage of the differential amplifier . Can the 14bit DAC of the 305 be used to drive Vcom and correct for offset error

    thanks

  • 0 in reply to Allen12
    TI__Mastermind 31655 points

    Hi Allen,

    Despite what the datasheet says, there is no way to permanently disable the internal reference buffer while the PGA305 is in its running mode. This is an update to the datasheet that will happen in the next revision.

    Basically, while you could technically disable the reference buffer while the PGA305 is in digital interface mode, as soon as it is put back into compensation mode, the internal digital circuitry will switch it back to using the internal reference buffer, so it will not be able to accept an external reference.

    Depending on your output needs, and the voltage range for the power to the pressure sensing front-end, you may look into the PGA302 instead. It does have some built-in analog offset compensation for the inputs.

    Regards,
    Scott

  • 0 in reply to Scott Cummins
    Intellectual 435 points

    Hello Scott, the 305 DAC drives the Vocm of the THP210,  therefore the  reference buffer   will remain "On" 

    If required,  can the DAC output be set to 1.25V, independent  of the "Third-Order TC/Third-Order NL Sensor Compensation block", 

    Doing so allows level shifting the sensor +5V  domain to ADC 2.5V domain. In addition, 210 would apply additional gain ~2     

    thanks 

  • 0 in reply to Allen12
    TI__Mastermind 31655 points

    Hi Allen,

    My apologies. I was mistaken, and I thought you were referring to controlling the internal reference of the PGA305. Yes, you would be able to use the DAC output to set the Vocm of the THP210. The trick would be in getting the DAC to output the desired voltage. When in running mode, the DAC will output a voltage based on the input voltage and whatever coefficients you have set. If you want a compensated output through I2C, this setup may not work, because the only way I can think to set a static DAC output would be to use the DAC clamping values, which will also affect the compensated digital output. 

    For example, if you want the DAC set at voltage x at all times to compensate for offset, then you can set the normal low value to the maximum value, so the PGA305 will think that it is always below the normal range. Then you can set the low clamp value to your desired voltage x, and it will permanently output that voltage. (The same could be done by setting the normal high value to the minimum, and setting the high clamp value to your desired voltage).


    If you set the DAC this way, the only PADC data you will be able to get is the raw PADC data with no compensation through the I2C output.

    Regards,

    Scott