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[FAQ] PGA900: Questions DAC Related to the PGA900, PGA300, PGA302 and PGA305 Devices

Bob Benjamin
Guru** 113765 points
Part Number: PGA900

Tool/software:

I have some questions regarding the use of the DAC on the PGA devices as follows:

  • How do I get my +/-20mV input signal from my sensor to correspond to 0 to 3V on my DAC output?
  • What does it mean by ratiometric DAC output, and how do I achieve this?

Thanks!

  • 0
    TI__Guru** 113765 points

    The DAC used is 14-bit.  The output can be either a voltage or expressed as a current.  Note that the resolution of the DAC is less than the ADC resolution.

    Bob Benjamin said:
    How do I get my +/-20mV input signal from my sensor to correspond to 0 to 3V on my DAC output?

    Here you would calibrate your system based on the input voltage values so that the output of the DAC corresponds to the desired levels.  For example, -20mV would correspond to 0V, 0mV input would correspond to 1.5V and +20mV would correspond to 3V on the DAC output.

    Bob Benjamin said:
    What does it mean by ratiometric DAC output, and how do I achieve this?

    This is unclear in the datasheets.  The DAC reference can be supplied by the internal precision reference or from a voltage divider from the supply.  In either case the reference is limited to a specific range based on the AVDD supply to the DAC.  For the DAC output to be ratiometric with the supply, the supply voltage must be limited to 4.5 to 5.5V.  When using a supply voltage greater than 5.5V, the output of the DAC will become non-linear.