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PGA116 Gain settings not working

Benjamin Brooking
Prodigy 20 points

Other Parts Discussed in Thread: PGA116, PGA112

I have a design where I have used a PGA116 for it's multiplexing/calibration capabilities and have had this working happily for a while. Now I need to use the gain block and so I have modified the firmware on the processor which controls it (over SPI) to send the new gain setting with the channel setting.

The channel is still changed successfully as before, and if I do a read config afterwards the config byte is as I set however this doesn't appear to have any affect on the gain being applied to the signal.

I set the config by sending the 2-byte SPI command 0x2A31 to get channel 1 and a gain of 8.

I have tried other gain settings with the same result. I figure that the SPI comms are ok given both that the channel is clearly being chenged and that I can read back the config successfully. The only oddity is that the datasheet lists the gain config bits in a table with PGA112/113 next to it but doesn't mention anything about if/how the gain bits differ for the PGA116/117.

Any help gratefully received!

  • 0
    TI__Mastermind 27218 points

    We will set this up and try to reproduce your problem.  We should be able to get back with you soon.

     

    Art Kay

  • 0 in reply to Art Kay
    Prodigy 10 points

    I configured the part as you did and I did not have any issue with the gain.  Ox2A31 should be the SPI command to send to configure the PGA to used ch1 and have a gain of 8.  What are you using for DVdd, AVdd, Vref, and what is the range of your input signal?  

  • 0 in reply to Paul Frost
    Prodigy 20 points

    Both analog and digital sides are on a 3.3V single supply. The reference is set to 1.68V supplied by an op-amp buffer. The signal I am currently trying to measure is excursions of approx 100mV either side of the bias (1.68V).

    I have tried this on two boards to rule out a component fault, and also tried with a 0V reference to make it more obvious whether gain was being applied, all showing positive evidence that only a gain of 1 is applied.

    In case it is relevant my acquisition sequence runs as follows:

    • Chan: CAL3 Gain : 1
    • Chan: CAL2 Gain: 1
    • Chan: 1 Gain: 8
    • Chan: 2 Gain: 8
    • Chan: 3 Gain: 8

    But I can't see anywhere that you can't change gain repeatedly, or gain and channel at the same time?

     

    Many thanks,

    Ben.