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PGA112 input impedance?

William Tolli
Prodigy 40 points
Other Parts Discussed in Thread: PGA112

Hello,

I am using a PGA112 with the following configuration:

VCCA = 3.0V

VCCD = 2.5V

VREF = 1.024V

AIN0 = 0-2V (100K impedance)

AIN1 = 0-2.5V (20K impedance)

With the gain set to 1:1, both channels seem to work fine.  However, on AIN1, when I increase the gain, the voltage on the input goes down (measured with high impedance DVM).  It looks like the high input impedance only works on AIN0, although I have only tested gain 1:1 and 1:2 on this channel.  As I increase the gain on AIN1, the input voltage drops incrementally at each gain increase.  I can find no reference to this type of behavior in the datasheet.  I have not tried a second PCBA yet, but I am starting to suspect that the part may be bad.  Is this behavior a good indicator that the part may have been damaged during assembly or handling?

Thanks,

William

  • 0
    TI__Mastermind 27715 points

    Hello William,

    This is odd behavior, since the input impedance on CH1 is much higher (10GΩ) than on VCAL/CH0 (100kΩ). What kind of voltage drop do you observe on CH1 as you adjust the gain? Can you provide a schematic of your circuit, including the input source and load?

    Best regards,

    Ian Williams
    Linear Applications Engineer
    Precision Analog - Op Amps

  • 0 in reply to Ian Williams
    Prodigy 40 points
    Hi Ian,
    The circuit going into CH0 is a bit complicated and has several configurations using different resistor population options. I think I fooled myself on the input impedance to the CH0 input in my current configuration. I will try a different configuration that definitely has ~100K input impedance to the CH0 and report the results tomorrow.

    The circuit is pretty simple going into the CH1 input. It is just a resistor divider with a 27K-ohm pull-up to 3.0V and a 50K pull-down to ground. The 50K resistor is a NTC thermistor. The data I have is below:

    Vref = 1.024
    AVCC = 2.976V
    DVCC = 2.501V

    Config Register - AVIN1 , VOUT
    0x00 - 1.967V , 2.03V (CH0)
    0x01 - 1.970V , 1.948V
    0x11 - 1.943V , 2.500V
    0x21 - 1.928V , 2.500V
    0x31 - 1.896V , 2.500V
    0x41 - 1.846V , 2.500V
    0x51 - 1.815V , 2.500V

    I will also try another PCBA tomorrow and report the results.