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PGA116: Floating inputs

Emanuel Greco
Prodigy 30 points
Part Number: PGA116

we are using the PGA116 in the following configuration

PGA with A-Switch

3 analog switches (DG4051EEQ) are connected to 3 channels of the PGA116 (CH0-CH2), further 7 analog inputs are connected directly to the PGA (CH3-CH9).

When the analog inputs are connected to a fixed voltage there is no problem.

When one input is left floating and that channel sampled at the output, VOUT of the PGA is subject to a sort of weak pull up effect (see oscilloscope picture, with CH1 VOUT and CH2 DIN signals). It happens with all channels (CH0 to CH9).

oscilloscope

As soon we try to measure the inputs with an oscilloscope this “effect” disappears (the signals are then LOW), probably because the output is pulled down by the oscilloscope.

Are floating inputs allowed or they should be pulled down (100kOhm)? Why when we measure VOUT with the oscilloscope the signal remains still HIGH (so pulled up)?

Thank you in forward for any answer, best

  • +1
    Guru 140200 points

    Hi Emanuel,

    letting an input float is never a good idea. I would always add a pull-down resistor.

    Kai

  • 0
    TI__Mastermind 49435 points

    HI Emanuel,

    The PGA116 input consists of a multiplexer, and a non-inverting, programmable gain amplifier. The internal amplifier offers very high input impedance (10-GOhm), and each channel exhibits some input capacitance: the multiplexer channel input capacitance of 2pF and amplifier input capacitance of 3pF. 

    As Kai has mentioned, the best practice to use a pull-down resistor to GND on the unconnected channels; otherwise, the floating channels may be prone to noise and AC signals coupling into the unused channel input pins and charging any of the associated small parasitic capacitance.

    Thank you and Regards,

    Luis

  • 0 in reply to Luis Chioye
    Prodigy 30 points

    Hi Luis,

    There are no unused channels: they are floating because the external sensor could also not be connected. OK so we will add weak pull downs to all inputs.

    Thank you for your answer, best regards

    Emanuel