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ADS7828: Negative analog input voltage range

Amjed Hallak
Prodigy 10 points
Part Number: ADS7828

Hi Team,

We are trying to implement sampling for multiple analog voltages ranging from as low as -2.5V to +2.5V. We are unsure of whether this is possible with the ADS7828 given that the electrical characteristics specify that the analog input lower limit is between -0.2V and +0.2V, but we are unsure of whether this is in reference to COM or to GND. We have our COM pin wired to the -2.5V supply line. Will the following schematic be feasible?

  • 0
    TI__Mastermind 24940 points

    Hello

    The Common pin is not meant to be used as a common mode pin, but instead as a near ground pin. You should not be putting -2.5 volts into the device.

    This device will not provide the range you are looking for, as this device cannot measure voltages below ground. The input range is 0V to Vref.

    The common pin is really meant to help with sensors that are on a different board than the ADC, which means the grounds on the ADC and the ground on the sensor board might not be perfectly at the same voltage. Connecting the COM pin to the sensor board ground then provides a more accurate reading of the sensor measurement.

    it sounds like what you are looking for is a fully differential ADC.

    I did a quick parametric search, based on I2C and differential input and found 11 possible devices

    Regards

    Cynthia

  • 0 in reply to Cynthia
    Prodigy 10 points

    Thank you for your input Cynthia.

    Really what we need is an 8 channel ADC that can sense between about -1v and +1v. With a differential ADC we end up wasting space with the redundant “-IN” pins and would prefer to have something where that reference can be common across all channels, since this were very constrained with space on this design.

    Are there any single ended ADCs with this kind of negative common reference?

  • 0 in reply to Matt Gallion
    TI__Mastermind 24940 points

    I think I understand what the application is now.

    There are very few devices that do what you are looking for, and none of which are I2C protocol.

    I would suggest shifting the analog signal up before it gets to the ADC.

    We have, what we call a cookbook, that speaks to this. It uses an fully differential amplifier (FDA) to convert the single ended input to a differential input and feed into the ADC. with the FDA you can set the common mode voltage to 1V, and shift the +/-1V input to 0-2V

    Regards

    Cynthia