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ADS122C04: Issues reading Pyranometer output

Part Number: ADS122C04

Hey guys,

Recently we've been trying out the ADS122c04 in some applications, and recently we've been trying to make some Pyranometer readouts.

Due to a request from the final client, there are no biasing resistors on the differential output and the system is not connected to local grounding.

An example circuit is as follows, note that filter component do not represent the final values:

As far as we're concerned, the differential input reads return close to zero as the input slowly creep from 0V to 1mV in a 5 minute interval, and after that will not show measurements when exposed to voltages as high as 10mV. Yet, measurements will eventually pick a signal and will output correct readings until there's a 0V input again.

Are there any ways to use a circuit without biasing resistors for common mode voltage?

  • Hello,

    It is not possible to interface a truly floating source with the analog inputs of an ADC, op-amp, INA, etc.  The inputs must have a voltage potential with a known relationship to the power and GND voltage potentials of the measurement system they're interfacing with.  Outside of violating the input common-mode range for the measurement system, there is also no way for DC analog input bias currents to return from a truly floating source and some local path must be provided (most INA datasheets will show this).  The ADS122C04 datasheet provides the requirements (copied below) for the absolute voltages applied to the inputs.  You will need to bias the inputs such that they meet these requirements or the system will work unreliably.  Input isolation is often used to avoid having to connect the potentials of the input source directly to the measurement system.