Other Parts Discussed in Thread: ADS8684, LM2904
Hi TI,
We're seeing some odd behavior while connecting an INA827 to an ADS8684. The attached, simplified schematic shows how we 1) take measurements from photodiodes 2) connect the photodiodes to transimpedance amplifiers, 3) MUX the multiple amplifier outputs into a single INA827, and 4) connect the output of the INA827 to an input on an ADS8684. This allows us to multiplex many, many photodiodes to a single INA/ADS measurement system. However, we are finding some odd behavior. Specifically, the output of the INA827 (and input to the ADS8684) gives different output values for the same input (on its inverting input). To put numbers to it: with the photodiode in the dark, we always get 5V on the inverting input (measured with a multimeter). The output when this is the case is either 4.01V (as expected; the ADS reference voltage is 4.096V) or, sometimes, ~4.3-4.4V. We only see errant values after switching through many MUX channels, and we are only performing multimeter measurements with the MUX held on a static channel. We're wondering: is there any way we may have damaged or misconnected the INA such that we could see different outputs for the same input? We have confirmed the value reported by the ADS with a multimeter in both cases. We have also confirmed a stable 5V power supply via oscilloscope measurements. In terms of diagnosis, we think that our next steps will be to 1) measure the inverting input of the INA with an oscilloscope to confirm its stability and 2) sever the connection between the INA and the ADS and see if we can reproduce the problem. Do you have any other suggestions?
Thank you in advance for your time!
Dan