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Hi,
i'm revising a PCB board based on ADS1256, it has a very noisy digital conversion ~4mV, and i'm working in it's filter, cause it doesn't have the filter indicated by the datasheet (VrefP-VrefN and analog input filters).
I saw it had some noise generated by the ADS in the Vrefp, in the sampling frequency, so i made some wire-ups and tested the circuit with the 49,9ohms and capacitors between Vrefp and Vrefn, it had a great improve, reducing the noise by 50%, and even with those wire-ups-noise-antennas.
So, now i'm re projecting the circuit, it's ok for vrefp and vrefn filter, but it uses three single-ended analog input for conversion, the datasheet just show filter for differential, so, should i make three filters (based on the 301ohms and capacitors (datasheet)) between AIN's and AINCON (4 resistors, 6 capacitors)? Is it advisable?
Should i re-project for differential? How should be the hardware in this case?
Thanks,
Rodrigo
Hi Rodrigo,
I'd be glad to help with your question.
Just to clarify, the ADS1256 measures all input signals differentially.... The only difference then between a single-end and differential signal has to do with whether or not the negative (or common) input is fixed to known voltage potential or if this voltage fluctuates with the signal amplitude.
Therefore, regardless of which signaling implementation you decide to use (single-ended or differential), I would always advise filtering the input signal differentially, meaning that you place a differential filter capacitor between the two input pins (whether those be between AIN0-AIN1, or AIN0-AINCOM, etc).
If you have the ability to measure a truly differential signal from your sensor, then yes, I would try to implement differential signalling to the ADC. However, if your sensor only provides a single-ended voltage output (referenced to a fixed potential), then I would stick to your current implementation. Building a single-ended to differential output amplifier circuit often adds additional cost and degrades the overall noise performance, when compared to measuring the signal directly with a high-resolution delta-sigma ADC.
One other question I would have for you, is can your sensor output negative voltages? If so, then it might make sense to bias AINCOM to a mid-range voltage to allow the sensor to swing around that common voltage.
I hope that helps,
Chris
P.S. We'd be happy to review your revised schematic and offer any suggestions to help you get the best performance out the ADC!
Hi Rodrigo,
Thanks for sharing your schematics!
The analog input filtering looks okay. However, I do see some potential issues with the the power supplies and reference that might lead to poor noise performance...
Best regards,
Chris