32Hz noise on MAX4194 output, apparently caused by ESP32

[Emil]

I'm hoping someone can give me some hints in troubleshoothing this issue.

I have a board with an MAX4194 Instrumentation Amplifier. It has a gain of 1515 to amplify the 0 to 660µV output from a load cell. On the same board there is an ESP-WROOM-32 module. The ESP32 and the analog parts are powered by seperate linear regulators from the same battery. There are no switching regulators on the board.

When the ESP32 is running I get a 32Hz waveform as seen in the attached image (scope.png). (BTW, that should be 20mV per division, not 2mV as the screenshot shows.) When I hold the ESP32 in reset the waveform disappears, so the ESP32 appares to be the cause.

When the ESP32 is running I get a 133kHz sine wave ~60mVPP when measuring across the battery, but it's not detectable after the regulator powering the load cell and amplifier, and there is nothing at lower frequencies.

I used a 1.6KHz low pass filter when capturing that waveform. Using a lower frequency filter and averaging multiple samples I can get usable data, but I would like to find a better way of getting rid of this noise and I'm curious about the cause.

Why would the ESP32 cause noise on such a low frequency? If it matched the clock of the cpu or a peripheral I would not be surprised, but 32Hz?

[Neganur]

Not knowing your circuit, but WiFi modules pull significant amounts of pulsed current when transmitting. Enough to cause all kinds of fun stuff on the supply rails and ground plane which ends up all over the pcb. Your high amplification may be picking that up or maybe the psrr of the InAmp s too poor as well.

[_Wim_]

As Neganur said, most likely WIFI bursts. You could try to communicate heavily with the device over WIFI as see if your frequency changes...

[magtux]

Given the low frequency it is possibly the DTIM beacon of the router. The ESP needs to respond to this in order to keep alive.

Check this, for the esp8266 but should be releavnt

https://www.espressif.com/sites/default/files/9b-esp8266-low_power_solutions_en_0.pdf

[jmelson]

I'm hoping someone can give me some hints in troubleshoothing this issue.

I have a board with an MAX4194 Instrumentation Amplifier. It has a gain of 1515 to amplify the 0 to 660µV output from a load cell. On the same board there is an ESP-WROOM-32 module. The ESP32 and the analog parts are powered by seperate linear regulators from the same battery. There are no switching regulators on the board.

When the ESP32 is running I get a 32Hz waveform as seen in the attached image (scope.png). (BTW, that should be 20mV per division, not 2mV as the screenshot shows.) When I hold the ESP32 in reset the waveform disappears, so the ESP32 appares to be the cause.

You may need to put a matched set of RC filters on the input to the 4194.  So, the caps would be matched against each other, and the R's could just be 1% parts.  This will suppress RF at the input to the in-amp and prevent it from being rectified.

Jon

[Emil]

I'v been doing some more testing: I enabled one pheripheral at a time and checked for noise. The noise starts when I enable the capacitive touch sensor. WIFI and other peripherals do not appear to be a problem. I'm reading the documentation for the touch sensor peripheral now to see if I can find the reason.

[_Wim_]

I'v been doing some more testing: I enabled one pheripheral at a time and checked for noise. The noise starts when I enable the capacitive touch sensor. WIFI and other peripherals do not appear to be a problem. I'm reading the documentation for the touch sensor peripheral now to see if I can find the reason.

Thanks for the update, and keep us informed. This would indeed not be the "usual suspect"...

[Emil]

Thanks for the update, and keep us informed. This would indeed not be the "usual suspect"...

I have posted a question on the ESP32 forum, but no answers yet. https://esp32.com/viewtopic.php?f=2&t=3807