Noise on ADC1 When Streaming Data Over UDP

[connordo]

Our lab has been working on a device to wirelessly measure electromyographic signals (EMG) from the body and stream them wirelessly to a computer. The circuitry is based on the backyard brain spike shield, but we've implemented it using an ESP32 to allow for wireless interfacing.

The single EMG channel was read on GPIO 35 (ADC1) since ADC2 is used in wireless communications. We're able to stream the data, but we're getting a lot of noise if we sample the data using ADC1 while the WiFi is running. I tested the device under three conditions (shown below in the graphs)

1) Data sampled on ADC1 and streamed over UDP to a computer (Noisy, shown in blue)
2) Data sampled on ADC1 while WiFi is active but sent over serial to the computer (Noisy, shown in orange)
3) Data sampled on ADC1 while WiFi is INACTIVE, with data sent over serial to the computer (Clean, shown in green)

These tests indicate that the problem isn't part of the data stream itself since it happens on wireless and serial streams but is inherent to ADC1 when the WiFi functionality is enabled. Does anyone have any idea why I'm getting these noisy signals?

[xien551]

When WIFI is active and transmport wireless signal, the noisy caused by high frequency switch signal would couple into the grond or signal line itself.

Usually, for an anlage and digital mixed ciruit, one key point for design is to seperate the analoge and digital ground and just connect both in one point, just below the ADC chip. However, this design may unwork due to the performace of chips itself.

Another method is using isolationg such as opto coupler with another seperate ADC or MCU, if the data must be transport during data acquisition.