We are designing a low-cost two channels data acquisition system based on PCM2901. The device acquires the signals form IEPE accelerometers.
I attach the schematics. of the device. The USB interface is isolated by the use of the chip ADUM4160BRWZ
The prototype has been built on a 4 layer PCB The outer layers have signals and power-supply traces. The inner layers have groud planes. There is a clear separation between the two sides of the isolation
When observing the data acquired, harmonics at 1,2,3kHz are clearly visible even when no load is applied. The signals at the two opamp outputs (looking at pin 7 of U12 and U13) shows pulses with a period of 1ms when the software is acquiring the data.
By making some research, I notice that 1kHz noise is quite a common issue in USB audio device.
I have already made some tests by connecting a 100uF tantalum capacitor close to the USB connector and close to the 3.3V regulator output but the behaviour does not change.
I have also built on a breadboard a common mode filter to be placed near the USB connector. The filter was made by the common mode choke 744226 and two 100uF 10V capacitors. In this case the device is not recognized by Windows.
Below are attached the time and frequencies domain acquisitions by connecting to one channel a 6MOhm resistor and to the other channel a 20MOhm resistor
Which kind of filter do you suggest in order to reduce the harmonics multiple of 1kHz?
Thanks in advance
