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KevinS

Analog Discovery 1 Hardware Fault Low Frequency Harmonics

Question

My trusty AD 1 seems to have developed a fault. If I connect the AD to the USB and run Waveforms and use the spectrum view I see some peaks at 13.02kHz, 26.04kHz, 39.06kHz on channel 2 that are not present on channel 1 and are not present on a different AD unit that I have. The units are both Rev C. Take a look at the attached screen capture. There is another peak at 52kHz but this is down in the noise.

I have also noticed that if I generate a signal on the AWG2 with no external connections, say 50kHz, this then appears on the spectrum of the channel 2 input. If I do the same with AWG1, then nothing appears on either channel. Perhaps there is some connection between these 2 issues.

Unfortunately I am looking at some small signals in the audio band and these peaks are getting in the way. I suspect that I may have somehow damaged channel 2 in the past and not realised. Does any one have any ideas where to look in the unit? I have some of the schematic from the documentation.

Thanks,

Kevin

 

2017-02-08 14_57_21-WF1 - Spectrum Analyzer 1.png

Edited by KevinS

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Hello,

In case the scope inputs are floating, not connected to anything, it can capture any internal or external signal.

Try shorting the 2+ and 2- to ground, see if these components are still present.

Here Wavegen 1 and 2  is set to generate 20/30kHz 10V pk2pk signal. Trace 1 is the capture with floating inputs, trace 2 with shorted inputs:i1.png

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Thanks for the reply. I've tried it with the inputs shorted and it makes no difference to the frequency or amplitude of the 13kHz harmonics. It does however prevent pick-up from the AWGs so unfortunately it looks like I have a hardware fault somewhere on channel 2.

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On the PCB the Scope channel 2 is closer to the AWG. I see crosstalk of 0.1% only when scope input is not connected to anything.

I can't imagine any hardware fault except some dirt between AWG 2 and scope channel 2 + input.
i1.png

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I've given the board a visual inspection and clean and it has made no difference. If I spray some freezer spray on IC11 the 26kHz peak disappears. Not sure what to do next other than mark channel 2 as U/S :-(

Kevin

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