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2khz noise

luke67

Mar 18, 2012
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Hi people !
I am doing some input-referred noise tests on a Pcb. This Pcb is composed by 8 quad-op amp that filter and give gain to signals. Actually I made this test also on his twin boards and I didn't notice frequency noise. Instead in this Pcb there's a big noise at ouptut at 2Khz frequency even when the signal input is linked to the ground (see the attachment). I give supply to the board with a laboratory dc power supply (7 Volt) and obviously there are 2 electrolytic capacitors on the board. Anyone has some idea to how reduce this noise?
Thank you very much!
 

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Harald Kapp

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Try a 9V battery to eliminate any chance of noise from the power supply.
Show us the schematic and the PCB layout. It is possible the noise is coupled from the outside. Also: what kind of OpAmp do you use? Not one with a built-in autozero feature, do you?
 

Harald Kapp

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The inputs of your OpAmps are all open circuit. This can cause pickup of noise. Add a large resistor (1MOhm) from each "+" input to GND.

Actually I made this test also on his twin boards.
Do both boards have an identical layout? Or are the breadboarded?
 

luke67

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breadboarded... do you think that covering the circuit that 2khz noise will disappear ?
 

Harald Kapp

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Try it. Har d to say from a distance.
Also: are the OpAmps' supplies well buffered by capacitors as near as possible to the supply pins?
 

luke67

Mar 18, 2012
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yes. Here is the exemplum of one quad-op amp. What do you think ? Also you think there may be the presence of parassitic capacitor ?
THX
 

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Harald Kapp

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One end of the bufffer capacitors is well near the OpAmp. The other end, however, seems to be connected to VCC or GND, respectively, by a comparatively long trace. Or do you have Power and GND planes underneath?

There sure will be parasitic capacitors, but rather small ones and the presence of parasitic capacitors alone does not explain a 2 kHz noise.

Did you try the battery instead of the bench supply?

What is the output load of the OpAmps? Are there any capacitors (that are not shown in the exceprt from your schematic)? The ad8644 should not be operated with more than 200pF load capacitance. Otherwise it can become instable.

Have you looked at the waveform of the 2 kHz signal with an oscilloscope? Is is a sine or some distorted waveform?

and what is the auto-zero feature ?
This is a feature where the OpAmp automatically nulls the offset. This is done by measuring and calibrating the offset internally or by chopping Dc and low-frequency signals. This feature can inroduce switching noise. The ad8644 you are using is not of this type, so this can not be the source of the 2 kHz signal.



btw: that's not what I consider a breadboard.
 

Harald Kapp

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That OpAmp should also not be critical.
What happens if you short circuit the capacitors C9 ...C12? Still 2kHz noise? If yes, the phase shift of the lowpass filter in the feedback loop is responsible for oscillation. Again: what is the waveform of the 2kHz signal? Is it a reasonable periodic waveform (sinusoidal or square), or is it very noisy?
 

luke67

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short-circuit replacing them with a wire, that is putting in parallel to a wire as if there weren't? Can you make a picture of what do you mean ?
THX!
The waveform: I'am gonna to analyze it this afternoon! I'll post it !
 

Harald Kapp

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No picture: yes, replace these capacitors by a piece of wire.
 
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