OPA858: Unwanted spurs

Olaf Wohlmann1

Part Number: OPA858

Hello,

I have designed a AC-coupled non-inverting amplifier circuit as shown in the picture below. Thanks to the help of this forum the amplfier itself is stable and so far the results are quite promising. However when i do a spectrum analysis at the output without any input signal there is an unexpected spur at 250kHz. I have already inspected the power supply but no matter which power supply i use, the spur is still present. Is there anything in my circuit which would create such an issue? Any help would be appreciated.

over 3 years ago

0 Olaf Wohlmann1 over 3 years ago

Prodigy 230 points

Here is the spectrum showing the spur.

0 Michael Steffes over 3 years ago in reply to Olaf Wohlmann1

Guru 48395 points

Turn the power off to the DUT and see if it is still there, if so, ambient pickup

0 Olaf Wohlmann1 over 3 years ago in reply to Michael Steffes

Prodigy 230 points

Well, when i turn off the Vopa voltage then the spectrum is flat without any spurs. Same goes when i remove R67 to remove the DC bias.

0 Sam Burnett over 3 years ago

TI__Expert 3620 points

Hi Olaf,

These are not fun to track down, but there are a few things you can try:

-Shorter supply cables
-Faraday cage around the board / DUT
-Replace ferrite bead with zero ohm jumper

You mentioned changing supplies, I'm assuming you mean lab bench supplies? Or are we talking about switching converters or wall adapters?

Best,

Sam