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High frequency noise at the output of a 4th order filter

Question asked by hydntr on Mar 28, 2017
Latest reply on Mar 29, 2017 by harryh

On a board, we have several identical channels of an analog signal generation chain. A single channel consists of an AD5452, followed by ADA4807-4. We use all 4 opamps on the ADA4807 package to construct a 4th order filter at a cutoff between 150-200khz. Our DAC's are driven by an FPGA.

 

AD5452 is in bipolar mode. It's reference voltage is 1.5V. AD4807 is driven by 1.5V and -1.5V.

 

In order to test the board, we generate a sine-wave around 60khz frequency from all channels. Our SPI speed is 50MHz. For a period of a sine-wave, we generate 32 samples only.

 

When we test the board, we see that *some* of these channels have a peculiar noise on the output, especially towards the negative part of the sine wave. Attached, you can see the waveform shape. If we zoom in to the bottom peak of the sine-wave, the "noise" frequency is approximatly 50MHz

 

noisy opamp output

 

Schematics of a single channel is below:

 

dac output with 4th order filter

 

Also, we look at the output of the 3rd opamp in the above schematic, (to the left of R4), and the signal appears to be very clean at that point.

 

Our supplies appear to be clean as well. We tried adding a small capacitor (~100pf) at the output of the last op-amp, but it made the noise worse.

 

Any ideas what might be going wrong here?

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