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AD8244 Oscillating with buffered High Pass RC Filter

Question asked by jimbles on Jul 17, 2017
Latest reply on Jul 21, 2017 by christoliver

Hey, I am having trouble with using the AD8244 for buffering a RC high pass filter. As I saw the datasheet describes creating a sallen-keys filter I figured this chip would be a great option - particularly as I need to do this for 32 channels, the reduced footprint would be a great help. Using an SMD adapter I hooked up a single chip like this: 

 

VIN = yellow trace - EEG signal generated from Arduino Due using full 0-3V range

VFILT = Green trace - High Pass filtered Vin

VOUT = Blue trace - Buffered VFILT

When I disconnect the VFILT from in the input of the second buffer things make sense, with VFILT looking like VIN with the DC removed:

When everything is wired up, rather than the blue trace VOUT matching the green trace VFILT, I get an oscillating mess

If I remove the capacitor then all three traces are the same

 

Having read the datasheet I cant see anything that I might have missed - the example applications for sallen keys or notch filters dont mention anything about avoiding this behaviour as far as I can see. I also attach a LTSpice model which didnt show any of this behaviour.

 

Can anyone see anything I might have missed?

 

Cheers

Jimbles

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