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MFB Antialiasing Filter Problem AD8137/ADA4932

Question asked by Michael.S on Jan 9, 2013
Latest reply on Jan 9, 2013 by charlyelkhoury

Hi all,

 

I created a lowpass 3-pole Antialiasing Filter in MFB fully differential Topology to drive a differential ADC. The cutoff frequency is 30kHz and the Gain is 11.

When I use the AD8137 for the filter the output of the AD8137 is oscillating at 180MHz when I switch on the supply voltage.

The feedback capacitor is 1nF, is the feedback capacitance to huge for the AD8137? I reduced the Gain to 1, but the output ist still oscillating.

When I add a resistor in series to the capacitor  to prevent oscillation it works fine.

 

When I use the ADA4932 for the filter the output doesn't oscillate and I don't need a series resistor. When do I need a resistor to prevent oscillation?

Why is the output oscillating with the AD8137 and why not with the ADA4932?

When I simulate both filters in Spice they work fine.

 

What must I consider in designing of active fully differential filters / choosing the right differential amplifier for my application?

 

Thanks

 

Michael

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