# Oscillating active filter

I built an 2nd order active highpass filter with cut-off frequency at 800Hz using an OPA847 (and R18=39.2k, R19=16.9k, R16=11.3k, R17=11.3k, C7=20nF, C8=20nF). Simulation works fine. In real life however, the active filter didn't work with these values. My assumption was, that OPA847 has a too low input impedance (2.7k), so I divided all resistors by 1k and multiplied the capacitors with 1k.

Now the oscillator shows me a square wave with frequency 450 KHz. It seems that the active filter is an oscillator now. How can I remove oscillation? Where do I have to look at?

• Voltage supply (single, double)? Why do you use such low resistor values ? Why do you select such a "critical" topology with a fixed gain of 1.43 (hard to meet in reality)? Did you find in the data sheet the mentioned input resistor value (2.7k) ?
– LvW
Commented Dec 9, 2014 at 9:59
• Double voltage supply with a voltage buffer in between. I found the 2.7k input impedance of OPA847 in the datasheet, which was the reason why i chose low resistor values. I selected this topology based on the book "Active Filter Cookbook" by Don Lancaster. Commented Dec 9, 2014 at 10:17
• Yes - it is a wideband opamp (do you really need it?) with a rather low input impedance. However, it cannot be used in S+K filters because it requires gain>12. Use another opamp or another filter topolgy (Multi-feedback or phase shift).
– LvW
Commented Dec 9, 2014 at 10:25

The OPA847 says on page 1 of the data sheet that it is stable for gains > 12.

Your circuit diagram appears to indicate that it has a gain of about 1.43.

Also, you have a connection on your model that appears to go to ground. If this is the /disable input then you might want to connect it to Vcc. Where it is shown will disable the device.

You might also want to indicate where the power rails connect and what voltages they are.

Supplementing my short comment I recommend to use a Sallen-Key topology with unity gain. However, you are required to change the opamp type, because the OPA847 is stable for gain values above G=12 only (datasheet). That is the reason for your problems.

Thus, select a unity gain stable opamp. In this case, the formulas for the case G=1 are simple:

Equal capacitors C7=C8=C

Pole frequency wp=1/[C(SQRT(R16*R17)]

R17/R16=4(Qp^2)

EDIT:

Here is an alternative filter circuit (highpass). For three equal capacitors C the formulas are:

wp=1/[C(SQRT(R2R5)] and Qp=SQRT(R5/9R2).