Low Pass filter

Question

I am making a Sallen-Key topology second order low pass filter with 10Hx cutoff frequency:

I am using the LM248 op amp. The resistor and capacitor values are specified in the drawing. However, when I input a waveform of 5Hz with 100mV amplitude(and a power supply voltage of 3V), the resulting output from my circuit is a straight line at -1 V:

The interesting observation was that when I raised my power supply to 4V, the output became a straight line at -2V, when I increased it to 5V, output became a straight line at -3V and so on. So the difference between the output voltage and power supply differed by 2.

I have checked my breadboard circuit four times and all the connections are right so I am very confused as to what is happening.

I've never used an LM248, but from the datasheet, it's not likely to run well on a 3 V supply. I'd shoot for +/- 10 or +/- 12 as minimum supplies to use with this part. Also, it's a quad part and you only show one op-amp being used. What are you doing with the other 3? — The Photon, Feb 04 '19 at 04:19
but if you leave them totally unconnected, they could oscillate and muck up the performance of the 1st one. — The Photon, Feb 04 '19 at 04:31
The usual thing is connect them as voltage followers with input connected to ground (or mid-supply in single-supply applications). — The Photon, Feb 04 '19 at 04:47
See How to disable a single op-amp in dual packaging? for more details. — The Photon, Feb 04 '19 at 04:57

score 4 · Accepted Answer · answered Feb 04 '19 at 04:08

4

You have your OP-amp backwards.

You are giving it positive feedback instead of negative. It’s simply saturated.

answered Feb 04 '19 at 04:08

Edgar Brown

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But I got this circuit from a textbook. I am also seeing some of the same circuits online for second order sallen key. Could you please let me know how to correct my error – user510 Feb 04 '19 at 04:30
@user510 Either you transcribed it wrong or the textbook made a mistake. Do an image search for Sallen-Key, and you will see the error. – Edgar Brown Feb 04 '19 at 04:33
yes you are right – user510 Feb 04 '19 at 04:34

Low Pass filter

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