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

enter image description here

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:

enter image description here

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.

user510
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    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
  • The other three are just not being involved at all – user510 Feb 04 '19 at 04:30
  • 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
  • Ok then what should I do? – user510 Feb 04 '19 at 04:33
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    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
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    See [How to disable a single op-amp in dual packaging?](https://electronics.stackexchange.com/q/41611/6334) for more details. – The Photon Feb 04 '19 at 04:57

1 Answers1

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You have your OP-amp backwards.

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

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