I've been trying to get a multiple feedback lowpass filter to work. With this schematic.


The filter works in the simulation but when i try to build it, it gives no output.

The op amp used to build it is a MCP6002 dual op amp with a rail to rail supply. I'm using 5v and 0v to power the op amps and im supplying a 1v sine wave.

I've read online that i might have to use a vref bias voltage to get it to work but i can't find too much about this and i'm unsure as to why it's not working.

Any help would be appreciated.

  • \$\begingroup\$ Do you have the inputs to your opamps backwards? It looks like you have the non-inverting inputs grounded, but you are using a single supply. If the inverting input is higher than the non-inverting input, the opamp will try to sink current. This means the output will always try to be zero. \$\endgroup\$ – evildemonic Apr 24 '19 at 21:55
  • \$\begingroup\$ I've used this datasheet ww1.microchip.com/downloads/en/DeviceDoc/21733j.pdf and wired it up with vdd being 5v and vss being 0v And followed the schematic as shown in the gyazo. \$\endgroup\$ – Chriserke Apr 24 '19 at 21:59
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    \$\begingroup\$ There is no way this is going to work as drawn without using dual supplies. Think about how an opamp works: If the inverting input is higher than the non-inverting, the opamp will SINK current until they match. You have the non-inverting input grounded. \$\endgroup\$ – evildemonic Apr 24 '19 at 22:19

It depends on the input signal, just quickly glancing at it the DC portion feeds through so the input signal would need to be centered around 2.5V. If the input signal is not centered, then it would be better to add a reference voltage of 2.5V to the first opamp stage. I didn't check at the gain on the circuit but it looks ok.

  • \$\begingroup\$ Please see my comment to the OP. Am I off-base with my analysis? I can't see how without dual supplies the output of the first opamp wouldn't just sit at zero volts with any positive input. I am scratching me head. \$\endgroup\$ – evildemonic Apr 24 '19 at 22:21
  • \$\begingroup\$ So adding a voltage divider on the +input of the first op amp and setting it to 2.5v would do the trick? \$\endgroup\$ – Chriserke Apr 24 '19 at 22:26
  • \$\begingroup\$ Yep, the way to add references is on the +input \$\endgroup\$ – Voltage Spike Apr 25 '19 at 2:16

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