Right now I’m designing a single-ended to full-differential OPA filter.

But I don’t know how to measure or add the differential output from the filter in the spice simulator. I want to see the transfer function from the single-ended input to the differential output. You think you can help me? Thanks

The eeschema-ngspice interface is still rather limited. You cannot simply plot the voltage difference between two nodes.

So you have to create (design) your own voltmeter that provides a differential to single ended conversion.

Place a three terminal symbol with nodes 1, 2, 3. Assign 1, 2, 3 to in+, in- and out nodes. place a resistor from out to ground (to not have an unconnected output).

Assign an ngspice subcircuit to your new device that uses a ngspice voltage controlled voltage source vcvs inside, which produces a voltage difference between its in+ and in- nodes and amplifies the difference by factor 1 and sends this to node out.

.subckt meas in+ in- out
Emeas out 0 in+ in- 1
.ends

Now measure (plot) the ac voltage at node ‘out’. This will be the ratio to V1 if you assign ‘DC 0 AC 1’ to V1, not just AC, and you will get the bode plot (gain and phase versus frequency), if you run an ac simulation.

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Hello Holger.
Thanks a lot for your help, you rock jaja. I think I get the gist of what you’re saying. Basically I need a unity gain differential amplifier, a buffer at the output. But I’m a beginner at this so I don’t know much about designing devices with ngspice, I’ve been trying to find some documentation in the topic with no success, do you think you could give me some?