AC simulation issues

Schematic Simulation (Ngspice)
1

I need to verify if a passive network between active components A (TI PCM2704) and B (Maxim MAX9792) will provide the correct levels to the components B.

Component A is simulated trough an ideal AC voltage source, component B is simulated trough some OPAMPS (I tried to mimic the internal blocks structure provided in datasheet).

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I tried with several OPAMP spice models without much luck then I found on this forum an universal opamp mode uopamp_v1.1.lib l that at least is not giving me errors.

I’m running an AC sweep simulation between 20Hz and 20kHz.

The simulated circuit is this one:

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The result graph is this (only mag plots):

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that’s not satisfying at all of course … but since I’m not sure if the opamp model is correct neither if I correctly defined the circuit I don’t know if I can trust the result or not.

I can share anything about this project.

Can anyone help me to check if the simulation is running correctly ?

2

I see a voltage source for VCC, but none for VSS.

3

VSS is just a label if you look into MAX9792 block diagram VSS is just GND level trough a capacitor.

4

So you are supplying the negative dc power rails of the opamps through a capacitor?

Please check the data sheet on the role of the node CPVSS and the role of the charge pump which you have not integrated into your simulation circuit.

I guess that the role of the charge pump is to supply a negative voltage to node CPVSS to internally power the operational amplifiers.

5

After several trials I found that using the same resistors (20k) on all MAX9792 input pins (HP_INL, HP_INR, SPKR_IN) will level all inputs on -3.6 dB level… that’s not so suprising since we are splitting the signal in equal portions.

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6

I don’t see that.
What do you think charge pump is doing there?

7

of course it’s providing negative supply for internal op amps but from the simulation purpose I don’t think I need such detail. I’m not interested in xxx_OUT levels just to SPK, HP_L, HP_R,
that in turns should only be related to amplifier input impedance. The use of the opamp model is just to fulfill such need not to clone MAX9792.

Anyway taking your point of view into account I modified the circuit to simulate the charge pump also (datasheet states CPVSS is up to -6V).

The circuit becomes this:

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BTW I’m not really sure the polarity of the DC power supply simulating charge pump is placed correctly.

The output plot will not change.

8

You are inconsistent. You should not provide positive supply also :slight_smile:
I have never used KiCad Spice. Last time I simulated anything with Spice was may be 5 years ago, and last time I simulated OpAmps was 20 years ago. I used PSpice I got demo version at Cebit in 1993. It was limited (number of nets) version so when I needed to simulate circuit with 4 OpAms I used theoretic amps that could give me infinite output voltage (but added no internal nets) and had no supply pins.
As your OpAmp models have supply pins than they are probably not ignored by model. If negative supply is ‘supplied’ by capacitor then probably model consumes some current and shortly ends with VCC=VSS and not working as you expect.
Of course I’m sure of nothing.

If remember well PSpice I used didn’t accepted ideal voltage source connected in paralel with ideal capacitor - problems with calculations. I don’t know - may be here models incorporate for example ESR and ESL for capacitors.

9

the simulation model (at least one of the mode provided but the library) did not take into account opamp power supply :slight_smile: