No op amp output voltage during simulation

Schematic Simulation (Ngspice)
1

I tried simulating an LMC6484 op amp in a simple test circuit (inverting input grounded, noninverting input either grounded or tied to VCC+), but the output shows zero volts for the noninverting input tied to VCC+.
Other op amps I’ve tried work ok for this test. Could someone please suggest what might be the problem?

I’m using several op amps and regulators in a complex circuit, and I’ve been first simulating each of the IC components individually to make sure that their SPICE models work.
Thanks!

LMC6484 op amp.zip (1.2 MB)

2

You already confirmed other opamps are working. I downloaded your project and tried it. But indeed the output is zero volts. Looks like the simulation model is not correct.

3

The issue is in the LMC648x-quad.lib. The following works:

  • LMC648x quad opamp model
  • filename: LMC648x-quad.lib
  •              1   2    3    4   5    6    7    8    9    10  11   12  13    14

.subckt LMC648xc out1 in1- in1+ vcc in2+ in2- out2 out3 in3- in3+ vee in4+ in4- out4
.include LMC648x.lib
XU1A in1+ in1- vcc vee out1 LMC648x
XU1B in2+ in2- vcc vee out2 LMC648x
XU1C in3+ in3- vcc vee out3 LMC648x
XU1D in4+ in4- vcc vee out4 LMC648x
.ends

Do you spot the differences?

4

Yes. Being new at ngspice, this is the second time I prefixed a character string with a number (due to typos in this case), will need to be careful with avoiding that in the future.

Thank you!

5

There is another important issue in ngspice (all spices):
1M is 1 milli (1e-3)
1Meg is 1 mega (1e6)
If you force the output of the opamp to GND with 1 milliOHm, the model will fail or the simulator will bail out.

EDIT: Not so in KiCad/Eeschema/ngspice. See below.

6

Some time ago I verified that 1M was interpreted as 1Mega Ohm resistor. I assume it has something to do with one of the compatibility modes.

Edit: / addition
Holger (See follow up) is wize, I’m quite a noob at the spice stuff.

7

Yes, you are right, that 1M is interpreted as 1e6, and 1m as 1e-3. But this is done in Eeschema, not in ngspice, so has nothing to do with compatibility modes.

Good to know and to remember.