I have downloaded the SPICE model file for the OP177 but found that it actually contains the model for the OP1177.
Where can I get the proper SPICE model file for the OP177 ?
thank you for your continued interest and support, especially for the hint to AN48.pdf. That really looks like a valuable and informative paper.
My inquiry for an OP177 SPICE model was just another attempt to find out (by comparison of the models) possible significant differences to the older OP77, which I could not spot at first glance in present day datasheets available on the internet.
In the meantime I have dug out an original copy of the old PMI volume 10 Databook dating back to 1990, where the OP177 appeared as a new device. Comparing those datasheets it seems that the two devices differ only by an additional selection grade of the OP177 with only 10µV offset voltage, a better bias current cancellation, less thermal bias current drift and somewhat better noise specs. The "rest", especially CMR and PSR seem to be the same.
I have included two simplified schematics that illustrate my first approach to plate current tracing and the next version that should provide better CMR.
Plate voltage is supplied by fullwave rectified 250Vrms 50Hz derived from the mains power lines as proposed in other tube curve tracer circuits for economical reasons. Peak voltage reaches up to 350..370 V. The current sensor consists of a half bridge of 100 Ohm resistors plus two 50:1 attenuators consisting of 0.1% resistors of 100k91k3.9kOhm. They reduce the maximum common mode voltage from ~350 to ~7 V. My first idea was to amplify and buffer the differential output voltage with one of the cheaper instrumentation amplifiers of the INA126 variety. Its architecture is sketched on the right side, set-up for a gain of 5.
After assembling and testing the circuit it turned out that this type of circuit has a major flaw (compared to the classical 3 OpAmp instrumentation amplifier):
The two OpAmps are driven to very different output levels. The lower one amplifies the common mode signal by 5/4 to ~8,75V which is then attenuated back to original level to compensate the common mode signal at the upper OpAmps positive input, resulting in almost zero output. However, the slight delay and nonlinear distortion generated by the highly driven lower OpAmp are not cancelled as in the symmetrical 3 OpAmp instrumentation amplifier. In this case, after trimming the CMR with Rtrim a residual signal with ~5 mVpp amplitude was left. With the present plate current scaling this corresponds to a resolution of 0.5 mA which is worse than what I hope to acchieve.
Actually there is no need to employ an instrumentation amplifier with high impedance inputs behind the attenuator circuit. Considerably better performance can be acchieved by replacing it with a classical differential amplifier. The second schematic sketches what I am going to test next. This circuit avoids the extra delay and distortion in the common mode cancellation bridge. The version sketched has been additionally modified to provide an overall gain of 1 (instead of 1/10) .
The second OpAmp serves to increase the open loop gain to acchieve higher bandwidth. Precision OpAmps OP77/OP177 have a gain-bandwidth product of only 0.6 MHz. At a gain of 50 the bandwidth would be only ~12 kHz. The 2nd OpAmp, set-up for a gain of 13..15 increases the bandwidth to ~240 kHz, which is even slightly more than that of the INA126 at a gain of 5 (200 kHz).
Hopefully this circuit will perform in reality just as well as in simulation.