Electronics > Projects, Designs, and Technical Stuff
Op Amp transient analysis
Vettett15:
I created a lt spice model of my circuit which consists of an op amp in a unity gain voltage follower configuration which feeds a SAR ADC. I also included the flywheel circuit in front of the adc. There are two models, each one having a different op amp.
The one labeled "bad" uses a TI 4111 which isn't really designed to handle the capacitive load my circuit has. I originally designed with this op amp before I knew the capacitance values. Thus I made a second circuit that uses an op amp with unlimited capacitance drive.
Using the analog suggestion on how to measure phase margin I plotted the following.
Not sure how to interpret these, it seems that the tlv4111 has more phase margin than the analog ada4807 which seems backwards to me given the load circuit. Any suggestions?
I had some suggestions from others that said to put a 0.5V source in series with the non-inverting input to lift the inputs off ground and repeat the measurement... Didn't make a difference in the result.
Method used: https://www.analog.com/en/education/education-library/videos/5579254320001.html
TLV4111 model: drive.google.com/open?id=16f1DOy9MGa5eAqLpUB9AkA7d-5P2t6DK
TLV4111 ac analysis: drive.google.com/open?id=1yB7cSt02a8esAloV1XLMuPj0H3VwHI5K
ADA4807 AC analysis: drive.google.com/open?id=1XzN9bI2dqWq7XrIicgfRNYllvPiZDeZM
T3sl4co1l:
Try setting +in to 1.0V or something like that. As shown, the opamp is saturated negative and AC results are meaningless. Or, heh, they should be, but who knows if the model obeys the laws of physics or not.
Capacitive load doesn't matter with a series resistor that huge. Offhand, I don't even know of any amps that can't drive a 6.8k load to a full 5V.
Tim
iMo:
The switches will not work in the AC analysis, imho.
Vettett15:
I've tried it with .5v, 1v, 2v input and it doesn't change the results. One thing I just realized is the tlv4111 is only rail to rail on the output, they only spec it to Vd-1.5v on the input. I know I had the circuit on a bench and got the input to 4.6V so they must be pretty conservative. I just need to get an oscilloscope on the device to understand why I'm having issues with the tlv4111 if it's not due to the capacitive load.
T3sl4co1l:
Often they use a complementary input stage which changes over near +V. The input offset voltage changes suddenly (there will be a graph showing V_ios vs. V_cm), usually going out of spec -- but the amp continues to work normally.
What issues are you having?
Tim
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