Tool/software: TINA-TI or Spice Models
Hello,
I’m trying to simulate a timing behaviour with an OPA171 operational amplifier. It is referenced as a rail to rail output.
In the TINA simulation, the output only reaches 3.08Volt by a symmetrical power supply of +3.3 and -3.3 Volts.
What is the reason for this?
Furthermore, the simulation stops, when the switch is opening again, with an error at 83%
Thanks for your help.
Rene,
Even though some amplifiers are called rail-to-rail input/output, it is literally meant only on the input side but NOT at the output – no linear op amp can have output reaching its rails because to do so would require no current in the output stage - this is shown in the datasheet table - see below.
For more info, please watch the TI Precision Lab video under the link below:
Also, I have attached a newer macro-model of OPA171 - use it for your simulations.
Hello Marek,
thanks for your help. Like I wrote, the macro seems not to run in the simulation. With the new model you send me, the simulation stops now at 3% with an error.
Can you help me with this please? Thank you very much.
Kind regards,
René
Hello René,
I've made some tweaks to your TINA file in order to allow the transient simulation to converge, modifying the model of the OPA171 itself, re-drawing the circuit so I could better understand the connections, and changing the analysis parameters.
TINA offers various analysis parameters that can be changed to resolve a number of simulation issues. To see these, click "Analysis" then "Set Analysis Parameters." Since this is a transient simulation, I clicked the hand icon in the lower-right and loaded the "Transient Convergence Solutions" settings.
Download the new circuit here: capameasurement 20170117_iw.TSC
Best regards,
Ian Williams
Linear Applications Engineer
Precision Amplifiers
Hello Ian,
thank you very much for your help. The simulation runs through, like here below. But...
Are you sure that the OPA171 model is reacting normal? Look at the developpement of the Vin(-) of the Opamp. It can not be, that this voltage is rising immediately.
Look the behaviour of the circuit by applying a LM318. You can see, that the capacitor of 5µF is charged slowly via the feedback resistor.
Thanks for your help
Best regards,
René
Hello René,
The OPA171, like many bipolar input op amps, has diodes across its input pins (commonly called back-to-back diodes). Please see the figure below, copied from section 7.2 of the OPA171 datasheet.
When the differential voltage across the OPA171's input pins exceeds the diodes' forward voltage in either direction, one diode will become forward biased and provide a low-impedance path directly across the inputs. This is how Vin- changes so quickly with the OPA171 model. If we place a current probe on each input, we can see that 60mA is being sourced directly into Vin+ and out of Vin- because of the short across the inputs.
The LM318 is a much older and simpler model and does not model any of the input or output ESD devices, so you do not see this behavior. However, any op amp with back-to-back ESD diodes, which is most bipolar-input op amps, will show this type of behavior. Keep in mind that this amount of input current violates the absolute maximum rating of 10mA, so you might want to reconsider your application circuit or your choice of op amp. Many high-voltage CMOS op amps such as OPA191 or OPA192 do not have these diodes.
Best regards,
Ian Williams
