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I am using the OPA858-Q1 op-amp in my circuit. Without providing any input signal (input source grounded), I am observing an inverting terminal voltage of approximately 18 mV and its changing . According to the datasheet its not the case , and output voltage is also not zero . its 50mV
Offset voltage: 0.8 mV
Input bias current: 0.4 pA
Input offset current: 0.01 pA
This output voltage seems unusually high given these specifications. I am trying to understand whether this behavior is expected or if something is wrong with my setup.
I’ll check to see if any of our techs and engineers that are more familiar with this can help assist you on this, though they won’t be back till after the holidays. In the meantime, could you provide the full circuit diagram you’re using this in?
so for this circuit without giving any input when its open circuited my voltage is getting saturated ,and at the input i am getting 150mv at inverting terminal , can you tell me what went wrong , ideally it have to be zero .
I am thinking that the resistors in your set up would need to be swapped to get positive gain values unless you are looking to not have positive gains values.
The below shows the diagram of what it would look like with those resistances swapped.
As @heke has implied, the magnitude of the feedback resistor is much too large for this op-amp. Parasitic capacitances along with the large value feedback resistor for this high-speed op-amp can cause phase delay in the feedback loop, which is likely causing oscillations.
Improvement can likely come from using the feedback pin (pin 1) for the feedback connection rather than the output pin, but the magnitude is still much too high. Note that most test circuits in the datasheet use an RF value of 433Ω. A 1MΩ feedback resistor is more than three orders of magnitude larger than this.
I would recommend starting with a value close to 433Ω for RF and go from there.
I did my amplifier design on pcb board with the thickness of 4.2mm a transimpedance amplifier with 1 mega ohm feedback resistance ,with non inverting terminal to ground and inverting terminal to signal with shorting 1mega ohm feedback resistance and without giving any input to inverting terminal I am getting 36mV and without shorting when 1mega ohm is connected and not giving any input at the inverting terminal is giving 12mv , but the offset of opamp is 0.8mv where can be the issue be , but without giving anything i am get 36mV .by shorting ideally it have to be offset but y 12mV and these both have to be equal shorting feedback and opening input , where can be this leakage coming from
so you are telling that only for that resistor values it will work without offset , remaining with 1mega ohm feedback resistance with gain 7V/V the offset have to be constant right?but why its too high and varying continuously. and i want to operate this opamp in TIA mode direct pumping of current to inverting terminal with gain 10^6
The engineering staff at Texas Instruments were out of the office for the Holidays. Please give them a little time to catch up to their support requests.
If they do not get back to you in a timely manner kindly let us know, we can follow up with them on our end.
Use of an oscilloscope is necessary for testing your circuit. If you are not using one, I would highly recommend you find access to one, as a multimeter will be quite insufficient.
Make sure you thoroughly read through all of the documentation for this part. There are some rather complex stability issues one must deal with, particularly when configured as a TIA. In particular, take a look at page 21 of the datasheet and the two other documents mentioned from on that page:
I am encountering an issue with my transimpedance amplifier (TIA) design using the OPA858 op-amp, and I would appreciate your insights.
Circuit Details:
Op-amp:** OPA858
Configuration:** Transimpedance amplifier (TIA)
Feedback Resistor:** As specified in the datasheet
Input Resistor:** As specified in the datasheet
Testing Environment:** Custom PCB
With no input signal applied to the inverting terminal (just powering the circuit), I measure 18 mV at the inverting terminal.
When I ground the inverting terminal, the voltage at the inverting terminal increases to 23 mV.
The output voltage also reflects deviations from the expected DC behavior from this extra volatge at inverting terminal not only offset .
What could be the source of this extra voltage at the inverting terminal?
Could PCB design factors (e.g., stray capacitance, parasitic effects) or op-amp input bias currents cause this behavior?
Are there specific considerations for the OPA858 op-amp in TIA designs that might explain this discrepancy?
I have followed the datasheet-recommended values for the feedback and input resistors, and my testing is conducted on a PCB.
