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OPA858-Q1: OPA858-Q1

deepthi kankanampati
Prodigy 40 points
Part Number: OPA858-Q1

Tool/software:

Issue Description:

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.

Circuit Details:

  • Configuration: Inverting amplifier.
  • Feedback resistor (Rf): 1 MΩ.
  • Input resistor (Rin): 2 MΩ.
  • Non-inverting terminal: Connected to ground.
  • Power supply: [2.5 , -2.5]
  • Input signal: Grounded.
  • 0
    TI__Genius 11505 points

    Hi,

    There are a couple things that could be happening with the circuit to show why you are seeing these results. I would first start by reducing the feedback resistor to something in the hundreds of ohms. Large feedback resistances can lead to many offset and stability issues. I would also suggest adjusting the gain this device is in as this is a decompensated amplifier it requires a minimum gain of 7V/V to remain stable. In its current configuration the device will likely oscillate and cause many other issues.

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    Prodigy 40 points

    While connecting 1meg to 0ohm in feedback without applying any input I am getting offset of 42mV at the output,but in data sheet it's mentioned 0.8mV is the offset .so it's. It working for 0 ohm feedback with inverting configuration.can you please tell me where might be the mistake 

  • 0 in reply to deepthi kankanampati
    TI__Genius 11505 points

    Hi,

    Setting a 0-Ohm feedback resistor will surely result in erroneous results as this device is decompensated. The offset measurement was found using a specific configuration and is a guaranteed spec only when the device is configured that way. For better results, use a 453Ohm resistor for your feedback and use 75Ohms for Rg.

    Best Regards,

    Ignacio 

  • 0 in reply to Ignacio Vazquez Lam1
    Prodigy 40 points

    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