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OPA2187: About the functions of OPA2187

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Intellectual 260 points
Part Number: OPA2187

Tool/software:

Could you confirm the below question for OPA2187?

  • Regarding the input bias current of the OPA2187, do both amplifiers (from the IC's perspective) flow in the same direction? Or does the direction change randomly during operation (e.g., one flows to the negative side, the other to the positive side)?
  • Is it possible for the value of the input bias current to fluctuate due to the operation of the internal chopper circuit in the OPA2187?
  • Could the input bias current momentarily increase due to some trigger (e.g., temperature changes, internal circuit offsets) during operation?
  • At room temperature (20°C to 28°C), is there any possibility that the input bias current could increase due to the operation of the internal circuits of the amplifier?
  • If the answer to question (4) is "YES," at what timing and through what mechanism might the internal circuits cause the bias current to increase

Regards

Kohei Nagaoka

  • 0
    TI__Mastermind 22587 points

    Kohei,

    Here are some answers to your questions.  Optimizing Chopper Amplifier Accuracy covers topics related to the current transients introduced by the chopping technique.

    • In general, for op amps you can look at the I­OS and IB and to understand the direction of IB.  In the case of OPA2187 IB  has a positive and negative polarity.  This means that IB can flow in either direction.  I­OS is equal to ±500pA and IB is ±350pA.  This means that the difference between the two currents can be quite large (e.g IOS = IBP - IBM = (+350pA) - (-150pA) =+500pA).  This behavior is generally true for CMOS devices and the bias current polarity can be in either direction on either input.  This is random and uncorrelated.  There is not a relationship between the two amplifiers in the dual package for bias current polarity.
    • The DC bias current is what the data sheet specifies.  Chopper amplifiers have very large fluctuations in bias current caused by charge-injection and clock-feedthrough of the input switching circuit.  The transients can be in the microamp range.  The magnitude of these transients will be different for different models of chopper amplifiers.  The document referenced above covers this.
    • The bias current will have a temperature coefficient but this will not trigger at one particular temperature.  The DC bias current is due to leakage of the input ESD diodes.  The transient bias current is due to the parasitic in the input switches.  Both bias currents will increase gradually over temperature (DC IB doubles every 10C).  I would not expect a rapid increase at any temperature inside the specified temperature range.

    Summary: Chopper amplifiers are the best choice when very good DC accuracy is required (i.e. low Vos and Vos drift).  However, many choppers can be loose this important benefit when large feedback or source resistors are being used.  Some choppers are fairly tolerant of large resistors so this isn't always a problem.  Optimizing Chopper Amplifier Accuracy provides guidance for the largest recommended source impedance and feedback impedance.  The list in the document shows that generally the wide bandwidth / low noise choppers tend to be more sensitive to source and feedback impedance.  However, as the document points out, if the source impedance is large than noise isn't a major concern as the source impedance itself will generate a lot of noise.  The point is that using the list in the document, you should be able to find a chopper that will work with your source impedance requirement.

    I hope this helps!  Best regards, Art