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OPA124 (obsolete) question

Gregory Duperon
Expert 6680 points
Other Parts Discussed in Thread: OPA140, OPA145, OPA196, LMP7701, OPA191

I have a customer that had used OPA124 in a previous design.

They are looking at the OPA140AID as a possible replacement. One significant difference between the two is that the OPA124UA has a substrate pin (pin 8). According to the datasheet, it is supposed to be connected to a guard trace if there is one and to ground if there isn’t. In the customer's application, there is a guard trace. Do you know what the function of this “substrate” pin is? Does it somehow provide extra shielding? Why does the OPA140AID not have a substrate pin? Customer is trying to determine if Ithey need to do RF immunity testing when they convert over to the OPA140AID.

I've attached the OPA124 datasheet.

OPA124_Burr-BrownCorporation.pdf

  • 0
    Guru 140200 points

    Hi Gregory,

    the OPA140 is ten times faster than the OPA124. So a schematic would be needed to be able to answer, whether the OPA140 can be used as a direct replacement for the OPA124.

    The substrate pin is just some sort of shielding. But more hum shielding and shielding against DC leakage currents, not so much HF shielding. Therefore, when it comes to RF immunity testing, I don't think that the substrate pin is what counts, but more that the OPA140 is way faster than the OPA124.

    Kai

  • +1
    TI__Guru 63670 points

    There is no direct replacement for OPA124 BUT the closest would be OPA145, OPA191 and OPA196, or a lower supply voltage LMP7701 (max Vs<12V or +/-6V).

    The substrate pin 8 was used as a guard shield to minimize the input bias current leakages inside the op amp by biasing internal shielding at the input common-mode voltage as shown below.  With the minimization of the IC's in the last 30 years this feature is no longer internally needed or used.

    However, in the applications requiring external shielding like TIA, a "guard" may be needed to drive a shield that physically surrounds the PCB input trace and feedback circuitry that is held at a potential equal to the average input signal potential, which minimizes the input bias current leakages - see below.