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OPA4376 Amplifier Interaction During Input Overload

Martin Ford
Prodigy 10 points
Other Parts Discussed in Thread: ADS8345, OPA4376, PGA204, OPA376

I am looking into a design that requires several outputs from bipolar instrumentation amplifiers (PGA204) to be connected to a 0 - 5V ADC (ADS8345) - this ADC only accepts inputs in the range of 0 - 5V.  I am proposing to use a OPA4376 quad 5V single supply ampifier to buffer the IN-amp outputs to the ADC thus removing the problem of interfacing the bipolar signals (+/-10V) to the ADS8345 input channels.  Note an input resistor will protect the amplifier inputs and offset resistor to +2.5REF added to provide a 0 - 5V amplifier output for a +/-10V IN-amp output to the amplifier.

One problem we have however is that any of the IN-amp outputs could saturate when the external sensor input is disconnected and hence an overload voltage (+/-13.5V) could be applied to the OPA4376 amplifiers within the same package.  Does anyone know whether the individual amps within the OPA4376 package are isolated from each other such that an overload on any amplifier input will not affect the operation of any non-overloaded amplifiers within the same package, i.e. no channel interaction occurs under all operating conditions.

Secondly, the OPA4376 data sheet gives a CMRR figure for input common mode voltage range of (V-) < VCM < (V+) - 1.3V however the VCM range is (V-) -0.1 to (V+) +0.1.  I'm was wondering what the CMRR is for VCM > (V+) -1.3V and whether any problems would be seen in the circuit operation when operating above this upper VCM level. 

  • 0
    TI__Guru 60570 points

    Martin,

    First and foremost, next time please include the schematic while communicating the problem - as they say, a picture is worth million words.

    On the technical side, if I understand you correctly, you plan to use OPA4376 to buffer analog +/-10V input signal coming from a bipolar op amp to 0 - 5V ADS while using a resistor network to provide the needed voltage translation from +/-10V to 0 to 5V signal fed into the input of OPA376.  As long as the resistor at the input of OPA4376 limits the current to +/-10mA, especially in case of transients exceeding the 0 - 5V supplies (see the max rating table below), this should work just fine.

    Of course, the input to OPA4376 may never directly see +/-13.5V without limiting the input current with an input resistor to +/-10mA or the input stage of OPA4376 would be  damaged.  As may be seen in the table above, the inputs may never see any voltage exceeding supplies by more than 500mV.  Having said that, all four op amps in OPA4376 package are completely isolated from each other with the exception, of course, of shared supplies. Therefore, any signal overdrive of one op amp channel could only be seen  by another channel through a possible dipping in the supply voltage (thus need for good decoupling) and/or possible heating up of the shared package due to sinking or sourcing of large output currents.

    The OPA4376 will work just fine with the input common-mode voltage at positive rail.  However, for (V-)<Vcm<(V=)-1.3V a typical CMRR is 90dB (P-channel input differential pair is operational) while closer than 1.3V from the positive rail N-channel differential pair takes over resulting in a random change in Vos that results in a lower CMRR performance (see below).