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OPA1642: OPA1642 buffer input bias voltage drop

Kristian Solberg
Prodigy 30 points

Part Number: OPA1642
Other Parts Discussed in Thread: OPA1652, OPA1662

Hi. 

I have problems with the bias voltage of a buffer circuit. 

I can´t manage to make the bias Vin/2. My supply is 5V. And I have tried to adjust R2 with a higher value,

but even with 5V out of the voltage divider the bias voltage won´t go over 2.1V.  (See att.)

What is wrong with the circuit? Why is the input current so high? According to the datasheet the input bias current should be around 2pA.

I also tried with a input impedance of 470k (R3), but I still couldn´t get it over 2.2V.

Are there any problems with attenuating the output back to the voltage devider? I thought that would work as a virtual ground with sink/source capability.

 

  • 0
    TI__Guru* 93105 points

    Hello Kristian,

    You are exceeding the positive common-mode voltage (Vcm) limit of the OPA1642. The maximum positive Vcm is listed in the Electrical Characteristics table as (V+) - 3.5 V. Thus, with V+ set to +5 V, the maximum positive Vcm is +1.5 V, and the applied +2.5 V exceeds the limit by +1 V. I suspect that since the maximum Vcm is being exceded that something internal to the OPA1642 is preventing the applied voltage from reaching the +2.5 V level. The 1 Meg resistor is developing a 0.4 V drop so the current flowing into the OPA1642 input is about 400 nA, which far exceeds the normal +/-2 pA input bias current.

    If you have the flexibility, you may want to try one of the other SoundPlus op amps in your circuit that have a wider input Vcm range. The OPA1652 is specified for a positive Vcm limit of (V+) -2 V. It is a JFET input op amp like the OPA1642. Then, there is the bipolar input OPA1662, which has a positive Vcm limit of (V+) - 1V. Note that its input current is about 600 nA so the 1 Meg resistor would have to be decreased a lot to prevent too much of a voltage drop. Either one of these amplifiers will accommodate a +2.5 V Vcm, when used with a +5 V supply.

    By the way, you can take the top end of R4 and connect it to ground, instead of back to the +2.5 V U1 output. 

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    Prodigy 30 points

    Thanks. I overlooked the common mode volatge limit, and I was not aware that it would actually limit the voltage on the input.

    I solved the problem by applying 8V on the opamp and then limit the output with a zener diode at 5V. 

    R4 is a digipot, and can only work between 0V and 5V.

    Thanks for the help..