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OPA2141: Problem with this OP Amp configured as a follower

Jose M Fernandez
Prodigy 110 points
Part Number: OPA2141
Other Parts Discussed in Thread: OPA320, OPA350, OPA340

Hello.

        I am using OPA2141 as a follower to increase the input impedance of the microcontroller´s ADC. All is ok if the (+) input is tied to GND or is tied to a voltage, but if the input is left floating, I see 0.9V in the OP Amp output. Is this a normal behaviour? I have attached an image where the circuit I am using is described.

                        Thank you in advance for your help.

  • 0
    TI__Guru 64670 points

    Jose,

    The specified input common-mode voltage range of OPA2141 is from the negative rail to 3.5V below positive rail which in your single supply 5V application means from 0V to 1.5 V. Because of the input bias current slowly charging the input cap, leaving the input floated causes the input to move toward the positive rail.  It will move about 300mV above the specified input voltage range and In the end will turn off the current biasing the JFET input differential pair and thus stop moving. At this point the input voltage will be at 1.8V and thus the output with two 56k resistor divider will see half of the voltage directly at the output of the op amp; thus 1.8V/2=0.9V you see.

  • 0
    TI__Guru* 93105 points

    Hello Jose,

    If you leave the input to the circuit floating there isn't any return current path for the input bias current. The bias current must return through the source resistance, or through an added bias resistor.

    The OPA2141 may not be a good choice for this single-supply application. The positive input common-mode range is limited to (V+) - 3.5 V, so with a +5 V supply the maximum positive input voltage will be +1.5 V. I suspect that a single supply op amp such as the OPA320, OPA340 or OPA350 would be a better choice because their common-mode input voltage range extends form supply rail to supply rail. For your 5 V supply the common-mode input voltage range would then be 0 to 5 V.

    Also, the RC filter you have at the output has a cutoff frequency of about 28 Hz. If that is what you intent it to be, then okay. Do keep in mind that the input of most ADCs presents a dynamic load to the op amp and requires the input current be supplied from the op amp that drives it. The 56 k series resistor will limit the op amp output current to low levels so that maybe something you need to consider. 

    Regards, Thomas

    Precision Amplifiers Applications Engineering