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TLE2037; Impedence/Resistance Values

Prashanth
Prodigy 350 points
Other Parts Discussed in Thread: TLE2037

Hello Team,

I have a customer looking for input impedance/ resistance values of device TLE2037.

Can you help me by providing these values to assist him?

Thanks
Prashanth

  • 0
    TI__Guru* 93105 points

    Hello Prasanth,

    The data-sheet is the only resource available and it provides only small hints about the TLE2037 input impedance. The schematic shown in the data-sheet shows that the amplifier has a bipolar transistor input . Also, there appears to be a mirror circuit that supplies a portion of the input bias current. My thought is it is a bias current cancellation circuit. The input bias current is in the 15 to 90-nA range. Based on those points, likely the input resistance is on the order of Megohms. I realize that is only a ballpark range, but that is about the best we can do with no other information. The input resistance isn't usually much of an issue unless the amplifier is being applied in a very high impedance application. In such cases, then a JFET input, or CMOS operational amplifier is applied.

    TLE2037 input capacitance is listed in the Electrical Characteristics tables as 8pF. That doesn't  reveal much. An operational-amplifier has both common-mode and differential-mode capacitances. If we are using the amplifier in differential-mode, then ideally only the differential-mode capacitance would be seen. Likewise, for common-mode operation; onlt the common-mode capacitance is seen. About all we can assume from is that whatever mode the amplifier is operating in that the total capacitance will be about 8pF.

    Regards, Thomas

    Thomas Kuehl

    HPA - Lnear Products Applications Engineering

     

     

  • 0 in reply to Thomas Kuehl
    TI__Prodigy 350 points

    Thanks Thomas,

    Customer has come back asking more details on impedance calculation as below:

     Kindly let me know if input impedance can be calculated from the input bias current vs common mode input voltage graph given in the datasheet (page 18) , i.e. input impedance = (change in common mode voltage/ change in input bias current). By this method, I am getting around 320 Megaohm range. Please let me know if input resistance of op amp ( TLE2037 ) can be calculated this way.

    Can you advice?

    Regerds, Prashanth

  • 0 in reply to Prashanth
    TI__Guru* 93105 points

    Hello Prasanth,

    The common-mode input impedance is a dynamic, small-signal parameter. It is the measure of the input impedance from either operational-amplifier input to common. Like any impedance definition:

    Zcm = ΔVS / ΔiS

    where ΔVS is a small change in the common-mode input voltage, and ΔiS is the corresponding change in input current. TLE2037 data-sheet, Figure 11, does appear to contain enough information to approximate Zcm - at least the resistive portion. The figure's input Vic (common-mode voltage) vs. IIb (input bias cirrent) curve is relatively straight-line, so Zcm appears to be nearly constant throughout the common-mode voltage range. It appears that the IIB changes about 4.5nA, for every 4V of common-mode voltage change, which equates to a Zcm of 890M-Ohm. Note that I am using a large voltage change and not a small change, but because the line is nearly straight the small-signal Zcm should be similar.

    I checked the Zcm of other bipolar operational-amplifiers and the Zcm runs form the hundreds of megohms, to tens of gigaohms. So a 890M-Ohms Zcm appears plausible for the TLE2037.

    Regards, Thomas