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TLV9052: Input impedance

Shunsuke Yamamoto
Expert 6951 points
Part Number: TLV9052
Other Parts Discussed in Thread: TLV906X

Hi team,

My customer is looking for the input impedance of TLV9052. Do we have the data of input impedance of TLV9052?

Best regards,

Shunsuke Yamamoto

  • 0
    Guru 306330 points

    A shown in figure 7-7, the inputs do not behave like resistors.

    You can compute an impedance value from the maximum input voltage and the input bias current specification, using Ohm's law, but that value would not be really helpful for designing your circuit.

  • +1 in reply to Clemens Ladisch
    TI__Genius 13256 points

    Hello Yamamoto-san,

    Clemens is correct, this impedance value is not as useful as it may seem.

    Generally speaking, the CMOS input impedance is going to be very high relative to system impedances. For example TLV906x is in excess of 5Tera-Ohm looking into the inputs. 

    For most customers, it is good enough to say that this impedance is very high in nature.

    CMOS input impedance for GPAMP amplifiers ranges generally from hundreds of Giga-ohm to tens of Tera-ohm. Interestingly, this bias current is dominated by the leakage current flowing through the ESD diode structure rather than the current flowing into the input stage transistors. 

    I find it is easier to think in terms of bias current changing (IIB vs Vcm) or (IIB vs temp), but you can certainly think in terms of input impedance changing as well. 

    Here is a thread where we calculated the input impedance of a CMOS amp using the equation Clemens mentioned. TLV9064 Input Impedance

    Please let us know if you have any questions,

    Best,

    Jacob

      

  • 0 in reply to Jacob Nogaj
    TI__Expert 6951 points

    Hi Jacob-san,

    Thank you for answering the question. may I ask you an additional question though I have clicked "solved" button once.

    May I know how the input bias current is measured? Is there a measurement circuit?

    Best regards,

    Shunsuke Yamamoto

  • 0 in reply to Shunsuke Yamamoto
    TI__Genius 13256 points

    Hello Yamamoto-san, 

    Great question. Input bias current can be a challenging parameter to measure for CMOS devices. 

    Some picoammeters go low enough to give an approximation measurement of bias current. An expensive but accurate way of measuring IIB involves using an electrometer on the input of the device. 

    The way we measure IIB in lab involves using a small capacitor connected to the input of the amplifier and relying on the IIB to charge this capacitor. We then sample the capacitor voltage at various points to interpret the voltage of the capacitor. We can then calculate the IIB current by knowing the charge time and the capacitor size. 

    A similar method is mentioned in this E2E thread: IIB measurement

    Generally speaking, IIB is a challenging characteristic for customers to measure. If the customer wishes to measure this value, I recommend using the method noted in the E2E. 

    If the customer has issues measuring this value, I recommend that they refer to the datasheet for typical and max IIB values. 

    Thank you,

    Jacob