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Keithley DMM - Canceling Input bias

Discussion in 'Electronic Design' started by Paul, Sep 25, 2007.

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  1. Paul

    Paul Guest

    Hi,

    I'm the proud owner of a Keithley 160B, which can measure down to 1uV
    DC and has less than 10 pA offset current. Does anyone know if the
    Zero Adjust knob affects the *input* offset current? I will need to
    measure a few microvolts from various devices where the Keithley
    cannot produce more than 0.2 pA into the device.


    I appreciate the help.
    Paul
     
  2. Keithley could probably answer that, but I'd be very suprised if that
    adjustment was done anywhere very near the front end.

    Best regards,
    Spehro Pefhany
     
  3. John Larkin

    John Larkin Guest

    Connect it to a capacitor, let it measure its own input current, and
    try it!

    Does it use a mechanical chopper?

    John
     
  4. Paul

    Paul Guest


    John,

    Thanks for the help. I'll see what voltage it measures across a
    discharged cap. I'm sure it will charge the cap far beyond its 1uV
    resolution, correct? I believe the 160B has a solid state chopper,
    but I believe the older models such as the 150B have mechanical
    choppers. The 160B manual says,

    http://www.keithley.com/support/data?asset=3423

    "A typical Model 160B has an internal offset of about -100uVolts in
    the chopper amplifier. Hence, adjusting the Front Panel Zero from end
    to end will typically produce about +125uVolts and -325uVolts. This
    "extra" adjustment at the negative end of the range allows sizeable
    positive offsets to be compensated for, as is the case when the Zero
    is used for test lead compensation on the low-ohms Ranges. 200m ohms
    of test lead resistance will produce +200uVolts of zero offset, which
    can be "bucked out" by the Zero control. (See also Section 3-5d).
    Zeroing of the chopper amplifier output is actually accomplished by
    having the "Zero" circuit either source or sink whatever current is
    present in feedback resistor R160D. For example, an Offset voltage of
    -111uVolts on the feedback line produces a current of -1uA in resiseor
    R160D (111.135 ohms). A voltage of -1uVolt at the wiper of Zero
    potentiometer R114 will produce an equal current of -1uA in resistor
    R115 (1M ohm).
    With this zeroing scheme, for the chopper amplifier output to be at
    zero, the voltage at the feedback
    line will always be offset from the voltage at the input by a
    constant, say for example. -111uVolts.
    (The feedback line connects to the junction of R160C and R160D)."
     
  5. Paul

    Paul Guest


    I was wondering, what about using the Keithley to measure current? I
    don't see any reference to bias current in the manual for measuring
    current.

    Paul
     
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