Keithley DMM - Canceling Input bias

[Paul]

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

 

[Spehro Pefhany]

On Tue, 25 Sep 2007 08:47:29 -0700, Paul <energymover@gmail.com>
wrote:

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

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
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

 

[John Larkin]

On Tue, 25 Sep 2007 08:47:29 -0700, Paul <energymover@gmail.com>
wrote:

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

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

Does it use a mechanical chopper?

John

 

[Paul]

On Sep 25, 2:22 pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:

On Tue, 25 Sep 2007 08:47:29 -0700, Paul <energymo...@gmail.com
wrote:

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

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

Does it use a mechanical chopper?

John

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)."

 

[Paul]

On Sep 25, 8:47 am, Paul <energymo...@gmail.com> wrote:

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

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