Keithley 3720 User Manual

Section
5: Basic Digital Multimeter (DMM) Operation Series 3700 System Switch/Multimeter
User's Manual
5
-26 Document Number: 3700S-900-
01 Rev. A / August 2007
Dry circuit ohms measurement considerations
Dry circuit ohms uses a constant current source with voltage monitoring that is used to clamp
the current source voltage. The current source will remain constant as long as the monitoring
voltage is <20mV. When the voltage exceeds 20mV, the current source shunts current internal
to the DMM until 20mV is maintained at the DUT.
The voltage is measured at the SENSE HI terminal. There is a secondary voltage monitoring
circuit on INPUT HI. This is needed if SENSE HI or INPUT HI have different voltages. This could
occur if measuring 4-wire resistance and INPUT HI becomes an open circuit. In this condition,
SENSE HI would indicate no voltage and the secondary voltage monitor would clamp the
current source to <20mV for 10 to 10k at less than 27mV/, 99k|| < 1.0F for the 1 to
100ranges, and 10M< 0.015F for the 1k to 10kranges.
When using dry circuit ohms, the DUT is shunted by 100k and 0.9F. This allows the current
source to have minimal overshoot voltage under transient conditions. When used with a
switching system, the overshoot is <40mV in 20sec.
Measurement methods
The Series 3700 uses two methods to measure resistance:
 Constant-current source method (1through 1M ranges): Sources a constant-
current to the DUT. Voltage is measured by the Series 3700 and resistance is then
calculated (R = V/I).
 Ratiometric method (10M and 100M ranges): Test current is generated by a 6.4V
reference through a 10M reference resistor.
Constant-current source method
For the 1 to 1M ranges, the Series 3700 uses the constant-current method to measure
resistance. The Series 3700 sources a constant current (I
SOUR
) to the device under test (DUT)
and measures the voltage (V
MEAS
). Resistance (R
DUT
) is then calculated (and displayed) using the
known current and measured voltage (R
DUT
= V
MEAS
/I
SOUR
).
The constant-current method is shown below. The test current sourced to the DUT depends on
the selected measurement range. For example, for the 100 range the test current is 1mA.
Since the voltmeter of the Series 3700 has very high input impedance (>10G), virtually all the
test current (1mA) flows through the DUT. For DUT 1k, 4-wire ohms measurements should
be used as shown. Since, the voltage is measured at the DUT, voltage drop in the test leads is
eliminated (this voltage could be significant when measuring low-ohm DUT).

Keithley 3720 Specifications

General

Brand	Keithley
Model	3720
Category	Multimeter
Language	English