Keithley 2700 User Manual

Model 2700 Multimeter/Switch System User’s Manual Basic DMM Operation 3-27
Ratiometric method
For the 10MΩ and 100MΩ ranges, the ratiometric method is used to measure resistance.
Test current for this method is generated by a 0.7µA current source (I
SOUR
) in parallel
with a 10MΩ reference resistance (R
REF
) as shown in Figure 3-11.
Figure 3-11
Ratiometric method to measure ohms (10MΩ and 100MΩ ranges)
Basic circuit theory dictates that the sum of the branch currents (I
REF
and I
DUT
) is equal to
the source current (I
SOUR
). Since the voltmeter of the Model 2700 (V
MEAS
) has very high
input impedance (>10GΩ), current through the voltmeter branch is insignificant and can
be discounted. Therefore, as shown in Eq. 1, I
SOUR
= I
REF
+ I
DUT
.
2700
V MEAS  R REF
R DUT
(I SOUR  R REF )  V MEAS
V MEAS
R REF
Eq. 1:
R DUT
Input Hi
Input Lo
A) 2-wire ohms ( W2 ) measurements (10M W and 100M W
ranges)
B) 4-wire ohms (
W 4) measurements (10M W and 100M W
ranges)
I DUT
I SOUR = I REF + I DUT
=
+
V
MEAS
R DUT
=
V
MEAS  10M W
(0.7µA  10M W )  V MEAS
=
Eq. 2:
V
V MEAS
I SOUR
0.7uA
R REF
10MW
IREF
2700
R DUT
Input Hi
Input Lo
I DUT
V
V MEAS
I SOUR
0.7uA
R REF
10MW
IREF
Example:
Asume: V
MEAS = 3.4V
3.4V
 10M W
(0.7µA  10M W )  3.4V
=
= 9.444M
W
R DUT
Sense Lo
Sense Hi
V
V LEAD
R LEAD
R LEAD
W2 Function
W4 Function
V DUT = V MEAS - 2(V LEAD )
For Eq.1, Eq. 2 and the Example,
V
DUT is used in place of V MEAS .

Keithley 2700 Specifications

General

Type	Digital Multimeter
Measurement Functions	DC Voltage, AC Voltage, DC Current, AC Current, Resistance, Frequency, Temperature
DC Voltage Range	100 mV to 1000 V
AC Voltage Range	100 mV to 750 V
Frequency Range	3 Hz to 500 kHz
Temperature Range	-200°C to +1372°C
Display	Vacuum Fluorescent Display
Connectivity	IEEE-488, RS-232
Channels	10
Power Supply	100V to 240V AC, 45Hz to 66Hz
DC Current Range	10 µA to 3 A
AC Current Range	10 µA to 3 A
Temperature Measurement	Thermocouple, RTD