Keithley 2750 User Manual

E-10 Measurement Considerations Model 2750 Multimeter/Switch System User’s Manual
Meter loading
Loading of the voltage source by the Model 2750 becomes a consideration for high source
resistance values. As the source resistance increases, the error caused by meter loading
increases.
Figure E-5 shows the method used to determine the percent error due to meter loading.
The voltage source, V
S
, has a source resistance, R
S
, while the input resistance of the
Model 2750 is R
I
, and the voltage measured by the nanovoltmeter is V
M
.
Figure E-5
Meter loading
The voltage actually measured by the meter is attenuated by the voltage divider action of
R
S
and R
I
, and it can be calculated as follows:
This relationship can be modified to directly compute for percent error:
From the above equation, it is obvious that the input resistance of the Model 2750 must be
at least 999 times the value of source resistance if loading error is to be kept to within
0.1%.
V
M
Measured
Voltage
R
S
R
I
V
S
Source
Voltage
+
–
V
M
V
S
R
L
R
I
R
S
+
-------------------=
Percent error
100R
S
R
I
R
S
+
-------------------=
2750-900-01.book Page 10 Wednesday, August 3, 2011 7:56 AM

Keithley 2750 Specifications

General

Interface	GPIB, RS-232, Ethernet
Switching Speed	Up to 500 channels/second
Maximum Voltage	300V
Operating Temperature	0°C to 50°C
Measurement Functions	DCV, ACV, DCI, ACI, 2-wire Resistance, 4-wire Resistance, Frequency, Temperature
Resolution	6.5 digits
Current	1A
Maximum Current	1A