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Using the Digital Multimeter
2–42
VX4101A MultiPaq Instrument User Manual
might receive errors resulting from this impedance. A voltage divider circuit is
created by the measurement source impedance and the VX4101A input
impedance. For example, a 1000 W source impedance loaded by the VX4101A
10 MW input impedance will result in a .01% error.
For voltage measurements less than 3 VDC, this error may be essentially
eliminated by sending the command INPut:IMPedance 10e9. Since an attenuator
is not required for the lower voltage ranges, this command is provided to switch
out that attenuator, if desired.
The VX4101A in the 30 mV DC measurement range has a resolution of 100 nV.
The accuracy at 0 VDC is between 8 mV and 15 mV depending on aperture
selection. At this low level of operation, the primary contributions to the error in
ambient temperature applications are the DC low range input amplifier voltage
and current noise and drift with time and temperature, and thermal EMF drops in
the range changing relays. These errors are minimized with the use of low drift
amplifiers and low thermal EMF relays in the VX4101A.
Cabling and aperture selection are vital in order to attain these measurement
accuracies. The primary source of additional error when you make an actual
measurement is normal mode power line noise. Normal mode power line noise is
differential noise between the high and low input that can easily be coupled into
your cabling. Since it is differential noise between the high and low input it
looks to the DMM like an actual input voltage.
To minimize these errors at low-level measurements, you should use shielded
twisted pair cabling to connect to the DMM. Since the VX4101A has a floating
front end, the low input of the VX4101A is not grounded. If your measurement
is referenced to ground, the best place to provide the ground reference is at your
equipment. If the measurement is referenced to ground, it is probably already
grounded within your unit somewhere. The DMM low side should not be
grounded. This will only create a ground loop adding potential error. Finally,
using an aperture that is a power line period multiple will provide significant
power line rejection as discussed in About Power Line Noise, below.
Low level errors are also minimized by using the autozero capability of the
VX4101A. Autozero minimizes long term drift errors in the input amplifier by
comparing a null reading taken during autozero with a reference null reading
taken when the VX4101A was last calibrated and compensating for any
difference. The autozero function also eliminates a portion of temperature drift
errors in applications where the temperature drifts slowly.
About Low Level DC
Measurements