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Technical Low Voltage/Low Resistance
Information Measurements
How to Select a Voltmeter From this equation, it can be observed that
HI
Many kinds of instruments can measure voltage, Johnson noise may be reduced by lowering the
Technical information: Low voltage/low resistance measurement products
Experiment Nanovoltmeter
VS VIN
including digital multimeters (DMMs), electrometers,
(source)
R
LO
temperature and by decreasing the bandwidth of
and nanovoltmeters. Making voltage measurements the measurement. Decreasing the bandwidth of
successfully requires a voltmeter with significantly Ground 1
I
Ground 2
the measurement is equivalent to increasing the
higher input impedance than the internal impedance
Ground bus response time of the instrument; thus, in addition
(source impedance) of the device under test (DUT).
VG
to increasing filtering, the bandwidth can be reduced
Without it, the voltmeter will measure less potential Input voltage to the nanovoltmeter is: by increasing instrument integration (typically in
difference than existed before the voltmeter was
VIN = VS + I R multiples of power line cycles).
Resistance of input LO connection
connected. Electrometers have very high input impe- (typically around 100mW) Ground Loops
dance (typically in the order of 100TW [1014W]), so Current passing through input LO
connection due to ground When both the signal source and the measurement
they're the instrument of choice for high impedance voltages (VG) in the ground bus instrument are connected to a common ground bus,
(magnitude may be amperes).
voltage measurements. DMMs and nanovoltmeters a ground loop is created (Figure 2a). This is the case
Source voltage (desired signal)
can typically be used for measuring voltages from when, for instance, a number of instruments are
10MW sources or lower. Nanovoltmeters are appro- I R may exceed VS by orders of magnitude.
plugged into power strips on different instrument
priate for measuring low voltages (microvolts or less) racks. Frequently, there is a difference in potential
from low impedance sources. Figure 2a. Multiple grounds (ground loops)
between the ground points. This potential differ-
ence--even though it may be small--can cause large
Low Voltage Measurements currents to circulate and create unexpected voltage
HI
Significant errors may be introduced into low Experiment Nanovoltmeter drops. The cure for ground loops is to ground the
(source) VS VIN
voltage measurements by offset voltage and R
LO entire measurement circuit at only one point. The
noise sources that can normally be ignored when easiest way to accomplish this is to isolate the DUT
measuring higher signal levels. Steady offsets can I
(source) and find a single, good earth-ground point
generally be nulled out by shorting the ends of the ZCM Single
System for the measuring system, as shown in Figure 2b.
test leads together, then enabling the instrument's Ground bus
Ground
Avoid grounding sensitive measurement circuits to
zero (relative) feature. The following paragraphs VG the same ground system used by other instruments,
discuss non-steady types of error sources that can machinery, or other high power equipment.
Input voltage to the nanovoltmeter is:
affect low voltage measurement accuracy and how to VIN = VS + I R
minimize their impact on the measurements. Magnetic Fields
Current passing through ZCM (MW or
GW) due to VG and currents in the Magnetic fields generate spurious voltages in two
Thermoelectric EMFs source (magnitude is typically nA's).
circumstances: 1) if the field is changing with time,
The most common sources of error in low voltage VIN VS, since I R is now insignificant compared to VS.
and 2) if there is relative motion between the circuit
measurements are thermoelectric voltages (thermo- and the field (Figure 3a). Changing magnetic fields
electric EMFs) generated by temperature differences Figure 2b. Single system ground can be generated from the motion of a conductor
between junctions of conductors (Figure 1). in a magnetic field, from local AC currents caused
Minimizing temperature gradients within the by components in the test system, or from the
A B A circuit also reduces thermoelectric EMFs. A way to deliberate ramping of the magnetic field, such as for
T1 T2
HI minimize such gradients is to place all junctions in magnetoresistance measurements.
VAB close proximity and provide good thermal coupling a. Area A (enclosed)
LO
to a common, massive heat sink. If this is impracti-
Nanovoltmeter
cal, thermally couple each pair of corresponding DUT
junctions of dissimilar materials to minimize their
Voltmeter
temperature differentials which will also help
B
The thermoelectric voltage developed by dissimilar minimize the thermoelectric EMFs.
metals A and B in a circuit is: The voltage developed due to a field passing
Johnson Noise through a circuit enclosing a prescribed area is:
VAB = QAB ( T1