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introduction FeAtured reSourceS
Many electronic test systems use relay switching to connect multiple VoltAGe SwitchinG
devices to sources and measurement instruments. In some cases, Many different applications involve switching a voltmeter or voltage source
n Switching in Multipoint Testing
multiple sources and measuring instruments are connected to a single to multiple devices, including testing batteries, electrochemical cells, n Testing Devices with
device. Switching allows automating the testing of multiple devices, circuit assemblies, thermocouples, etc.
High Voltage and
thereby reducing error and costs.
The types of switch cards and the techniques used in these applications High Current
Designing the switching for an automated test system demands will depend on the magnitude and impedance of the voltages switched.
an understanding of the signals to be switched and the tests to be The approximate level for low voltage switching is in the millivolt range
performed. Test requirements can change frequently, so automated or less, mid-range levels are from 1V to 200V, and voltages greater than
test systems must provide the flexibility needed to handle a variety of 200V demand the use of high voltage switching methods.
signals. Even simple test systems often have diverse and conflicting
switching requirements. Given the versatility that test systems must Switching a Voltmeter to Multiple Sources in Series
offer, designing the switching function may be one of the most complex Figure 1 illustrates switching a voltmeter to a series string of 30 batteries
and challenging parts of the overall system design. or voltage sources (VS). To avoid
short-circuiting one or more of
As a signal travels from its source to its destination, it may encounter these sources, it is necessary AdditionAl reSourceS
various forms of interference or sources of error. Each time the signal to open a given channel before V Ch. 1 VS1
n Optimizing a Switch System for
passes through a connecting cable or switch point, the signal may be closing a second one (break- Mixed Signal Testing
degraded. When calculating the overall system accuracy, the engineer before-make operation). To
n Tips and Techniques for Designing Cost
must include not only the effects of the switch itself but all the switching guard against short-circuiting,
hardware in the system. Effective, Efficient Switch Systems (webinar)
add fuses in series with each Ch. 2 VS2
The quality of a switch system depends in large part on its ability to voltage source to prevent
preserve the characteristics of the test signals routed through it. For damage to the switch card. be
example, when the test signal is a low voltage, the switching system sure not to exceed the common-
must minimize errors such as offset voltage and IR drops. Leakage mode rating of the switch card. oPen
current may be a problem for high resistance and low current switching in this example, each battery is
applications. Depending on the type of test signal involved, specific 12V and the total voltage across Ch. 30 VS30
switching techniques must be used to maintain signal integrity through the string is 360V. a channel-
the switch system. This e-handbook describes switching techniques to-channel voltage rating and a
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for a variety of test signals. common-mode voltage rating of Figure 1. Switching a voltmeter to multiple
at least 500V is desirable. sources in series
Switching a Voltage Source to Multiple Loads
Figure 2 shows a single voltage source connected to multiple loads, such
as lamps. if two or more loads are connected to the source, the voltage
at each load may be less than expected due to current flow through the
common impedances (R), such as the test leads and trace resistance. cloSe
as additional loads are connected, the total current will increase, thereby
increasing the voltage drop across the common impedances (R).
2
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Switch Resistance R Ch. 1 500mA
When switching a HI
voltage source to
multiple devices, it may Ch. 2
Contact resistance
become necessary to 5V Source is 1 for each switch IC <4V
compensate for voltage Ch. 3
drops due to switch LO
resistance. in particular, Ch. n
if the devices have low
Figure 3a. Voltage drops across the contact resistance cause improper results
resistance, the current
f lowing through the
switches may cause a
500mA 1
significant voltage drop. HI
To prevent this problem, Figure 2. Switching a voltage source to
multiple loads
remote sensing can be
used to correct for any voltage drops in switches and wiring. With remote HI Sense 1
sensing, external sense connections are made across the load. Therefore, 5V Source
the subsequent programmed output voltage will be the actual voltage with
across the load. Remote 6V IC 5V
LO Sense 1
For example, Figure 3a shows a 5V source being switched to an integrated Sense
circuit (iC). The contact resistance for each switch is 1. if the current
drawn from the source is 500ma, the voltage drop across each switch
LO 1
will be 500mV, and the voltage at the integrated circuit will be reduced
by a total of 1V. Operation of the iC will likely be unsatisfactory. Figure 3b
shows a 5V source with remote sense. in this case, sense leads are also Figure 3b. Using remote sense through a switch ensures the proper voltage oPen
connected to the load. This will ensure that the actual voltage across the delivered to the IC
load will be 5V, and the iC will operate as intended. note that the voltage at
the source output terminals is 6V.
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Do not use hot-switching with remote sensing since the voltage from the
source may become excessive during switching.
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low VoltAGe SwitchinG
When switching low voltage signals (millivolts or less), special techniques E
must be used to avoid unwanted voltage errors. However, the required
accuracy will determine if these techniques are necessary. These
unwanted voltage errors may be due to thermoelectric offset voltage in the
switch card and connecting cabling, switch film contamination, magnetic + +
interference, and ground loops. For more information on low voltage
measurements, refer to Keithley's low level Measurements Handbook. VS VM V S = VM
Thermoelectric Offset Voltage
The contact potential or thermoelectric offset voltage is the key