Channel-to-Channel Isolation: This specification is related to leakage
and crosstalk between adjacent signal paths through the switch assembly.
Troubleshooting problems caused by leakage and crosstalk can be a
difficult task. It’s much easier to start system development with the
proper switch design and specifications than to spend precious time
troubleshooting an elusive problem, which applies to other potential
Most switch assemblies are printed circuit board (PCB) cards that are
inserted into a switching type of measurement instrument, or into a
switching mainframe used with separate instruments. Therefore, the
electrical isolation between any two adjacent switches may be expressed
in different ways depending on the intended use of the switch card.
Normally, the switch channels on the PCB are aligned in order to achieve
proper voltage isolation and accommodate the physical dimensions of the
switches and other components, such as connectors. This spacing and the
PCB material provide a certain level of isolation between channels. The
higher the isolation, the lower the chance of crosstalk or leakage.
Typical values of channel-to-channel isolation are up to 10GW with
capacitance of less than 100pF see figure 2.
Fig 2: Representation of channel-to-channel isolation with shunt capacitance and resistance.
In high-frequency applications, leakage capacitance becomes an important
consideration. For these applications, isolation is usually stated in
dB. For instance, 60dB would be an isolation of 1000 to 1 from channel
to channel, meaning that a 1V signal on one channel could bleed over and
become a 1mV signal on an adjacent channel. Keep in mind that open
contact isolation resistance must also be taken into consideration when
developing the switching portion of a test system. The higher the
isolation between open contacts and between adjacent channels, the
better the integrity of signals passing through the system.
This current can occur on switch cards even when no test
signal is present. The largest magnitudes are due to finite
coil-to-contact impedance in electromechanical relays. It is also
generated by triboelectric, piezoelectric, and electrochemical phenomena
on the switch card, regardless of relay type.
Offset current is important, for example, in low level and high
impedance measurements taken during semiconductor parametric testing on
wafers and individual devices. Low offset current becomes a critical
specification when conducting leakage current measurements on
semiconductor devices and materials. It is also important when doing
semiconductor C-V characterization.
Depending on card design and intended usage, the offset current
specification could range from less than 1pA up to 1nA. Manufacturers of
cards designed for relatively high level DC current and voltage
switching may not provide an offset current spec, because it is normally
unimportant in those applications.
Relay Switching Speed:
A relay’s operating speed has a direct affect on
the throughput of a switching type of test system. The system developer
has to pay attention to relay speed specifications to also ensure that
measurements are accurate. A typical test scenario is to apply a
stimulus to the device under test (DUT), wait a short amount of time for
the test system and DUT to react and settle to a final value, and then
measure the DUT’s response. If measurements are taken before the system
has settled sufficiently, results can be inaccurate.
Relay operating speed is a measure of the rate at which its contacts can
be cycled and still obtain reliable operation. This rate is limited by
the relay’s actuation and release times. Actuation time is measured from
when power is applied to the coil until the contacts have settled.
Thus, actuation time includes contact bounce time. Release time is the
opposite of actuation time. It is measured from the time power is
removed from the coil until the contacts have settled to their open
position and includes bounce time.
A significant amount of system settling time is associated with relay
bounce time, which must have settled out before a solid reliable
connection is established in the signal path. Settling time varies from
relay to relay with typical times in the millisecond range. Sometimes
relay switching cards have a built-in delay to avoid problems associated
with contact bounce. In addition, some switching equipment may even
have a user programmable delay time.
Use of Solid-State Switches:
Standard electromechanical relays can
switch from one state to another in as little as a few milliseconds,
which is fast enough for some applications. However, in production
applications where test time carries a significant dollar value, this
switching time may be too long. Solid-state relays (e.g. transistors,
FETs) have a much faster switching time, generally below one
millisecond. Going from a few milliseconds to a few hundred microseconds
could shave off substantial test time and increase test throughput.
Another advantage of solid-state relays is their reliability.
Solid-state relays have a switching life of almost 100 times that of
electromechanical relays. This would be on the order of about 10 billion
switch cycles instead of a good electromechanical relay’s life of about
10 million cycles.
One disadvantage is the “on” resistance of solid-state relays, which is
on the order of tens of ohms. Such a high resistance could lead to
measurement inaccuracies in a twowire resistance measurement. Trying to
measure a few milliohms with upwards of 10W of resistance in the circuit
from the “on” resistance would effectively bury the low-resistance
One way around this is to use a so-called golden or standard channel.
This is a channel with a short on the device side. The channel is
closed, the resistance measurement is made, and the measurement is
subtracted from all other channels. Therefore, the “on” resistance is
essentially zeroed out. The problem is that this holds for only the
golden channel and would be slightly different on each channel. Using
this method would depend on the magnitude of the resistance to be
measured and the accuracy required.
Another way to correct for this resistance is the four-wire (Kelvin)
measurement technique, which involves using two channels instead of one.
One channel is used to source the current and one to sense the voltage.
This is a standard method to measure low resistance. Using an
electromechanical or reed relay would only have a contact resistance of
tens of milliohms, which would be more advantageous in low-resistance
measurements using the two wire method.
Other Settling Time Issues:
In addition to mechanical issues, there are
electrical issues associated with the opening and closing of switches.
When a mechanical relay opens or closes its contacts, there is a charge
transfer on the order of picocoulombs that causes a current pulse in the
test circuit. This charge transfer is due to the mechanical release or
closure of the contacts, the contact-to-contact capacitance, and the
stray capacitance between signal and relay drive lines. This phenomenon
can affect both the signal settling time and signal integrity.
The nature of the signals must also be considered. Some signals
originating from a DUT take longer to settle than others. As a general
rule, the rise time of a DUT output signal is defined as the time for it
to rise from 10% to 90% of its final value when the stimulus rises
instantaneously from zero to some fixed value. If the signal originates
from an extremely high impedance (producing a very low current), then it
may require several seconds or even minutes to settle. The settle time
is directly related to the small current charging the cable or stray
capacitance in the circuit. The higher the impedance the lower the
current and the more time it takes to settle out.
Making sure a test system has settled sufficiently is the key to good
measurements. Specifications listing relay actuation time are only the
starting point in determining the total test time for a measurement
sequence. The mainframe or switching instrument holding the switch card
also contributes some overhead time, which is a function of its command
to connect times in a test sequence. This varies according to the test
sequence design, but some switching instruments and mainframes have
displays that depict when a relay has closed, providing some indication
of how fast a sequence is progressing. However, keep in mind test system
design always involves tradeoffs between throughput and accuracy
A much more detailed version of this paper which additionally covers
topics of "Switch System Architecture and Topology", and "Switching Test
System Design Tradeoffs" is available at Optimizing a switch system for mixed signal testing
About the author:
Dale Cigoy is a Senior Application Engineer at Keithley Instruments in Cleveland, OH. His major responsibility is helping customers with measurement applications that include Keithley equipment, especially DMMs. Prior to this he wrote technical instruction manuals for Keithley products. Cigoy joined Keithley in 1976 after earning a Bachelor of Science degree in Electronic Technology from Capitol College in Laurel, MD.