Precision Resistor

If signal differences are taken into account, they can reduce signal integrity and compromise the overall test system performance

The differences between the types of signals that a test of the system must get the handle switching hardware is not always well understood. But if these differences not taken into account Switch system design, they can reduce signal integrity and affect overall test system performance.

When designing a measurement system, the selection of the switch ascritical as the selection of test instrumentation system or the design of the interface. The intended use must be thoroughly considered, and the switch will be selected must meet the requirements of the application. Careful attention to detail and the fundamental principles of measurement can help a greater accuracy and performance system.

Voltage vs. current switching state
Voltage sources, usually a compliance current of up to the programmed voltage. As a result oftypical standard condition, a voltage switch is open (in other words, drawing very little power or high impedance).

Power switch, but usually requires the default configuration to be a complete circuit. That is, turned on the current needs of the full path to. Typically, the exchange component (ie the relay) is a normally closed relay or the HI and LO terminals are shorted in the default state. A variety of topologies are suitable for use incurrent switching state applications: scanner, multiplex, and matrix switching. The scan configuration or scanners is the simplest arrangement of the relay in a switch system. It can be thought of as a multiple switch.

As the scan configuration can multiplex-switching used to be a tool to multiple devices (1: N) or more instruments into a single device (N: 1) Connect connect. However, the multiplex configuration is much more flexible than the scan configuration.In contrast to the scan configuration multiplex switching it allows multiple simultaneous connections and also permits, either sequential or nonsequential switch closures.

The matrix switch configuration is the most versatile because it can connect multiple inputs to multiple outputs. A matrix is useful, if pin resistor network connections must be made between several sources and a multi-device, such as an integrated circuit or one.

Typical currentRemember
The most current measurement applications require that all current paths are continuous, even if a particular current signal is not connected to the ammeter. To achieve this, switch cards for current switching state often designed or SPDT form C relay.

Note that the current is briefly interrupted when the form C relay is energized. This could cause problems when used with sensitive high-speed logic or other circuits on a brief interruption of current flow. SuchProblem may provide a switch card, as it changes with isolated using the Series 3700 System Switch / Multimeter with a pair of type A for a make-before-break connection to overcome.

High-reflection
When designing a circuit for high current (> 1 A), pay special attention to the maximum current, maximum voltage and VA specifications of the switches and relay cards. It is also important to have a switch or relay your itinerary with low contactto avoid resistance to excessive heat, which may cause contacts to weld together, resulting in a failure to contact you. The heating is caused by contact I2R loss.

High-current switching can be used with either a power supply switching multiple loads or for switching a current meters to several sources. If a power supply to multiple loads with the help of a scanner multiplexer card, the power switched output 1 A to each of the four loads. This does not pose a problem when only oneChannel is closed at a time, but when all four channels are closed, the power output 4 A by the common path.

Unfortunately, although the maximum current is of a certain channel at a given A, then the common way on the switch card, and not be able to tolerate 4 A. This is usually not specified for a Switch card, but the limit is usually a function of latitude and plug ratings. One way to avoid this problem is a switch card used with independent(Isolated) relay and the connections with wires rated to take the lead throughout the flow.

Low-Current Considerations
When switching currents of 1 micro-A or less, have special techniques used to interference sources such as offset currents, leakage currents, electrostatic interference, to minimize triboelectric currents and electromagnetic currents. The interference can come from the switch card itself, the cable or the test fixturing.

Offset currentsspurious currents are switching from one card, even though no signals are applied is generated. They are usually caused by electrical sources on the switch card. Offset current is especially significant when measuring low currents if the size of the offset is comparable to the current is measured.

Leakage is a mistake A current flows through insulators, when a voltage is applied. It can be found on the switch card, the cabling and test facilities. Even high-resistancecan create channels between low-and conductors near power sources significant leakage currents.

To reduce these effects, always use a Switch card with high channel isolation and the use of the Guard ability of the instrument. Another method to reduce the leakage current is, keep clean the switch card. Dirt, body oils and the like, and a lower resistance path and pass it leakage.

To reduce the leakage in the text fixturing, always useadd good quality insulators such as Teflon and polyethylene avoid; materials such as nylon and phenols, the humidity can influence, their level of insulation.

Shielding is necessary because high-impedance circuit is prone to false picking up EMI. Relay contacts should be shielded from the coil to minimize induced noise from the relay power supply. The candidate and interconnect wiring should also be shielded to prevent noise pickup. All shieldsLO circuit should be connected.

Triboelectric currents are generated by fees generated by friction between a conductor and an insulator, such as between the conductor and the insulation of a coaxial cable. The friction can be reduced through the use of special low-noise cable with conductive coating (eg graphite) and by securing the interconnect cables to minimize movement.

Electrochemical currents are generated by galvanic battery action that the results of contamination andHumidity. remove Cleansing joints and surfaces thoroughly electrolytic residues (including PC caustics, body salts, chemicals and processing) is to minimize these parasitic effects battery.

Settling
When a relay opens or closes, a charge-transfer in the order of picocoulombs occurs that causes a pulse of current in the circuit. This charge transfer is due to the mechanical release or closure of the contacts, the contact-to-coil capacitance, and stray capacitancebetween the signal and relay drive lines. After a relay is closed, it is important that sufficient settling time before the measurement. This can be as long as several seconds, depending on the relay.

When a step voltage to the circuit, a transient current is generated. This current is gradually decay to a steady-state value. The time required to reach the constant value (or settling time) can be used to ensure proper measurement to determine latency.

Cold, hot, andSafe
The term "cold switching" means that a switch is turned on with no signal applied. Therefore, no current will flow when the switch is closed and no current is interrupted when the switch is opened. When turning on hot, voltage and current will flow the instant the contacts close. When the switch is open, this current will be interrupted and can lead to arcing.

Cold power switch allows the device under test will be applied in a controlled manner. His primaryAdvantage is more life as a switch with hot switching. Cold switching also account for the relay contacts can cause arcing and no RFI them.

Hot change might be necessary if an accurate control to be exercised in the period between the application of power and the production of the measurement needs. For example, hot switching is typically used where digital logic is involved, because devices may change when state power is interrupted even for a moment. Relay with relatively large, hotSwitching should be performed every so often to get a good contact to ensure closure. The connection may be without the "net" measures the flow of current through the contacts made reliable.

Many electrical test systems can produce dangerous amounts of power. These high performance levels make personal priority. Some protection components methods include:

– Design of test equipment that operators avoid contact with hazardous substances circuits
– Double-insulated, all electricalConnections that an operator could touch
– With high reliability, fail-safe interlock switch that to separate power sources, if a fixture is opened
– Appropriate training for all users, so they know potential dangers and know how to protect themselves from injuries

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Tags: current, Measurement, Switching, Systems