Optical Relays For Test and Measurement
Solid-State Relays In Test And Measurement Systems
To keep pace with advances in the electronics industry, test and measurement systems increasingly require advanced solid-state Relays that combine low capacitance, low on-resistance, physical isolation, and high linearity. All these characteristics play an important role as data acquisition devices become faster and more precise. Here’s why:
- Low capacitance improves switching times and isolation characteristics for high frequency load signals.
- Low on-resistance reduces power dissipation when switching high currents, and increases switching speeds to improve the precision of measurement.
When considering on-resistance values, pay close attention to the temperature range the Relay must withstand. Rising temperatures decrease the mobility of electrons, driving up the on-resistance. Starting with a Relay that has low on-resistance will minimize the effects of temperature drift.
- Physical isolation, sometimes referred to as galvanic separation, between the Relay’s input and output channels, it enhances precision by minimizing noise. Optically-isolated Relays offer a true physical separation of the input and output. The best of these products exhibit isolation voltages as high as 5,000 volts AC.
- High linearity ensures accurate measurements.
With a variety of signals at work in a typical test system, it’s particularly important to find Relays that offer the right combination of electrical characteristics. For example, many systems have both DC and AC switching needs, and will require Relays that combine low-on-resistance and low capacitance. The low on-resistance minimizes signal loss when switching DC signals, while low capacitance improves isolation when switching AC signals.
For Suitable Relays, Go Optical
Of all the types of Relays on the market, optical MOSFET-based Relays offer the most complete lineup of the electrical characteristics needed for test and measurement applications. Consider Panasonic's Low CxR PhotoMOS Model AQY221N2M as a prime example. It offers:
- Low capacitance of 1.1 pF. A laterally diffused metal-oxide-semiconductor (MOSFET LDMOS) lowers the Eelay’s capacitance.
- Low on-resistance of 9.5 ohm. A vertical-type, double-diffused, metal-oxide-semiconductor (DMOS) limits the Relay’s on-resistance.
- Fast Switching and Physical Isolation. Thanks to the low capacitance and on-resistance values, this Relay supports switching times as fast as 20µs, and provides the isolation required to switch high-frequency load signals.
- Linearity. Optical MOSFET-based Relays like PhotoMOS have highly linear input and output characteristics that outshine those of alternatives, such as Triacs or OptoCouplers. PhotoMOS Relays can also control small analog signals without distortion, unlike Triacs and Bipolar transistors, whose offset voltages distort and clip signals.
- Minimal Signal Propagation Delay. Measurement applications benefit from a reduced length of internal bonding and flat lead terminals, which results in reduced signal propagation delay.
Data Acquisition Circuits With Low CxR PhotoMOS Relays
Besides using Low CxR PhotoMOS Relays for switching signals and I/O lines to devices being tested, these Relays may also be employed in data acquisition circuits. For instance, they can be used to select the gain of operational amplifiers. With the help of an optically-isolated Relay, the device’s digital control unit and the analog signal system can be physically isolated, enhancing the precision of the device by minimizing noise.