Selecting relays for test and measurement equipment
One of the key challenges with developing test and measurement equipment is to ensure that the signal that gets measured is the same as the signal that you want to measure. It sounds obvious, but multiple factors in the path from the test point to the measurement point can distort the signal. These can include inconsistent physical connections to the signal wire; capacitances, resistances and inductances introduced into the signal path; impinging radiated and conducted electromagnetic interference (EMI); and even the signal-switching strategies in the tester.
Equipment makers understand these issues and go to great lengths to combat them through careful component selection, circuit design, PCB layout, and shielding strategies. One of the biggest challenges is to switch signals under test in a way that doesn’t distort them before they are measured. Electromagnetic relays are often used, but they can suffer drawbacks such as limited switching speed, contact wear that changes their electrical characteristics over time, and possible EMI generation during switching. Such relays can also be relatively large, compared to semiconductor alternatives, because of their coil and contact assemblies. And they can be relatively power hungry, due to the energy needed to energise the coil.
These drawbacks are felt particularly acutely in the design of automated test equipment for the semiconductor industry, where signals are small and the market is constantly pressuring vendors to add more measurement channels to keep up with increasingly complex IC designs. The simple physical constraints of routing hundreds of signals away from a dense IC package or bare die also demand a compact switching solution.
One alternative to electromagnetic relays is to use a switching circuit in which light provides galvanic isolation between the inputs and outputs. Panasonic produces a range of PhotoMOS® relays that work this way. The input side of these devices has an LED which emits light if current flows through it. This light then shines through a clear resin onto an array of photo cells. The incident photons become electron-hole pairs in the photo-cell material, which leads to a voltage drop across the array of cells. The voltage drop is then used to signal to controller circuitry that the state of the output should change, from On to Off or vice versa.
The advantages of the PhotoMOS® approach are manifold. The galvanic isolation afforded by the optical coupling from input to output helps suppress electrical noise that could distort the signal under test. The relays’ outputs are made up of two source-coupled MOSFETs, which have a highly linear transfer characteristic that helps maintain the integrity of the signal under test.
The other MOSFET characteristics that are advantageous for test equipment makers include low on-resistances and output capacitances. High on-resistances can affect the precision with which a signal can be measured, as well as dissipating large amounts of energy when switching large currents. High output capacitances can limit the relay’s switching speed and its ability to isolate high-frequency test signals.
Panasonic has engineered a special range of PhotoMOS relays, with an output stage made up of two DMOSFETs, which offers lower capacitances and resistances than its standard parts. The characteristics of these ‘low CxR’ PhotoMOS® relays rely on a number of factors, including an optimised MOSFET layout and improvements to the layout of the bonding pads, the way dice are wire-bonded to their packages, and the design of the terminal leads.
These characteristics make the parts useful for measurement and data-acquisition applications. For example, the AQY221N2V (left) has a typical on-resistance of 9.5Ω and an output capacitance of 1.0pF, enabling switching times of 20ms. They also offer good isolation for high-frequency load signals. The optimised die-connection strategy has also reduced signal propagation delay, allowing the parts to be mounted in smaller packages. The new ‘shrink small outline packages’ (SSOPs) are only 60% of the area of a conventional SOP, and just 40% of its volume.
Panasonic is also mounting four PhotoMOS® relays in one 16pin SOP, with useful area advantages. The diagram to the right shows four different ways to mount four PhotoMOS® relays on a PCB with a 1.27mm grid pitch.
Reading left to right in columns, we have four SOPs, four SSOPs, and a single SOP16. You can see that four SSOPs (centre column) take up roughly the same area as three SOPs (at left). Perhaps more importantly, one SOP16 (at right) includes four PhotoMOS® relays in the same area as three SSOPs. The electrical characteristics of the SOP16 device (type AQS221N2S) match those of the AQY221N2V mentioned above.
When Panasonic developed the PhotoMOS® range, it combined strong galvanic isolation and MOSFET output stages to create a low-power, compact relay that offered low noise and very linear output characteristics. This has made the range, especially in its Low CxR variant, suitable for use in applications such as measurement, telecoms and medical equipment.
It turns out, though, that it is also possible to achieve good galvanic isolation using capacitive coupling strategies, which is what Panasonic has done with its PhotoMOS® Series TSON parts (below left). These ultra-miniature parts use a TSON package and join the Low CxR part of the overall PhotoMOS® range.
The smaller package is made possible by the use of capacitive rather than optical coupling. Capacitive coupling extends the operating temperature range of the relays up to +105°C. It reduces the input drive current to a low 0.2mA at operating voltages of 3 to 5V. And it cuts switching times compared to the optically coupled parts.
These devices have all been optimised to help ensure the integrity of signals under test in advanced measurement equipment. But Panasonic’s PhotoMOS® range is much wider than this, including variants for general use, high-power, high-sensitivity and RF applications. Visit the page to find out more. Alternatively, if you would like to discuss your design with one of our technical specialists, click the Ask an Expert button to get in touch.