How do MEMS switches defeat traditional switches?

Auxiliary switching functions may also be required outside the test equipment, especially on the device interface board (DIB), which is sometimes also called the test interface unit (TIU). Figure 2 shows such functions and an example of switches for an ac/RF test setup for a device under test. On the test board for the device under test, signal filtering, amplification, and calibration paths are often required to provide sufficient test flexibility to improve test system performance, such as minimizing the noise floor and reducing printed circuit board (PCB) losses.

Figure 2. Example of an AC/RF DIB showing the complexity of the switch function.

The type of switch used depends on the signal type and the performance required. Many high-performance solid-state switches are also used in ATE test equipment. However, when dc PMU signals and high-speed digital/RF signals need to be transmitted on a common test path with only minimal signal loss and distortion, large EMR switches are still required. However, EMRs have some limitations. They are large, slow to drive, have a very limited lifespan, are difficult to design into a PCB from a routing perspective, require external high-power driver circuits, and are complex and cumbersome to rework.

Figure 3. ADGM1304 5 mm × 4 mm × 0.95 mm LFCSP package compared to a typical RF EMR.

In addition to the physical size advantages of MEMS technology, the electrical and mechanical performance of MEMS switches also offer significant advantages. Table 1 shows some key specifications of the ADGM1304 and ADGM1004 devices, compared to a typical higher frequency single-pole throw (SPDT) 8 GHz EMR. The ADGM1304 and ADGM1004 devices have excellent bandwidth, insertion loss, and switching time, with a life of 1 billion cycles. High bandwidth is key to driving switches into new applications. Low power, low voltage, and integrated power drivers are other key advantages of MEMS switches. The ADGM1004 has a high electrostatic discharge (ESD) rating of 2.5 kV for the human body model (HBM) and 1.25 kV for the field induced device charge model (FICDM), which further enhances ease of use.

Table 1. Comparison of ADGM1304 and ADGM1004 SP4T MEMS Switch Specifications with a Typical 8 GHz SPDT EMR

Figure 4 shows the insertion loss and off isolation of the ADGM1304 SP4T MEMS switch compared to a DPDT 3 GHz EMR commonly used in test instrumentation. Figure 4 shows the signal bandwidth advantage of the MEMS switch over the EMR.

Figure 4. Insertion loss vs. frequency for the ADGM1304 and 3 GHz DPDT EMR.

Figures 5 through 7 show how MEMS switches can eliminate these limitations, enhancing their value in ATE applications. Figures 5 and 6 show typical dc/RF switch fan-out application schematics using EMR switches and the ADGM1304 or ADGM1004 MEMS switches, respectively.

Figure 5. Example DC/RF fan-out test board schematic, nine DPDT relay solution

Figure 6. Example DC/RF fan-out test board schematic, five ADGM1304 or ADGM1004 MEMS switch solutions

Figure 7. Visual comparison of a DC/RF fan-out test board with a 16:1 multiplexing function using nine EMR switches (left) and five MEMS switches (right).

Figure 7 shows a photo of a visual demonstration PCB that implements both schematics. A fan-out 16:1 multiplexing function is used in this demonstration. The relays in Figure 5 are DPDT EMR relays. Nine DPDT relays and a relay driver IC are required to achieve an 18:1 multiplexing function (eight DPDT relays only yield a 14:1 multiplexing function). The physical relay solution is shown on the left side of Figure 7, which illustrates how large an area the relay solution takes up, how difficult it is to maintain symmetry between wiring connections, and the need for a driver IC.

The right side of Figure 6 and Figure 7 shows the same fan-out switch function simplified using only five ADGM1304 or ADGM1004 SP4T MEMS switches. The right side of Figure 6 and Figure 7 shows the reduced PCB area and reduced routing complexity of the switch function. The MEMS switches reduce the footprint by more than 68% in terms of area , and by volume, a reduction of more than 95% is possible.

The ADGM1304 and ADGM1004 MEMS switches have built-in low voltage, independently controllable switch drivers; therefore, they do not require an external driver IC . Due to the small height of the MEMS switch package (0.95 mm for the ADGM1304 and 1.45 mm for the ADGM1004), the switch can be mounted on the reverse side of the PCB. The small package height increases the achievable channel density. Figure 8 shows another example of the use of test equipment switches. The figure shows a typical schematic of a test interface to a high-speed or RF DUT, using EMR as the switching element. In this example, high-speed RF signals and digital/DC signals are required to evaluate the electronic equipment.

Figure 8. Example RF and digital/DC DIB using 14 EMR switches.

The solution shown in Figure 8 uses relays as a switch solution. 14 SPDT relays are required to implement the bandpass filter selection, digital signal routing, and DC parameter test functions. Cascaded relays are required. An equivalent solution using MEMS switches is shown in Figure 9. Figure 9 shows a simplified design of a functionally enhanced test interface when using MEMS switches. This design requires only six ADGM1304/ADGM1004 switches, significantly reducing wiring complexity and board area. Overall, the SP4T configuration of the ADGM1304 or ADGM1004 switches provides more functional channels and implements more digital and DC parameter test functions: eight functions can be implemented using MEMS switches, while only four functions can be implemented using relays. MEMS switches have a wide bandwidth of 14 GHz, 0 Hz/dc operating frequency, small package size, and low voltage control characteristics. This solution is more flexible, has a longer service life, and reduces the occupied area, and can simultaneously implement high-precision high-speed digital signal routing and wide-bandwidth RF signal routing.

Figure 9. Simplified and enhanced RF and digital/DC DIB using six MEMS switches.

As device complexity and test requirements increase, it is difficult to implement ATE solutions from the perspective of optimal performance and space efficiency. As DC/digital and RF functions are now common requirements, switches are also becoming an essential part of ATE automated test solutions. ADI's MEMS switch technology is unique, it improves test functions and performance compared to traditional RF relay solutions, and occupies less PCB area. The ADGM1304 and ADGM1004 SP4T MEMS switches have precise DC performance and broadband RF performance, adopt small size SMD packaging, low drive power requirements, long service life, and enhanced ESD reliability. These features make ADI's MEMS switch technology an ideal universal switch solution for all modern ATE equipment.

Thumbs up for MEMS switches!