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ADI reveals next generation for MEMS switches for test

lanka 2020-11-20 14:42:58

Analog Devices revealed details of its second generation of micromachined electro-mechanical relays for automated test.


The aim is to provide an alternative to relays – which have good DC performance and poor RF performance – and semiconductor switches – which have poor DC performance and good RF performance – in a single device with good DC and good RF performance – ADI is claiming 34GHz operation.

Given good RF and DC characteristics, both a power tester and RF instruments can be connected to a terminal of a unit-under-test through a cluster of identical switches. Generic loop-back testing is another potential application.

ADI-MEMS-switch-gen1Several years ago ADI introduced its first generation of MEMS relays, typified by the ADGM1304 (left).

The MEMs switch in these is a cantilever with a contact on the end which is pulled down to complete a circuit by bending the cantilever using electrostatic force. In the diagram are four switches on the MEMS die, which is co-packaged with a high-voltage chip that provides ‘gate’ drive for the cantilevers.

ADI-MEMS-switch-cct-gen1and2For  generation 2, the company has gone away from a cantilever and adopted a see-saw movement operated by two gate electrodes (schematic right) which has at least three advantages:

Firstly, on and off times are now balanced – at ~30μs each, and secondly there is a option to do change-over switching instead of only SPST switching. The third is more subtle – if kept operated, the cantilever in gen1 would gradually bend and eventually stay closed when the drive voltage was removed – leading to a ‘max constantly on lifetime’ specification being introduced – seven years in the case of gen1, which is now >10 years in gen2.

80V is needed to electrostatically create generate sufficient operating force, once again provided by a co-packaged IC (diag top right). The package is 4 x 5 x 0.95mm.

The new integrated driver chip can also now offer an SPI serial interface – compared with the parallel interface of the first generation, and there has been a move to an internal circuit board (which was a lead-frame in gen1) to allow passive components to be integrated, including decoupling capacitors and 10MΩ resistors on the contact ports – the latter to drain away charge build-up on any connected parasitic capacitance to prevent high (damaging) current surges when the contacts close.

The driver chip needs 3.3V at 2-3mA to turn a contact on.

ADI-MEMS-switch-gen2-mechPhysically, the see-saws and contacts are gold, built in a back-end process after support frames are etched from high-resistivity silicon in a CMOS fab.

Specs for the contacts  (drawn left) include 3Ω contact resistance, ±6V maximum make and break voltage, ±20V open withstanding voltage, 200mA continuous current and 16fF open-circuit capacitance.

RF-wise, -10dBm can be switched into 50Ω without reducing lifetime from its purely mechanical switching lifetime, and 33dBm can be handled.

ADI-MEMS-switch-gen2-graphAdding the internal circuit board has also allowed internal circuit paths to be improved by turning them into transmission lines. A summary of RF specs is available in the graph (right). A highlight here according to ADI apps manager Eric Carty is a 1.5dB insertion loss at 34GHz.

Linearity is 69dBm and isolation is around -20dB.

With SPDT switches and space for passive components on the board, will ADI be making non-reflective switches?

“We could do them,” is all Carty was prepared to say at the time, when ADI spoke to Electronics Weekly during virtual Electronica.