Keysight SSR Scan Card DAQM900A

[voltsandjolts]

The DAQM900A is an SSR scan card for Keysight's new-ish DAQ970A.

Looking at the board, I'm puzzled about a couple of things:

1/ Unless I am mistaken, there seems to be a flyback step-up followed by Cockcroft-Walton style voltage multiplier on board, with high enough voltage to warrant caution symbols. What would that be for?

2/ Why does each SSR have a "000" resistor on the switch side? Are they polyfuses?


https://www.keysight.com/gb/en/product/DAQM900A/20-channel-solid-state-multiplexer-module-for-daq970a-daq973a.html

[thm_w]

Those are just normal low inductance resistors. The SSR will fry long before the resistor.

https://www.batronix.com/files/Keysight/DAQ/DAQ970A-973A-Datasheet.pdf#page=10

Oddly the PDF above shows 100R resistors in the COM lines, but I don't see those on the PCB. Maybe there are custom versions where people want to specify a series resistance in there?
But yeah the specs don't say why a voltage multiplier is needed here. Seems way overbuilt.

20-Channel General Purpose Multiplexer
•450 ch/s scanning
•Two-and four-wire scanning
•Built-in thermocouple reference junction
•120 V switching
The DAQM900A module is divided into two banks of 10 two-wire channels each. All 20 channels switch both HI and LO inputs, thus providing fully isolated inputs to the internal DMM or an external instrument. During 4-wire resistance measurements, channels from Bank A are automatically paired with channels from Bank B to provide the source and sense connections. The module has a built-in thermocouple reference junction to minimize errors due to thermal gradients when measuring thermocouples.

edit: I don't think thats a voltage multiplier, its some sort of clamping circuit. The SMA parts are SMBJ75(C)A so maybe two in series for 150V clamping of the 120V max input? That or its limiting the total floating voltage, 6 * 75V = 450V.

[voltsandjolts]

Thanks for the comments.

I agree on the zero Ohm resistors, perhaps there just to give the user the option of adding inline resistance for SSR protection.

Good idea, clamping circuit would make much more sense than a step up.
I notice ZGND labelled in that area, which is the chassis.
Could be limiting the common mode voltage to within a certain limit of chassis potential, and the differential to 150V.
Perhaps two SMBJ75 for the measurement bus differential, and same for sense bus, then the centre of those to another two SMBJ75 to ZGND.
Hence HV warnings because its connected to the measurement and/or sense bus.
Still not sure about the inductor, but yeah, clamp circuit it is.