Potential risk of performing continuity test on DC coupled inputs

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Hawaslsh

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Hello,

I had a question regarding the potential risks of using the continuity or resistance measurement functions in a multimeter on non-reflective RF switch ports. For example, using a SPDT switch from mini-circuits (datasheet), after assembly I want to make sure I didn't short out the signal lines to the adjacent ground pads before attaching the SMA connectors. Perhaps not such a big deal with the 3 GHz connectors, but we also use much higher frequency SPDTs (datasheet). For the mini-circuits part, the datasheet says "RF pins 3, 8 and 13 must be at 0 VDC. The RF pins do not require DC blocking capacitors for proper operation if the 0 VDC requirement is met". Also, un-powered I don't know what state the chip's switches will be in, whether open or terminated in 50 Ohms.
Clearly the continuity or resistance measurement function will apply a test voltage. Will the limited current supplied during the resistance measurement be OK or do folks think any applied DC voltage will damage the switching FETs. Perhaps it's impossible to tell without knowing whats actually inside the chip, but I thought I'd ask before attaching the SMAs and testing the chips.

Thanks in advance,
Sami
 

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My analog VOM sends 60 mA from the leads on the lowest Ohmmeter range (labelled Rx1). It comes from a 1.5V battery. When measuring an unknown resistance I try to remember to start out on a higher range because those provide less current to the leads.
 

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I've seen test programs using "force current, measure
voltage" blow out pin circuitry when there's an actual
continuity problem on another pin (specific anecdote,
a tri-statable line driver got its output FET gate oxide
ruptured if the EN pin was open and the output was
hi-Z instead of logic-low). Probe contact repeatability
issues at the root of it all, but the specific test needed
to be changed to prevent punching out whole wafers'
worth of dice before anyone working the probe floor
on second shift noticed.

Stick to a voltage that should be survivable whatever
the other pin conditions are, maybe add some physical
current limiting like a source resistor in series, as
current limit (compliance) features on testers can be
slower to react than the damage mechanism on chip.

If using forced current then perhaps you should add
some shunt clamps to prevent pin overvoltage, and
look for a voltage result that is less than clamp-stop.
 

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i use a standard network analyzer set up in time domain mode with a step function.

if the reflected signal is a straight line, you have a good 50 ohm match with not short or open circuits, and little inductive or capacitive parasitics shown. it will work well easily up to 30 GHz

If there is an open circuit (like from a sliding contact), or a short circuit (some conductive epoxy, or some conductive debris) it is OBVIOUS in the time domain response. You can even test an empty housing that has had hermetic 50 ohm connector beads welded into it, before installing all the substrates components--just slip on the field replaceable coax connectors, and hold down a tiny 50 ohm chip resistor on the inside microstrip coplanar launch with a wooden Q-tip. it can save you the cost of installing all the internal parts and THEN figuring out there is something wrong with the basic housing

i suppose you could current limit your ohmmeter test with say a 10K resistor, so if you do inadvertently forward bias a semiconductor junction, there will not be enough current to damage anything? A 10K reading would mean a short circuit.
 

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Note 1 at page 4 mention: "RF pins 3, 8 and 13 must be at 0 VDC. The RF pins do not require DC blocking capacitors for proper operation if the 0 VDC requirement is met"
Which means, any DC voltage higher than 0V will damage the switch.

Usually, FET switches when no power supplies are present, the channel remains in the off condition, and the switch inputs are high impedance.
 

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GaAs FET switches are usually depletion mode JFETs and conducting in unpowered state.

I won't expect chip damage with test signals below maximum ratings.
 

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