Relay selection, input/output capacitance, trap for young players

[Fusion916]

Be very careful with your intended frequency of operation vs the input/output capacitance from your relays. I didn't pay too much attention to my relay selection other than cost and "on" resistance, but I didn't bother checking the "off" capacitance. I was getting a ton of feed-through on my relays when the relay was "off". After lots of debugging the root of the problem was too much series capacitance from the input and output terminals.

Here is a bode plot of two relays, the one I was using now (130pF), vs an older one I used that didn't have the off state feed-through issue:

130pF relay:



~1pF older relay:



The signal going through these relays have frequency components 5MHz+. Bode plots are self explanatory.

[T3sl4co1l]

How do you get 130pF in a relay?  Is it not photoMOS?

Got extras to tear down?

Tim

[dmills]

Indeed, 130pf is **Massive**, my usual issue with relays is more in the on state inductance line then off state cap.

What load resistance are you working into?

Another trap with relays is that they do not usually do well with very small signal 'dry' switching, you want to arrange for some reasonable amount of current to flow to puncture the layer of oxides and contaminants that the contacts tend to pick up, even the specifically "Low signal" parts benefit from some standing current. About the only type which is really immune to this are the mercury wetted ones, but they are getting hard to find these days (And have other issues). 

Regards, Dan.