Design to replace several jumper on a dense PCB

Question

I'm working on the design of a board which include a lot of Jumpers (around 40). They are used in order to provide a possibility to connect or disconnect parts of the schematics. Signals are all analogic.

As my board is small and I already have quite a lot of components, I'm thinking of a way to reduce the space used by jumpers (header with 2 pins , 2,54mm). It is possible for me to cut a group at a time (lets say, unplug 8 jumpers at time).

I am thinking of using of CMOS switches, such as CD4066, with output control pins themselves drived by jumpers, but I am not sure if it is the best way to go. I am also paying attention to the noise induced.

So my question is, do you have better suggestions to improve space reduction and / or noise induced ?

Additionnal information on signals used : +/-12 Vpp range, 15Hz-100kHz frequency range.

Answer 1

There are a number of factors that need to be considered here. Analogue circuits can be sensitive to routing things around just to make certain other features work. So the proposal of how to reduce jumpers has a lot to do with just how those jumpers are used.

A. If jumpers are used to simply isolate the circuit sections in the case a board is under debug trying to root cause a fault of some sort then it can be a good idea to simply use SMT components commonly called zero-ohm resistors to link the circuit sections. For the cases where an isolation is needed you simply remove the component with a tweezer type soldering iron.

B. If the jumpers are used for configuring the circuit so that it can be calibrated then it could be a great idea to use the analogue switches as long as the added component count and circuit complexity can be justified. There is a temptation with this approach to use multi-switch packages and the extra routing added to analogue signals could be a problem. Switches also add some capacitance and switch impedance that could become factors to consider in certain types of analogue RF or low signal level circuits.

C. If the jumpers are also used as test points to allow connecting scope probes and as signal injection points to apply signals from external testing gear then you really have no choice but to leave the jumpers in the design. You could consider using the smaller type of jumpers that have 2mm spacing or 50mil spacing.

It is possible that after you analyze your circuit that you will find that the current implementation is a mix of A, B, and C above and you can set to optimize each accordingly.

Answer 2

One trick that I've used is called a 'Cuttable Jumper'. Basically it's an SMT resistor footprint (0603, 0805, or whatever) with a trace connecting both pads. If you later need to disconnect the signals then just slice through that trace with an X-acto knife. If you want to connect them again then populate a zero-ohm resistor.

If I know that non-expert solderers will be doing the work then I try not to go any smaller than 0805; it's easier.

Answer 3

+/-12V signals mean rails of more like +/-15V so the 4066 is already not suitable due to absolute maximum supply voltage.

There are (relatively expensive) analog switch ICs that may do the job.

Keep in mind that a jumper is almost perfect at low frequencies- low 'on' resistance, very high off resistance, potentially very little leakage. Just to mention one possible issue, the on resistance of a switch might be of the order of 100 ohms and it will vary with the signal (and temperature) so it could introduce distortion in the signal.

The 4066 does not have very tight specs, but it still can be useful. One thing to keep in mind that it does not have level shifting built in so the control signal on, say, +/-5V will have to go below ground (maybe -4 to +4 to be reliable). The higher end switches usually allow 3V or 5V control signals to control a switch that can handle +/-15V or more.