Hi all,
I expect questions like this have been asked lots, so sorry if its old ground!

I have made a RFID reader using a pic and some op amps, while developing on the bread board I had my scope connected most of the time.

Some of you can see this coming, when I disconnect the earth clip if the scope the circuit doesn't work as well as it should, if I touch the metal case of my PSU, it works, if I connect to a battery it only performs as I hoped if I touch the earth/ground line with my finger?

I have 100nF decoupling caps on each ic, two on the pic as it has two power inputs.

Do folks think a PCB with a good ground plane will get rid of the problem? Should I use different value decoupling caps? As its going to be hand held I could just have a earth on the handle but this seems a bit hap hazard!

Cheers Geoff

 

Here's your problem:

developing on the bread board

A well designed PCB with a good ground plane should fix all of your problems.

 

Thanks,
I was hoping that was the case , my bread boards start of nice and neat, by the time I've got it working it looks like a drunk ferret made it.
Cheers Geoff

 

Separate from that, a wiring error could leave a portion of the circuit only partially connected when the extra ground connection is broken.

ak

 

Hi,
Its hard to diagnose the problem without the schematic. That said, one way to confirm if its an inadequate ground plane problem & not a wiring error (as mentioned in post #4) would be to to plug one end of a short length of wire into the breadboard ground socket. This wire will then act as the other half of the dipole (just as your scope probe lead did, when that was connected to the breadboard). I'm assuming that you are using some sort of monopole to both energise the RFID as well as read from it, the monopole being the other part of the dipole.

If on the contrary you are using an inductive antenna, then an inadequate ground plane wouldn't matter as much and the problem is more likely to be a wiring error.

 

Thanks for the replies,
Slightly embarrassed, it is inductive and was a wiring problem!

I'd pushed the scope probe into the breadboard and made the contacts too wide to make a proper connection to a resistor, quite why earthing it a bit made it work I don't know??

Thanks to everyone who looked.
Cheers Geoff