Offset issues when Relay is on

Hi guys, I have another temperature control project which is exhibiting an offset issue. I'm powering 2 5v regulators with a 12v switching power supply. 1 regulator powers an LCD screen and the arduino uno, the other powers the 2x5v relay board for switching AC.

The sensor input op amps and conditioning runs off of the 12v supply so I can use the full 5v range with a low noise floor going into the Uno. Everything has a common ground and there are a few decoupling caps on the regulators and in the arduino. My 12v bench supply can deliver 22A, so plenty of headroom, there.

The Problem: When the relays click on the input voltage to the Uno analog pin (from the sensor circuitry) drops slightly, giving a false reading. The offset changes depending how close I place the relay ground and power to the 12v supply. Is there an easy way to better-isolate the relay power from the rest of the system?

My other thought was to use a 24v supply and use a regulator to isolate the sensor circuitry at 12v and see if the relays effect the 12v regulated side. I worry that a bad ground would introduce a ground offset and effect the Uno reference. In the end I want a single power supply for the entire system so it is compact. Thoughts? Thanks for reading :)

You need to separate the grounds. Dedicate one of the Arduino ground pins as analog ground, and use it as the ground reference for the op amps, sensors and associated circuitry. Use a different ground pin to connect Arduino ground to the power supply grounds, relay board grounds etc. Also connect a resistor (perhaps 100 ohms, lower if necessary) between the 12V supply and the op amps, with decoupling caps local to the op amps. The purpose of the resistor is to prevent noise on the 12V supply getting through the decoupling caps to analog ground.

Very good routing procedure. I will try and reconfigure everything. I tested the 24v supply idea and all it does is pretty much confirm that it's a ground problem. I got the ground voltage offset down to about 8mV with the 24v supply and a separate 12v reg for the sensor circuit. Man those 5v regulators are cookin' with a 24v input!

As I move the ground for the relays all the way back to the 24v power supply terminals I get near 0 offset. I guess I could chalk this up to poor connections on the breadboard or something. It would probably be better all soldered up. Thanks DC42, that's my next move to test. Any other ideas on how to eliminate this problem?

DC12, could you clarify how to connect the resistor and caps you mention? I have a pretty good idea, but if you wouldn't mind explaining a little further...

kanurys:
DC12, could you clarify how to connect the resistor and caps you mention? I have a pretty good idea, but if you wouldn’t mind explaining a little further…

Like this.

Nice thanks for the scan. That makes sense.

Here is another thread concerning this same problem on eevblog. If you're not aware, Dave has a great video blog... http://www.eevblog.com/forum/projects/offset-issues-when-relay-is-on/

Confirmed: grounding the op amps through the Arduino eliminated the offset almost completely even without the choke. I'll throw the choke resistor and cap on there to see how the noise looks with that. Thanks again.