Noise with voltage suppliers

[guillermorf]

Hi

I have a circuit with an analogic part and with a digital part. The thing is that if i just turn on one of the voltage suppliers (doesnt matter wich one) i have a nice signal, but when i turn on the second voltage supplier i get a lot of noise even with the strobe in the air, not touching anything. I get a noise of 100mV peak-peak that adds to my signal.

How can i get rid of it?

thanks in advance

 

[Ian Rogers]

What are the voltage supplies? Are they tied together? are they both the same potential.. ie..5v, 3.3v etc..

 

[guillermorf]

I dont understand the question "what are the voltage suppliers?" you mean the brand?

They are not tied togheter as the digital part has its own ground, just like the analog. one is giving 5V(analog part) while the other is giving 3.3V(digital part)

 

[Ian Rogers]

Its ok, you've answered my question 5v and 3.3v. I was checking that there was no contention between the two voltages... All I can suggest is you make sure ALL IC's have 0.1uf decoupling caps.. I'm not so sure about the separate grounds though, I usually have a common ground that has a different path back to the supply.

 

[guillermorf]

I have decoupling caps everywhere... Maybe its that i have a lot of cables all over the circuit... im gonna try to reduce them

 

[crutschow]

Are the supplies switching or linear type?

What is the frequency of the noise?

What do the circuits do?

 

[guillermorf]

they are linear. The noise is 50MHz freq, and the circuit is a passband filter

 

[Ian Rogers]

Are you sure it's noise at 50mhz or is that the speed of your clock... Have you any wireless devices nearby?

 

[guillermorf]

No I dont have any clocks in my circuit.. Maybe its a wirless device... i dont know! im inside a lab and there are lots of machines

 

[Ian Rogers]

I was just thinking... 50mhz... its kinda high for noise.. especially one that's affected by a linear power supply (noise on these are usually in the order of 50khz)

With so much lab equipment on the mains, you may need an isolation transformer... IF you source power temporarily from a battery (I'm assuming the source is from the mains at the moment) it would eliminate your circuit.

 

[alec_t]

I'd guess this is a grounding problem. Try connecting both 'grounds' to a common point grounded with respect to the mains supply. Mounting your kit in a screening box also connected to that common point would probably help, too.

Alec

P.S keep all wiring as short as possible