50 Hz hum/noise/oscilation

[xmat]

Hello to all.

I have the following problem in my circuit.

A transducer with a high output impendance is connected into a high input amplifier which is connected to an absolute value circuit....all on a perforated board.

The transducer's input voltage ranges from 100mv to 6 Volts, lets say...
YET there is a noticable 50Hz noise in the circuit (120mV), that exists even when the transducer is unpluged from the circuit.

Considering all the components are on a perforated board, and that the IC used for the HII amplifier is CMOS technology, what could be the main cause of the problem and how can I eliminate it??

For instance, would metallic shielding be used to entirely solve the problem, or there is something else in the picture I am missing??

Thanx in advance,

xmat.

 

[Nigel Goodwin]

Hello to all.

I have the following problem in my circuit.

A transducer with a high output impendance is connected into a high input amplifier which is connected to an absolute value circuit....all on a perforated board.

The transducer's input voltage ranges from 100mv to 6 Volts, lets say...
YET there is a noticable 50Hz noise in the circuit (120mV), that exists even when the transducer is unpluged from the circuit.

Considering all the components are on a perforated board, and that the IC used for the HII amplifier is CMOS technology, what could be the main cause of the problem and how can I eliminate it??

For instance, would metallic shielding be used to entirely solve the problem, or there is something else in the picture I am missing??

Your details are a little vague?, but in general any high gain amplifier, particularly with a high input impedance, should be very well screened, and the input should be connected via a high quality screened cable.

If possible it's best to use the lowest input impedance you can, the lower the impedance, the less hum and noise it picks up - which is why 600 ohm mikes have now replaced 50,000 ohm ones.

 

[zachtheterrible]

hey xmat, im assuming that you're seeing this noise with an oscilloscope?

if an oscilloscope is connected to a long enough lead, it will pick up 50/60hz hum from the mains. It will also pick it up if connected to your body.

 

[xmat]

hey xmat, im assuming that you're seeing this noise with an oscilloscope?

if an oscilloscope is connected to a long enough lead, it will pick up 50/60hz hum from the mains. It will also pick it up if connected to your body.

Yeah, of course..

It is a digital storage oscilloscope, "Textronix" or something...

Considering the circuit is quite sensitive to noise, since it is on a board with no metallic shielding, is it most likely that it is peaking noise from the mains???

What can I do to solve that, if so??

Really appreciate your help,

xmat.

 

[zachtheterrible]

Like Nigel said, unless it is a high gain circuit, with high input impedance, it probably won't make a difference.

If your circuit works fine without shielding, then why shield it?

 

[Roff]

Stupid question:
Do you have the probe's ground lead connected to circuit ground? If not, that is the source of your hum. If it is connected, does the hum get worse when you disconnect it? If not, the probe ground wire may be broken. I've encountered that problem more than once.

 

[xmat]

Thanx...Help really appreciated.

 

[Roff]

Thanx...Help really appreciated.

Did you find the source of the hum? If so, what was it?

 

[xmat]

Thanx...Help really appreciated.

Did you find the source of the hum? If so, what was it?

Hi!

Sorry for the delayed response, I ve missed your answer..

All oscilloscope cables were ok, hum probably caused because they were pretty long ones, and the whole circuit is on a perforated board andwith many high impedance components...

Refining the circuit, using sorter cables, having matched-paired components on a pcb board...pretty much decreased the problem to elimination level..

Thanx for the help, it has been usefull.

xmat.