[SI-LIST] Re: Electrical wiring questions

Do you have control over the PCB and sensor? Or are these purchased
assemblies with fixed circuitry and components?
 
If they are fixed then you are correct that a separate cable with 5V 
and GND is required. If this is the situation I would tie sensor GND
to Signal- of the ADC PCB. This should help cancel common mode
noise at the sensor, and simulates a "4 wire" measurement scheme.
The connection of sensor GND to Signal- should happen at the sensor
and not at the ADC PCB.
 
If you have control of the assemblies consider what the RF guys do 
-- capacitively couple your signal on top of the 5V and decouple it at 
the other end.
 
                                                     Curt
 
Curt McNamara. P.E.
Senior Electrical Engineer
Logic Product Development
411 Washington Ave. N Suite 101
Minneapolis, MN 55401
Tel 612-436-5178
Fax 612-672-0443
curtm@xxxxxxxxxxx
www.logicpd.com <file://www.logicpd.com>  
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________________________________

From: si-list-bounce@xxxxxxxxxxxxx on behalf of Mike S.
Sent: Fri 2/17/2006 5:43 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Electrical wiring questions



Hello to all,
I would like to ask for some advice and orientation on electrical
wiring matters!
I believe this configuration is very common:
I have an analog sensor with 3 wire leads (Vcc -5 V-, GND, signal)
that will be sampled by a fully differential ADC that is on a separate
PCB. I only have one 5 V power supply to power both the sensor and the
ADC PCB (this card is also 5V). I believe that the best way to connect
this setup is to power the ADC PCB and then, with a single connector
from the ADC PCB to sensor, containing 3 pins (Vcc,GND and signal).
This way would avoid ground loops. The problem is that I don't have
such a connector in the ADC PCB. In fact, I only have a connector with
2 pins (Signal+ and Signal-, that are the ADC differential pins). So
the sensor must be powered from the same power supply, but with a
different cable that will not originate in the ADC PCB, but directly
in the power supply.
Since the connector to the sensor in the ADC PCB doesn't have any
ground pin the sensor return current would have to return in the power
supply ground wire. Although the ADC has fully differential inputs
(that would cancel any induced currents in the loop) I don't think
this is a good solution. Is it?

Any suggestions or comments are welcome.
References like books or other publications that deal with these
matters are also welcome.

Thanks in advance
Best Regards

B. Mike
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