Sending signal by voltage...or current?

Hello,

The basis of this thread is:

If you have a signal that you are sending across a PCB to a uC…..is it better to send the signal as a current , or as a voltage?

(the PCB has three ~5W SMPS’s on it.)
- - - - ---- - - - - - - --- - - - - - - - - - - - - -- -- - - -
….is it true that sending the signal as a voltage is bad because it may get corrupted by noise?

Here is the circuit diagram…it’s the signal from a thermistor which gets buffered by an op amp then sent to a uC……(the uC’s ADC reads the voltage)

CIRCUIT DIAGRAM:
https://i56.tinypic.com/125qqs9.jpg

(Vcc = 5V)
(-the actual thermistor is off the board, and is connected by wires and a shrouded header on the PCB)

The thing is, the PCB track from U4A output to PTA4 on the uC, is about 10cms long, ……..this is a voltage signal, from the opamp.

Would it have been better to put the opamp nearer the uC….and then basically the output of U4A would not have to travel so far to the uC ?……………..

…….the actual thermistor signal (current flowing through the thermistor) would then have to travel further to get to the opamp, but that’s ok(?), because that is a current…..the actual thermistor signal does not get converted to a voltage until it goes into resistor R31 , which is next to the opamp.


So which is best for sending a signal, current or voltage?

-and also, why is such the case?

In industrial environments the main way of reading sensors is by using a 4-20ma output which can travel through long wires without any losses because the output of the sensor
uses constant current output (4-20 for the full range of measurement) which includes in the loop the wires and the resistance in the ADC (destination) side.
This current is the output of the sensor circuit and is converted to a voltage drop close to the ADC which then measures the voltage drop on this resistor.
The 4-20mA circuit is used in the sensor side and also the ADC side has a low resistance about 500 ohm so it is also more difficult to pick up noise.

The thermistor and a resistor are connected in your circuit as a voltage divider, i consider this connection as a voltage connection,
voltage and current are tied together anyway because of the basic ohm law but i consider a connection having a current signal when the output signal is a current source,
in that case the current stays the same if the load resistance changes (as long as the source can provide the voltage needed).

Your circuit has the capacitors to create low pass filter and limit the noise picked by the NTC,
your ADC has an input resistance of 3K, it is not very high but i don't know if it picks any noise.

In the solution recommended by Texas Instruments (as shown in your post https://www.edaboard.com/threads/201312/#post847039) then the only thing that changes is the accuracy with which the sensor value is read, the signal will still travel as voltage from the output of the opamp to the ADC.

Alex