Signal transmission before or after A/D conversion...

shahidmk · Joined: 03 Sep 2010 Posts: 16

A general question. I have an analog sensor that will go through an I to V conversion before being submitted to an AVR ADC. The max distance is 65 inches. Is it bettr to run the cable straight from the sensor before the I to V conversion or after that!.. and perhaps a little more detail as to why it would be better in ether case!!!!!

Thanks all.

Frankeman · Joined: 11 Aug 2004 Posts: 948 Location: the Netherlands

Hi,

The resistor should in principle be as near to the ADC pin as possible.
Because in that case the wire resistance from the sensor to the ADC pin isn't added to the precision resistor wich would degrade the precision of the measurement.

But... This is not the way to go.

Between the precision resistor and the ADC pin there must be a buffer.
The buffer is an opamp with an amplification of 1 (but other values are also possible.
The buffer has a very high input impedance and a very low output impendance.

Before the precision resistor the must be a low pass filter to filter out higher frequency's.
The filter gives a better measurement and is also good for noise imunity.

shahidmk · Joined: 03 Sep 2010 Posts: 16

@frankeman,
Thank you fo your reply, My question related to the length of the wire which would connect the sensor to th signal processing circuit. Perhaps I didn't word it exactly as to what I was asking.. that is why I might be unable to comprehend your answer

here is another version of the same question. the wire which connects the sensor to the signal processing circuit is at max 65 inches long. Is it better to attach this wire between the sensor and the signal processor or is it better that the signal processor (the I to V section) is attached to the sensor and the 65 inch wire is run from the signal processor to the adc..???????

Frankeman · Joined: 11 Aug 2004 Posts: 948 Location: the Netherlands

Hi,

Read my post again and replace the word "resistor" with "signal processor".

It is better the have the wire between the sensor and the signal processor.
In this case the signal on the wire is in the form of a current, otherwise it would be a voltage.

With this cablelength it is not very important in this case.
When the wire gets really longer, let's say about 50 meter a current signal would be better to use.
With short distances (in your case) it doesn't really matter.

Frank.

hgrueneis · Joined: 04 Apr 2009 Posts: 902 Location: A-4786 Brunnenthal

Hi,
it depends on the amount of current and the resistance of the wire.
Also on the frequency of conversions and the accuracy you want. If there are not a lot of conversions you can add a LP-filter to the voltage side.
If the current is high, feed the voltage through a shielded cable (65") to the ADC.
If the cable is moved then keep in mind that you will most likely have small
changes in capacitance and also calculate about 1pF/cm on average for shielded cables.
It all depends on the aplication.
Hubert