potential divider as signal conditioning

[Neil.Pate]

Hello DAQ experts.

I have digital sensor switching between 0 --> 12 V. I wish  to feed this into a counter channel on a  PCI DAQ which only supports +5V on the digital input. I was thinking of using a simple potential divider network (two resistors) to scale the signal down to 0 --> 5V.

Can anyone suggest if this may or may not be a good idea? If it isn't I would love to hear why not (analogue electronics is not my stongest skill )

thanks
nrp

[AnalogKid2DigitalMan]

It should would just fine. Do you need help determining values for the divider. I think there is a tutorial somewhere on the DevZone. If you can't find it, let me know the manufacturer/model of the sensor (or attach a spec sheet) and the model of your DAQ card.

-AK2DM

[Neil.Pate]

AnalogKid,

Thanks for your offer of help, but luckily I kept my advanced electronics textbooks from university

It is good to hear that the idea has some merit.

In all seriousness though, I have no problem deriving the proportions of the two resistors, what always gets me is the absolute value (i.e. ballpark range).

I have no real gut feeling of e.g. 10k versus 100k or 1 M, I figure I want the resistance fairly high so that current will be low (and thus power dissipation low). I assume the impedance into the DAQ is quite high?

Any advice on picking a suitable range?


 

[encore]

I would put a 5 volt zener diode (1N751A or similar) across your digital input (cathode to input, anode to common) and then put a 10k resistor between your 0-12V signal and the diode-protected input. This will limit the voltage on your input to a safe 0-5V range, and the resistor will limit the current into the diode.  V = I * R, so I = V / R, where V= 12 - 5 = 7V, R=10k, current is 0.7mA

Tom

[Neil.Pate]

encore, do you mean something like this:

+12V ------
           |
           |
           10k
           |
           |----------------   o---- DAQ DI
           |
           -
           /\5V Zener
           --
           |
           |
GND -------------------  o----DGND