using LM324 instead of LT1491

[EmmKay]

Hi

I have the following circuit converting current from a psd to Voltage for input into an ADC.

I do not have LT1491's any more and want to use an LM324 as I have plenty of these in stock. Can people guide me with the redesign. For me the major block seems to be the +-5 V input of the LT1491. How would I go about implementing that into an LM324....

Secondly. the signal in has to be from a distance of 65 inches. My question is would it be better to run the cable before ICIA or have the connection cable running between IC1A and IC1B.

 

[MikeMl]

Do you have a more positive supply to tie the Vcc of the LM324 to? The LM324 can only pull its output to Vcc-1.5V, so if you power it off +5V, the highest it can pull its output is 3.5V

 

[EmmKay]

@MikeMl .. I have +10V max available on the board... but I would like to cap the output to 5V as that would be the vref for the A2D converter.

 

[Diver300]

You can put a zener in parallel with C2, or a diode from the top of C2 to the +5 V supply. Either will limit the voltage so that you won't damage the A2D. R4 will limit the current in the zener or the diode.

In normal running, the output voltage is set by the input level and the resistor values, and should not exceed the A2D range, but a clamp is a good idea to avoid damage if there is a larger signal for any reason.

 

[EmmKay]

Got the lm324 solution working by powering it with 6.5V. So that part is ok at the moment! have to do more testiing on it!!!! so thank you all!

Onto the 2nd question. is it better to run the connection cable from the sensor before the preamp stage or after the preamp stage.! the cable run is about 65 inches long!!!!!

 

[ronv]

I'm guessing the first stage has gain tho no input resistor is shown. If that is the case signal to noise ratio will be better if the cable is between the op amps.