Off Topic: AD to read 4-20mA with low input imepedence

[Guest]

Ok, newbie here.. I've done a lot of AD using the internal PIC, but recently I needed to measure 4-20mA. I can get resolution at high input impedence, but as I lower the input imepedence I lose resolution. Anyone have any recommendations or a smack in the head or a good external AD to look at?

Thanks.

[Joan]

Do you remember the Ohm law V=R*I and P=V*I ( or P=R*I*I )?

Use it. Find and R that gives you 5V for 20mA and put your Vref at 5V. Next, prove that the Power that gives your sensor it's enougth for the R you find before.

The max resolution is when you Full Scale it just in the limit of your measure. ( For example for measure 4.95 Volts is better a Vref of 5V that 10V ).

Regards;

Joan

[Guest]

Yes. The problem is when I increase R I increase the load resistance. I want to reduce R, but when I do that I decrease resolution.

I need minimum load resistance.

I am not asking for anyone to design this for me, I am just looking for a suggestion or a point in the right direction.

Thanks.

[Joan]

Hi:
A good hardware method is to put an Operational Amplifier in non-inverted configuration or a Diferencial Amplifier like the AD620 from Analog Devices. One time you have decided you best "R", amplify the Vr max ( max dropout voltage to R ) to 5V, doing the gain

G = ( 5 / Vr max )

Then you will give less impedance and high precission. Sometimes conecting the sensor directly to the A/D pin is not the best solution and we cannot jump the Ohm law. If we use some instrumentation basics we give the best result.

Understand that I'm not designing nothing only asking your question.

Best Regards;

Joan.

[SteveS]

I would look at an external sigma-delta converter if you can stand conversion rates of a few samples per second. Benefit is that you can set them up to filter 50 and 60 hz noise. I've used the AD7705 which has programmable gain. There are probably newer ones that are even better. Analog Devices and Linear Tech are good starting points.

- SteveS