ADC circuit help " is this possible "

[pyrogeek]

can the ADC in the pics test a circuit that only changes resistance in 3 ohm increments?

[mister_e]

3ohm on what? Input Voltage matter, not the impedance step....How about Vref VS Vdd?

[Charles Linquis]

The PIC measures voltage, not resistance. There is not enough information here to give an intelligent answer.

[pyrogeek]

ok, sorry, i know it measures voltage and resistance effects voltage. so, i guess i will try to explain what im trying to convey. lol

basically i need to test a circuit, to see if its open or closed. the item that closes the circuit is an ematch and they are only 3 ohms, and there could be multiple ematches in series or parallel, i was thinking it would be nice to be able to tell by the adc reading, how many and maybe how they are wired. i was also thinking of using just a simple ressistive voltage divider, where R2 would be the ematch and R1 would be 10k. Vdd 3.3 , Vref 3.3 maybe less. Vout would go to the adc pin. there would also be a diode in series with R1 and R2, the adc pin would connect after R1, have to shield the pic (18lf6622 ) from the 24Vdc that would also be there every now and then. When i do the math, it is a drastic change when going from R2 being 0 and R2 being 3 ohms, but a very small change when R2 changes from 3 to 6 ohms or even 9 ohms. So what im wondering is will the adc even register this small of a change?

[cncmachineguy]

Simple answer: If full scale =0 - 3.3V, and the adc is 10 bit, LSB = 3.3/1024 = 3.22265mV. I would think you should plan for 1LSB error so you could detect ~6mV worst case. Now there are plenty of debate around here about 1024 vs 1023, but I live in 1024 world

[Charles Linquis]

Even though the A/D is 10 bits, you won't get a lot of accuracy unless you have a dedicated Vref and you average a lot of readings. You might get 9 bits of accuracy in that case.

You can do a lot better if you amplify the signal with a rail-to-rail input / rail-to-rail output op amp.

If you don't want to go that route, you are going to have to get a bigger voltage change than you will get with your setup. First, R1 will have to be smaller than 10K. I don't know how many "ematches" you plan to have, but 330 ohms would seem to be a good value. Of course, the resistor would have to be a 5W type. This will give you 72mA through the resistor, and .218V across 3 ohms. You will also need a 2K 1/2W resistor between the junction of R1 and R2 and the PIC input to protect it when the switches are open. That should give you enough signal to work with.