Discussion in 'General Electronics Discussion' started by georgypaul, Nov 13, 2018.

1. ### georgypaul

1
0
Nov 13, 2018
I was working on an Automated Resistor Sorter Project with GUI found on internet. Here they have chosen the ADC output limits to be 99 (0.32v)
although the PIC 32 is using a 10 bit ADC which have levels upto 1023.
(http://people.ece.cornell.edu/land/.../anp56_bwg38_nol5/anp56_bwg38_nol5/index.html)

"The measurement autoranging feature and the CTMU is controlled by the PIC software. We first initialize the CTMU by modifying CTMUCONbits to turn on the CTMU module, disable the discharging, and enable the charging of the circuit. We initially start off with current level 1, the lowest possible current output by the CTMU. If the ADC reading is greater than 99 (~0.32 V) then we break out of the loop and use this value of current to calculate the resistance. Otherwise, the loop iterates through increasing current levels until an ADC value of 99 or the maximum current is reached. Therefore, we keep the current safely low and gradually step it up only when we know the next step will not exceed the tolerable voltage"
This is what is given in the project description. Can anyone let me know why 99 was taken and also how to find the value of max resistor that can be sorted. Please

2. ### CircuitMaster

48
1
Dec 17, 2016
This is a little unclear.

1. Whats your power supply?
2. 99 only means that you need 0.32V for whatever you are doing, if you need something else, change the voltage.

3. ### BobK

7,682
1,688
Jan 5, 2010
Probably because they don't want to put more than 0.32V across the resistor. Think of how many Watts it would dissipate if you were to put 3.3V across a 1Ω resistor, compared to what it would be with 0.32V. (hint P = V^2 / R) Using 1/10 th of the range gives you roughly 1% accuracy while limiting the current to a level that will not blow out your resistors. Actually, the PIC would not provide enough current to blow it out, more likely the PIC would be damaged.

Bob

4. ### kellys_eye

4,282
1,152
Jun 25, 2010
How fast is the loop? Looping through 1024 iterations takes 10 times as long. For a 'fast' sorter a fast loop is needed.