aljamri
Member
Hi,
I am trying to use ADC to read a signal of 4 to 20 mA. With the help from two superb ETO members in This Thread. , the software part is accomplished.
I've tested the circuit both using OshonSoft IDE and on a bread board using 0 to 5 volts (0 to 20mA) signal, The Vref- is connected to Vss (0v) and Vref+ to VDD (5V) . The circuit works as expected.
When I tried to shift the Vref- from 0v into 1v using two resistors as voltage divider between Gnd and 5V. The resistors were 10k and around 7.5k. Then applied the signal, (1 to 5V) into RA0, and it woks fine too.
THE PROBLEM:
When I tried to replace the input voltage with mA signal ( 4 to 20 mA ), and by using a 250 Ohm shunt resistor, the things went wrong, as I've connected the shunt resistor between the Gnd and AN0 ( as what I did in all the previous succeeded tests ).
I'm wondering what is wrong, is it where I have to connect the signal or something else.
I am trying to use ADC to read a signal of 4 to 20 mA. With the help from two superb ETO members in This Thread. , the software part is accomplished.
I've tested the circuit both using OshonSoft IDE and on a bread board using 0 to 5 volts (0 to 20mA) signal, The Vref- is connected to Vss (0v) and Vref+ to VDD (5V) . The circuit works as expected.
When I tried to shift the Vref- from 0v into 1v using two resistors as voltage divider between Gnd and 5V. The resistors were 10k and around 7.5k. Then applied the signal, (1 to 5V) into RA0, and it woks fine too.
THE PROBLEM:
When I tried to replace the input voltage with mA signal ( 4 to 20 mA ), and by using a 250 Ohm shunt resistor, the things went wrong, as I've connected the shunt resistor between the Gnd and AN0 ( as what I did in all the previous succeeded tests ).
I'm wondering what is wrong, is it where I have to connect the signal or something else.