ADC ref problem

[simon630]

dear friends,
i am now using the adc features of 16c73 chip. because the circuit is more or less finalized, my reference voltage has to stay at 2.5v at pinA3. and the monitored analog input is tied to pinA0. this is an output pin of a battery whose rating is 4.2v. now what i want to do is to monitor this pin so that when it drops to around 3v, i will go and charge it. however, the voltages from 3v-4.2v are higher than the refenrence one. i am wondering whether the compiler provides any additional function that makes the Vref 2 times of the one it detects on the pinA3.

main(){
....
setup_adc_ports( RA0_RA1_ANALOG_RA3_REF );
setup_adc( ADC_CLOCK_INTERNAL );
set_adc_channel(0);
while (1) {
....
value=read_adc();
} //end while
}
attached is portion of the program. thx for anybody's help in advance.
Simon
___________________________
This message was ported from CCS's old forum
Original Post ID: 10645

[PCM programmer]

:=dear friends,
:= i am now using the adc features of 16c73 chip. because the circuit is more or less finalized, my reference voltage has to stay at 2.5v at pinA3. and the monitored analog input is tied to pinA0. this is an output pin of a battery whose rating is 4.2v. now what i want to do is to monitor this pin so that when it drops to around 3v, i will go and charge it. however, the voltages from 3v-4.2v are higher than the refenrence one. i am wondering whether the compiler provides any additional function that makes the Vref 2 times of the one it detects on the pinA3.
:=
:=main(){
:=....
:=setup_adc_ports( RA0_RA1_ANALOG_RA3_REF );
:=setup_adc( ADC_CLOCK_INTERNAL );
:=set_adc_channel(0);
:=while (1) {
:=....
:=value=read_adc();
:=} //end while
:=}
-------------------------------------------------------------

Why don't you use the internal Vref of VDD ?
I assume you're running the PIC at +5v.
That would let you measure your analog voltage of 4.2v.

If you look at Figure 13-2 in the 16c73a data sheet,
you're currently using the setting with the CFG bits
set to 101. If you change it to use the setting for 100,
then RA3 becomes an analog input, and the Vref becomes VDD.
Since you already have an analog voltage (in effect)
of 2.5v on RA3, it's no problem. You still have the
same digital pins as before. Looks like it should work.

Use the following line to configure the PIC for that mode.
setup_adc_ports(RA0_RA1_RA3_ANALOG);
___________________________
This message was ported from CCS's old forum
Original Post ID: 10655

[Sherpa Doug]

:=:= i am now using the adc features of 16c73 chip. because the circuit is more or less finalized, my reference voltage has to stay at 2.5v at pinA3. and the monitored analog input is tied to pinA0.
:=Why don't you use the internal Vref of VDD ?

If you are worried that VDD may not be as stable as your 2.5V, you may be able to measure the 2.5V and correct for drift of VDD.
For example if A0 reads 123, and A3 reads 197 then
197/123=2.5V/Vbatt so Vbatt=4.0V even if VDD has drifted.

___________________________
This message was ported from CCS's old forum
Original Post ID: 10657

[PCM programmer]

:=:=:= i am now using the adc features of 16c73 chip. because the circuit is more or less finalized, my reference voltage has to stay at 2.5v at pinA3. and the monitored analog input is tied to pinA0.
:=:=Why don't you use the internal Vref of VDD ?
:=
:=If you are worried that VDD may not be as stable as your 2.5V, you may be able to measure the 2.5V and correct for drift of VDD.
:=For example if A0 reads 123, and A3 reads 197 then
:=197/123=2.5V/Vbatt so Vbatt=4.0V even if VDD has drifted.
----------------------------------------------------------
Or he could spend more money and put in a 1\% regulator.
___________________________
This message was ported from CCS's old forum
Original Post ID: 10658

[oscar]

:=dear friends,
:= i am now using the adc features of 16c73 chip. because the circuit is more or less finalized, my reference voltage has to stay at 2.5v at pinA3. and the monitored analog input is tied to pinA0. this is an output pin of a battery whose rating is 4.2v. now what i want to do is to monitor this pin so that when it drops to around 3v, i will go and charge it. however, the voltages from 3v-4.2v are higher than the refenrence one. i am wondering whether the compiler provides any additional function that makes the Vref 2 times of the one it detects on the pinA3.
:=
:=main(){
:=....
:=setup_adc_ports( RA0_RA1_ANALOG_RA3_REF );
:=setup_adc( ADC_CLOCK_INTERNAL );
:=set_adc_channel(0);
:=while (1) {
:=....
:=value=read_adc();
:=} //end while
:=}
:=attached is portion of the program. thx for anybody's help in advance.
:=Simon

Simon,

There is a simple solution to your problem. Create a voltage divider from your battery to ground. Use two 10k Ohm resistors in series to ground from battery. This will divide your battery by two. The divided battery signal = Vbat/2 should now be connected to pin A0. What you will now do is poll your A/D (pin A0) so when you read 1.5V (half of 3V) you will activate your charger.

The voltage divider will consume less than 200uA at full battery voltage. If this current consumption is too high for your application proceed to increase the voltage divider resistor values to 20K. Raising the resistance values will increase your A/D acquisition time.....remember the A/D can be modeled as a capacitor charge circuit so if your input impedance is high then your time constant increases (keep this in mind if you have issues as you raise the impedance).


Hope this helps,
Oscar
___________________________
This message was ported from CCS's old forum
Original Post ID: 10667