Measure value with A/D, but no accurate ref.

[hmmpic]

How to solve this.

My input to the A/D is stable and ok!

The PIC system itself is powered from a bat. that will say the power will drop slowly from about 4,3 down to 3,7, and then I power the system off.

How to make some measurement with the AD when the ref. voltage is not constant?

Maybe the ref. can be made with a 2,5V zener? but the power drain against the zener must be around 1ma. and this is too much.

Another solution is to use one port pin and feed this to the zener system, and then feed the zener voltage to the ref voltage on the PIC only some ms before making the measurement?

I don't want a lot of extra components!

Suggestion?

[Gabriel]

what are you measuring?
is it a sensor?
is the sensor running of the same battery pack?
is it a linear sensor?

if its something like a voltage divider with a LDR then then i believe you should be ok..... you wouldn't need anything.. the voltage source of your "sensor" varies proportionally to your Vref....

if R1(LDR)=R2 on your divider (assume 8 bit ADC)
Assume Vref= Vcc.
at 4V Vcc your output is 2V or ~127
at 3V Vcc you output is 1.5V or ~127 .... you cant still figure out the values of the LDR.... regardless of the voltage....


If i am terribly mistaken.... please feel free to correct me.... i have my doubts on this issue too....
_________________
CCS PCM 5.078 & CCS PCH 5.093

[Humberto]

IMO your best choice is to use a low power voltage reference in order to meet the imposed restrictions
of your project, a low voltage reference at very low power. For example the MAX6035, it can give a stable
2.5V draining less than 100µA of battery current and support Vin min=4.4V

Regards

[hmmpic]

Hi

The problem is that the output from the sensor is not floating with VCC but give a stable linear signal.

I think the solution with a low drain ref. can be a choice. Ex MCP1525 cost 0,45$ and work from 2,7-5,5 output is 2.5V

Thanks for reply!

[PCM programmer]

You didn't say what PIC you are using. If your PIC has it, you can use
LVD or HLVD. Here is an example program for the LVD (low voltage
detect) module:
http://www.ccsinfo.com/forum/viewtopic.php?t=24577