Hi,
Due to the high costs of VFCs and ADCs in one of our products we are planning to use a NE555 in astable mode as a VFC to convert the analog signal into digital and measure the frequency with a 52's timer2 in capture mode.
A similar application is described in one of st's application note: AN1755

http://www.st.com/stonline/.../10306.pdf

Altough the application note is titled as 'PRECISION THERMOMETER...' they do not mention how precise it is and they do not say anything abut the accurcy of the overall system.
When I looked at the NE555 datasheet

http://www.st.com/stonline/.../ne555.pdf

For timing error at page 3 it states that:
for astable operation the initial accuracy is 1.5% typically.
Is this the accuracy of the DUTY CYCLE or FREQUENCY of the generated output signal? if it is the accuracy of the frequency then this will not be a problem.
Because the analog to digital conversion will be based on the measurement of the duty cycle(duty cycle is proportional to the analog input).
But if it is the accuracy of the duty cycle then the overall systems accuracy will be 1.5% typically for stable temperatur and power supply.
Is such kind of analog to digital conversion reliable for a 0.1% accuracy?
Best regards
Ertan Balaban