Slope ADC voltage measurement

Hello,

Sounds like you would liek to know more about the technique rather than how it is acheived on the 430?

Slope ADC techniques to measure voltage can be implemented using a comparator and a timer. The basic theory is to charge a capacitor and measure the time it takes for discharge. The longer it takes to discharge the larger the input voltage.

Here are a few articles that discuss this technique:

http://www.faqs.org/docs/electric/Digital/DIGI_13.html

http://www.parallax.com/dl/docs/cols/nv/vol1/col/nv25.pdf

This application report is a little old (based on the MSP's 3xx family) but the concepts for voltage measurement remain the same.

Regards,

Priya

Measuring resistance with a slope ADC is the very same as measuring voltage. Only that in the first case the voltage applied to the resistor is known and the resistacne is unknown, in the other case the resistance must be known and the voltage is the unknown factor. The Formula to get the final result then is just an 1/x of the other.

There are, however, different aproaches to implement a slope ADC depending on the exact circumstances. The easiest case is to apply a known voltage to a constant capacitor through a constant resistor and compare the voltage at the capacitor with the signal to measure. If the comparator switches from low to high, the time it took is taken and equals the measured voltage. Then discharge the capacitor though low resistance (maybe you can just configure the comparator input to low output) and start over.

Unfortunately the voltage change is not linear.

It's better to charge the capacitor with a constant current. Then the voltage change on the capacitor is linear and the time required to charge it is equal to the measured voltage (with a constant factor based on the charge current and the capacitance).

Another approach can be used if the voltage you measure always is above a trigger level (comparator with internal reference, or digital input with schmitt trigger). It requires a resistor and a capacitor. Charge the capacitor through the resistor by the voltage you want to measure and count the time until it exceeds the reference level. THen discharge by switching the comparator input to low output for some time. The measured time corresponds to the voltage, but too is not linear. And it only works as long as the voltage you measure is above the reference (which would be in case of a low-batt sensor). It is, however, the cheapest of all options and works even if there's no 'real' comparator available.

The classical 'charge to voltage and then count the time to discharge' approach is the worst one when it comes to hardware requirements. You'll need to disconnect the sampling voltage while discharging. You'll need to make a controlled discharge and you'll need a comparator that compares to zero voltage.