This chart shows the frequency measured with respect to the input voltage. It is a linear relationship, except at the lower frequencies where the relationship becomes asymptotic. This is problematic becomes the voltage range is quite large at these low frequencies (~1.1 voltage differential), and the corresponding frequency range is extremely small. Recall that PBASIC handles integer values only. Fractional frequency values (values between 0 Hz and 1 Hz) for the voltage range of 0 to 1.1 volts would be a problem for the BASIC Stamp. The chart shows this problem clearly; there are no points between 0 volts and 1.1 volts because the corresponding frequencies are in between 0 and 1 Hz, decimal values that PBASIC cannot handle. One could possibly attempt to extrapolate those points but it is preferrable to find an alternative that would allow for direct measuring of those points.
We didn’t have time to experiment more and figure out the best way to get around this problem so we decided to go with only the ADC0831.
