June 2005, Issue 179

Accurate Capacitance Meter
Cypress PSoC High Integration Challenge 2004 Contest Winner

COMPARATORS

We used two comparators (CompLow and CompHigh) in this design to define the voltage range. Their threshold voltages (V_LOW and V_HIGH) are programmed internally using the criteria shown in Figure 4b.      

The CompHigh’s maximum value is tied to the normal operation of the current source (see Figure 4c). We used Ref_LOW = 0 and Ref_HIGH = 2, which is well below the undesired zone because V_HIGH = 0.45 V. It’s certainly possible to use a higher voltage, but the V_HIGH – V_LOW range will be increased, which will lead to longer measuring times. Our experience shows that using a wider range does not lead to a noticeable increase in measurement accuracy.