June
2005, Issue 179
Accurate
Capacitance Meter
Cypress
PSoC High Integration Challenge 2004 Contest Winner
by
Alexander Popov, Jordan Popov, and Peter Popov
PRINCIPLE
OF OPERATION
The
DC method we’ll describe requires the following well-known
relation:
[1]
where
t1 and t2 are the times the voltage across the measured
capacitor CX reaches the two reference voltages U1 and
U2, respectively. iC is the charging current from the
current source. We can measure capacitance by noting
the time it takes to charge CX from U1 to U2 with a
constant current iC. To implement the idea, we had to
provide a current source that outputs iC, a pair of
comparators with thresholds U1 and U2, and a means to
measure the time (T), as shown in Figure 2.
|
(Click
here to enlarge)
|
Figure
2—Our meter uses DC to measure capacitance. All
key components—the DC source, the two comparators,
and the timer—are implemented within the PSoC. The
UART and LCD controls are not shown here. |
After
the gator clips are attached to the terminals to the
measured capacitor CX, it begins to charge from the
current source (the InpBuf and DiffAmp unity gain PGA
buffers, the CurSet DAC, and the RSET resistor) and
the voltage across it rises linearly with time. When
this voltage reaches the reference voltage of the CmpLow
comparator, the output compare bus COMP0 triggers the
Enable input on Timer32. When the voltage across CX
reaches the CmpHigh comparator’s reference voltage,
COMP0 disables Timer32 while COMP1 triggers the end
of the measurement. The capacitance is linearly proportional
to the time measured in Timer32.
The
ADCINC12, which is only used for calibrating the internal
voltage band gap reference, doesn’t take part in the
actual measurement. The BufAmp provides an external
connection to the analog ground (AGND).
