handbook of modern sensors

188 5 Interface Electronic Circuits
(A)
(B)
Fig. 5.32. Simplified schematic (A) and timing diagrams (B) of a differential capacitance-tovoltage
converter.
Figure 5.32A shows a simplified circuit diagram of a switched-capacitor converter
[6], where the variable capacitance C x and reference capacitance C r are parts
of a symmetrical silicon pressure sensor. Monolithic MOS switches (1–4) are driven
by opposite-phase clock pulses, φ 1 and φ 2 . When the clocks switch, a charge appears
at the common capacitance node. The charge is provided by the constant-voltage
source, V PM , and is proportional to C x − C r and, therefore, to the applied pressure to
the sensor. This charge is applied to a charge-to-voltage converter which includes an
operational amplifier, integrating capacitor C f , and MOS discharge (reset) switch 5.
The output signal is variable-amplitude pulses (Fig. 5.32B) which can be transmitted
through the communication line and either demodulated to produce a linear signal or
further converted into digital data. So long as the open-loop gain of the integrating
OPAM is high, the output voltage is insensitive to stray input capacitance C, offset
voltage, and temperature drift. The minimum detectable signal (noise floor) is determined
by the component noise and temperature drifts of the components. The circuit
analysis shows that the minimum noise power occurs when the integration capacitor
C f is approximately equal to the frequency-compensation capacitor of the OPAM.
When the MOS reset switch goes from the on state to the off state, the switching
signal injects some charge from the gate of the reset transistor to the input summing
node of the OPAM (inverting input). This charge propagated through the gate-tochannel
capacitance of the MOS transistor 5. An injection charge results in an offset
voltage at the output. This error can be compensated for by a charge-canceling device
[7] which can improve the signal-to-noise ratio by two orders of magnitude of the
uncompensated charge. The temperature drift of the circuit can be expressed as
dV out
dT
= V C x − C r
PM (T Cr − T Cf ), (5.38)
C f
where T Cr is the nominal temperature coefficient of C x and C r , and T Cf is the temperature
coefficient of integrating capacitor C f . This equation suggests that the tem-