Signal Chain Adaptation for Piezoresistive and Capacitive Sensors
Adapting piezoresistive and capacitive sensors requires matching source impedance, suppressing parasitics, and isolating conversion noise floors.
Switched-Capacitor
Discrete Topology
Electronic circuit techniques emulate resistive behavior by rapidly moving charge between capacitors using high-speed semiconductor switches. A switched-capacitor circuit replaces traditional resistors with a combination of a small capacitor and two or more transistors. The equivalent resistance is inversely proportional to the switching frequency and the capacitance value.
This approach is highly effective in integrated circuit design because accurate capacitors are easier to fabricate on a silicon die than large resistors.
Clock Frequency
Tuning of the filter or amplifier characteristics is achieved by changing the rate at which the switches open and close. In a switched-capacitor filter, the cutoff frequency scales linearly with the master clock, allowing for precise control over the signal bandwidth. This frequency-dependent behavior makes the circuit versatile and easy to integrate with digital control logic.
High clock speeds are necessary to ensure the sampling rate is well above the signal frequency to avoid aliasing.
Matching Precision
Accuracy depends on the relative ratio of the components. Switched-capacitor designs benefit from the high level of matching achievable between adjacent units.
Charge Injection
Imperfections in the transistors cause a small amount of unwanted charge to leak into the signal path whenever a switch turns off. This charge injection creates an offset voltage that can degrade the performance of high-precision converters. Designers use differential architectures and dummy switches to cancel out these errors.
The resulting circuit provides the linearity and stability required for modern data acquisition systems.