Capacitive sensing techniques and considerations--The basics

The Problem
Sensing capacitance change from 10pF to 100pF is not difficult. Sensing capacitance change from 10.0pF and 10.5pF is not trivial but does not present significant challenges. However, sensing capacitance change from 10.00pF to 10.05pF is quite challenging, especially when doing so in the presence of environmental changes, system-level electromagnetic interference and measurement control circuitry variance. Challenges may be even more severe when also considering power consumption, response time, and usability.

The measured element in capacitive sensing applications is a capacitor formed between a sensor pad (an area of conductive material) and surrounding conductive material, which is typically a ground fill of some density and distance. The capacitance value of a sensor is given by:

Where A is the complete area of the entire sensor and associated routing and d is the distance between the sensor/routing and the surrounding conductors. As conductive objects (such as a finger) move in proximity to the sensor, they alter the electrical field lines of the capacitive sensor and change the capacitance that is measured by the control circuitry.

Charge Transfer
Charge Transfer is one category of capacitive sensing using charge amplification and filtering to measure the sum of all the individual charge contributors around a given sensor. A waveform is generated and converted to a representative capacitance value through a combination of timing and filtering means.

Charge Transfer is not a term specific to capacitive sensing. It refers to the transfer of charge from one location to another through a series of switches. This same technique is used in MEMS for physical measurements. Charge Transfer for capacitive sensing uses a switched capacitor network to accumulate charge onto an integrating capacitor. The potential across the integrating capacitor is then measured against a reference voltage or read using an analog-to-digital converter.

In the first of two non-overlapping phases, a sensor capacitor is connected to a voltage source, accumulating charge on that sensor capacitor. In the second phase, the charge that is stored on the sensor capacitor is discharged into the larger integrating capacitor. This process is then repeated a number of times, thereby increasing the voltage on the integrating capacitor. The amount of charge that is transferred to the integrating capacitor in each subsequent phase two cycle decreases exponentially. The sensing circuitry is a simple structure where the voltage across the integrating capacitor is compared to a fixed reference voltage.