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AD7142 Datasheet(PDF) 11 Page - Analog Devices |
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AD7142 Datasheet(HTML) 11 Page - Analog Devices |
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11 / 68 page  AD7142 Rev. 0 | Page 11 of 68 THEORY OF OPERATION The AD7142 and AD7142-1 are capacitance-to-digital converters (CDCs) with on-chip environmental compensation, intended for use in portable systems requiring high resolution user input. The internal circuitry consists of a 16-bit, ∑-Δ con- verter that converts a capacitive input signal into a digital value. There are 14 input pins on the AD7142 and AD7142-1, CIN0 to CIN13. A switch matrix routes the input signals to the CDC. The result of each capacitance-to-digital conversion is stored in on-chip registers. The host subsequently reads the results over the serial interface. The AD7142 contains an SPI interface and the AD7142-1 has an I2C interface ensuring that the parts are compatible with a wide range of host processors. Because the AD7142 and AD7142-1 are identical parts, with the exception of the serial interface, AD7142 refers to both the AD7142 and AD7142-1 throughout this data sheet. The AD7142 interfaces with up to 14 external capacitance sensors. These sensors can be arranged as buttons, scroll bars, wheels, or as a combination of sensor types. The external sensors consist of electrodes on a 2- or 4-layer PCB that interfaces directly to the AD7142. The AD7142 can be set up to implement any set of input sensors by programming the on-chip registers. The registers can also be programmed to control features such as averaging, offsets, and gains for each of the external sensors. There is a sequencer on-chip to control how each of the capacitance inputs is polled. The AD7142 has on-chip digital logic and 528 words of RAM that are used for environmental compensation. The effects of humidity, temperature, and other environmental factors can effect the operation of capacitance sensors. Transparent to the user, the AD7142 performs continuous calibration to compen- sate for these effects, allowing the AD7142 to give error-free results at all times. The AD7142 requires some minor companion software that runs on the host or other microcontroller to implement high resolution sensor functions such as a scroll bar or wheel. However, no companion software is required to implement buttons, including 8-way button functionality. Button sensors are implemented completely in digital logic on-chip. The AD7142 can be programmed to operate in either full power mode, or in low power automatic wake-up mode. The automatic wake-up mode is particularly suited for portable devices that require low power operation giving the user significant power savings coupled with full functionality. The AD7142 has an interrupt output, INT, to indicate when new data has been placed into the registers. INT is used to interrupt the host on sensor activation. The AD7142 operates from a 2.6 V to 3.6 V supply, and is available in a 32-lead, 5 mm × 5 mm LFCSP_VQ. CAPACITANCE SENSING THEORY The AD7142 uses a method of sensing capacitance known as the shunt method. Using this method, an excitation source is connected to a transmitter generating an electric field to a receiver. The field lines measured at the receiver are translated into the digital domain by a ∑-Δ converter. When a finger, or other grounded object, interferes with the electric field, some of the field lines are shunted to ground and do not reach the receiver (see Figure 16). Therefore, the total capacitance measured at the receiver decreases when an object comes close to the induced field. EXCITATION SIGNAL 240kHz Σ-Δ ADC 16-BIT DATA PLASTIC COVER AD7142 Tx Rx PCB LAYER 1 PCB LAYER 2 Figure 16. Sensing Capacitance Method In practice, the excitation source and ∑-Δ ADC are implemented on the AD7142, while the transmitter and receiver are constructed on a PCB that makes up the external sensor. Registering a Sensor Activation When a sensor is approached, the total capacitance associated with that sensor, measured by the AD7142, changes. When the capacitance changes to such an extent that a set threshold is exceeded, the AD7142 registers this as a sensor touch. Preprogrammed threshold levels are used to determine if a change in capacitance is due to a button being activated. If the capacitance exceeds one of the threshold limits, the AD7142 registers this as a true button activation. The same thresholds principle is used to determine if other types of sensors, such as sliders or scroll wheels, are activated. |
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