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AD7147 Datasheet(PDF) 11 Page - Analog Devices |
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AD7147 Datasheet(HTML) 11 Page - Analog Devices |
11 / 68 page AD7147 Rev. 0 | Page 11 of 68 THEORY OF OPERATION The AD7147 and AD7147-1 are CDCs with on-chip environmental compensation. They are intended for use in portable systems requiring high resolution user input. The internal circuitry consists of a 16-bit, ∑-Δ converter that can change a capacitive input signal into a digital value. There are 13 input pins, CIN0 to CIN12, on the AD7147 or AD7147-1. 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 AD7147 has an SPI interface, and the AD7147-1 has an I2C interface, ensuring that the parts are compatible with a wide range of host processors. AD7147 refers to both the AD7147 and AD7147-1, unless otherwise noted, from this point forward in this data sheet. The AD7147 interfaces with up to 13 external capacitance sensors. These sensors can be arranged as buttons, scroll bars, or wheels, or as a combination of sensor types. The external sensors consist of an electrode on a single- or multiple-layer PCB that interfaces directly to the AD7147. The AD7147 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 an on-chip sequencer that controls how each of the capacitance inputs is polled. The AD7147 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 affect the operation of capacitance sensors. Transparent to the user, the AD7147 performs continuous calibration to compen- sate for these effects, allowing the AD7147 to consistently provide error-free results. The AD7147 requires a companion algorithm that runs on the host or another microcontroller to implement high resolution sensor functions, such as scroll bars or wheels. However, no companion algorithm is required to implement buttons. Button sensors are implemented on chip, entirely in digital logic. The AD7147 can be programmed to operate in either full power mode or low power automatic wake-up mode. The automatic wake-up mode is particularly suited for portable devices that require low power operation to provide the user with significant power savings and full functionality. The AD7147 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 AD7147 operates from a 2.6 V to 3.6 V supply and is available in a 24-lead, 4 mm × 4 mm LFCSP. CAPACITANCE-SENSING THEORY The AD7147 measures capacitance changes from single-electrode sensors. The sensor electrode on the PCB comprises one plate of a virtual capacitor. The other plate of the capacitor is the user’s finger, which is grounded with respect to the sensor input. The AD7147 first outputs an excitation signal to charge the plate of the capacitor. When the user comes close to the sensor, the virtual capacitor is formed, with the user acting as the second capacitor plate. PLASTIC COVER AD7147 SENSOR PCB Σ-Δ ADC 16-BIT DATA EXCITATION SIGNAL 250kHz Figure 19. Capacitance-Sensing Method A square wave excitation signal is applied to CINx during the conversion, and the modulator continuously samples the charge going through CINx. The output of the modulator is processed via a digital filter, and the resulting digital data is stored in the CDC_RESULT_Sx registers for each conversion stage, at Address 0x00B to Address 0x016. |
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