CapTouch®Programmable Controller for Single-Electrode Capacitance Sensors
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To introduce the AD7147 capacitance to digital converter and its basic operation
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CapTouch®Programmable Controller for Single-Electrode Capacitance Sensors
- 5. Capacitance Sensing Theory 1 input (CINx)
- 6. Complete Solution for Capacitance Sensing Scroll Wheel Slider Buttons
Notes de l'éditeur
- Welcome to the training module on Analog Device’s CapTouch® Programmable Controller for Single-Electrode Capacitance Sensors . This training module introduces the AD7147 capacitance-to-digital converter and its basic operations.
- Cell phones, multimedia players, digital cameras, and other rapidly shrinking mobile devices, increasingly require new user interfaces to replace mechanical switches. The AD7147 capacitance-to-digital converter or CDC is a highly integrated touch controller that allows designers to quickly and easily add responsive user controls, such as scroll-wheels and touch pads, to portable electronic devices. The device features the addition of an active shield for improved noise performance. The configurable AD7147 CDC features 13 capacitance inputs and femto-farad resolution, allowing mobile electronics designers to implement a touch pad, scroll-wheel, slider, or up to 36 buttons per device. It includes a SPI interface, while the AD7147-1 features an I²C interface. The device also has on-chip calibration logic to compensate for changes in the ambient environment. It is intended for use in portable systems requiring high resolution user inputs. Examples of these portable systems include cell phones, personal music & multimedia players, smart handheld devices, and other consumer electronics
- This is the block diagram for the AD7147 CDC. The internal circuitry consists of a 16-bit, sigma-delta (Σ-Δ) converter that can change a capacitive input signal into a digital value. A switch matrix routes the 13 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 effects of humidity, temperature, and other environmental factors can affect the operation of capacitive sensors. The AD7147 has on-chip digital logic and 528 words of RAM that are used for environmental compensation. The device performs continuous calibration to compensate for these effects, allowing it to consistently provide error-free results.
- Capacitance sensing is a dilute-version to touch-screen-solution. However, the capacitance sensing solution is made easy because the design of the human interface can be done using the Printed Circuit Board (PCB). The diagram demonstrates the basic workings of a capacitance sensing function. To create the capacitor effect, the human finger acts as the grounding plate of the capacitor. The other plate of the capacitor is the sensor electrode on the PCB. The AD7147measures capacitance changes from single electrode sensors. When a user approaches the sensor, the total capacitance associated with that sensor changes and is measured by the AD7147. If the change causes a set threshold to be exceeded, the AD7147 interprets this as a sensor activation.
- There are many user input applications for capacitance sensing. A simpler application is switch buttons or keypad design. A software that runs on the host processor is needed if the application requires higher positional resolution sensors such as scroll-bars or wheels. The memory requirements for the host depends on the sensor and are typically 10 kB of code and 600 bytes of data memory, depending on the sensor type. The advantage of using AD7147 is its diversification in interface facility. It can operate using either SPI or I 2 C communication. Since the device is a supporting component, it is necessary to at least work with a microcontroller.
- The AD7147 has three operating modes. In full power mode, all sections of the device remain fully powered and converting at all times. It is suited for applications where power is not a concern, such as game consoles that have an AC power supply. When the device is in low power mode and the external sensors have not been touched, the AD7147 reduces its conversion frequency, thereby greatly reducing its power consumption. In low power mode, the AD7147 remains in a low power state until proximity is detected on any one of the external sensors. In shutdown mode, the part shuts down completely.
- The capacitance-to-digital converter on the AD7147 has a sigma-delta (Σ-Δ) architecture with 16-bit resolution. There are 13 possible inputs to the CDC which are connected to the input of the converter through a switch matrix. Each Cin input can be connected to the positive input , or to the negative input of the converter. Click 2 x Connecting to converter’s positive input results in an increase in CDC value when the user touches the sensor. Click 1 x Click 2x Connecting to the negative input results in a decrease in CDC value when the sensor is touched. The activation thresholds are stored in on-chip-registers. When the CDC value passes through a threshold, the sensor is activated. There are two programmable DACs on board the AD7147 to null the effect of any parasitic capacitances on the CDC measurement. This parasitic capacitance is the capacitance of the sensor itself – only the change in capacitance due to the user interaction is of interest to the devices.
- Up to 12 conversion stages can be performed in one sequence. Each of the 12 conversions stages can measure the input from a different sensor. Depending on the number and type of capacitance sensors that are used, not all conversion stages are required. For example, a button sensor generally requires one sequencer stage and a wheel sensor requires 8 stages, one for each segments of the wheel. The AD7147 converts on Stage 0, then goes onto Stage 1, Stage 2 etc. , and stores the result of each conversion in the CDC results-registers.
- The CDC provides on-chip capacitance sensor calibration to automatically adjust for environmental conditions that have an effect on the ambient levels of the capacitance sensor. The output levels of the capacitance sensor are sensitive to temperature, humidity, and, in some cases, dirt. The AD7147 achieves optimal and reliable sensor performance by continuously monitoring the CDC ambient levels and compensating for any environmental changes by adjusting the values of the STAGE-X_HIGH_THRESHOLD registers and the STAGE-X_ LOW_THRESHOLD registers. Figure 1 shows the typical behavior of a capacitance sensor when calibration is not applied and the ambient levels drift over time as environmental conditions change. Figure 2 shows a simplified example of how the AD7147 applies the adaptive calibration process, resulting in no interrupt errors even with changing CDC ambient levels due to dynamic environmental conditions.
- The AD7147 has an interrupt output that triggers an interrupt service routine on the host processor. There are three types of interrupt events on the device: a CDC conversion-complete interrupt, a sensor-touch interrupt, and a GPIO interrupt. Each interrupt has ENABLE & STATUS registers. The conversion-complete and sensor-touch (sensor-activation) interrupts can be enabled on a per-conversion-stage basis. The status registers indicate what type of interrupt triggered the INTERRUPT pin. Status registers are cleared, and the INTERRUPT signal is reset high during a read operation.
- The AD7147 has one GPIO pin. It can be configured as an input or an output. When the GPIO is configured as an output, the voltage level on the pin is set to either a low level or a high level, as defined by the GPIO_SETUP bits. The GPIO can be used to turn an LED on and off by setting it as either output high or low. As shown in the figure, GPIO output high turns on the LED; setting the GPIO output low turns off the LED. The GPIO pin connects to a transistor that provides the drive current for the LED.
- The AD7147 CapTouch™ controller must be tuned to offer the best performance for each user’s application. Tuning is dependent upon what type of sensors i.e. buttons, scroll bars, or wheels, are attached to the controller. Sensor size, surroundings, and overlay all have an effect on performance. The tuning process consists of the five steps show. However, do remember that before getting started on tuning the AD7147, it is important to ensure that the device is not already programmed. These steps can be accomplished by reading and writing to registers using a development system or using an AD714x evaluation board and software. The high and low clamp and offset values are used by the AD7147 in determining the activation thresholds for each sensor. Step by step instructions on how to tune the AD7147 can be located in the applications note AN-929 which is available at www.analog.com
- This figure shows the typical design for button sensors. The button size has to be equal to or grater than 3mm in diameter. Each button sensor is connected to one C-IN input of the AD7147. Button sensors can be connected to either positive or negative capacitance-to-digital converter (CDC) input as shown in the figure. Two buttons can use the same differential stage, with one button connected to positive CDC input and the other connected to negative CDC input. Note that buttons connected differentially cannot be activated together because one cancels out the other.
- A slider can be constructed using between five and eight discrete sensor segments depending on the sensor length. The scroll wheel is a special type of discrete slider. Each of the discrete segments in the slider is arranged into a circular shape. Sliders and wheels consist of eight separate sensor segments. Each segment is connected separately to positive CDC input, similar to button connections. When scrolling, the user interacts with more than one segment.
- The AD7147 can be used to implement a matrix keypad. The device can implement up to 36 keys. The keys are arranged in rows and columns, similar to a standard matrix keypad. The keys are constructed with one-half of the keys connected to the column input line, and the other half connected to the row input line. Each column and each row of a keypad use one connection to the positive CDC input. For matrix keypad operation, check the row and column status to find which key is active.
- Thank you for taking the time to view this presentation on “ CapTouch® Programmable Controller for Single-Electrode Capacitance Sensors ” . If you would like to learn more or go on to purchase some of these devices, you may either click on the part list link, or simply call our sales hotline. For more technical information you may either visit the Analog Devices site, or if you would prefer to speak to someone live, please call our hotline number, or even use our ‘live chat’ online facility.