AD7147 Touch Controller System Block Diagram
 ** Click on boxes in diagram for more information.
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Implementing a touch sensor solution using the AD7147 requires three components: the AD7147 CapTouch™ controller, sensors, and software for Error detection and high resolution navigation.
Our standard sensor library is available in a PADS Mentor Graphics™ format and .dxf format, allowing the design of your own unique sensor interface layout.
Free, open source software examples are available in C-code. There are I2C™ and SPI™ serial interface examples availble to download. Example code for buttons includes error detection, and decoding of the status registers to figure out which button is active, as well as configuration details for the AD7147. To get the example code for the slider and wheel, you must sign a software license agreement. This example code gives configuration of the AD7147 as well as an algorithm to implement high resolution navigation.
AD7147 Frequently Asked Questions
PCB LAYOUT
Can other components go on same PCB as sensor?
Yes, other components, for example LEDs, connectors, other I.C.s, can go on the same sensor PCB as the sensors. Components should be placed on the back side of the PCB, to ensure there is no air gap over the sensors.
Is an air gap between sensor PCB and covering material allowed?
No air gap is allowed between the sensor PCB and the covering material or case of the product. Less of the electric field extends above the covering material when there is an air gap, and the field dissipates in air. These combine to decrease the sensor response. Also, the covering material may bend on contact, and the user may interfere with more or less of the electric field than before. This results in a non linear response from the sensor. These two effects combine to give an unpredictable response from sensor. We recommend 3M 467MP double-sided tape or similar to glue or tape the sensor PCB to covering plastic.
When should I use AC shield?
Use instead of a ground plane around the sensors on the PCB. To shield traces to/from the sensors when the IC is not on the same PCB as the sensor, use around, above and below all sensor traces.
2-layer or 4-layer PCB?
Choose a 4-layer PCB design when there is no room to route from the AD7147 to the sensors, outside of the sensor active areas. Nothing should be routed directly under the sensors or sensor traces. If there is room on the PCB to route to and from the AD7147 without interfering with the sensor traces, then a 2-layer PCB can be used.
Is there a keep out area required around the sensor?
Yes. For grounded metal, the keep out is 0.2 mm. For floating metal, keep-out area is 5 cm. Floating metal closer to the sensors than 5 cm should be grounded.
What is the maximum distance between sensor traces and AD7147?
Maximum distance between sensor trace and AD7147 CIN input pin is 50 cm. The AD7147 can be placed on a different PCB than the sensors, provided ACshield is used to shield around the sensor traces.
How many sensors can I implement using one AD7147 device?
The number of sensors that can be implemented using one AD7147 depends on the type of sensors required. The AD7147 has 13 CIN inputs, and has 12 time based conversion stages. The table below gives a the number of input pins and the number of conversion stages required for each sensor type. Any number of sensors can be combined, as long as the total number of inputs required does not exceed 13, and the total number of conversions required does not exceed 12.
| Sensor Type | # CIN inputs | # conversion stages |
| Button | 1 | 1 or 0.5 |
| Keypad | 1 per row & 1 per column | 1 per row & 1 per column |
| Slider | 2 | 2 |
| Wheel | 8 | 8 |
SENSORS
Can all sensors connected to the AD7147 be active at the same time?
The AD7147 takes a measurement on all sensors connected to it in a round robin fashion. All sensors can be measured within 36 ms. In practice, this allows the AD7147 to detect if sensors are active at the same time, as it is very unlikely the end user can activate/deactive a sensor within 36 ms.
What happens when a finger is placed across two sensor buttons?
If the two buttons are adjacent to each other and never meant to be activated at the same time, the AD7147 CDC converter stages can be configured differentially to null out the effects of a user simultaneously touching two buttons. Alternatively, the AD7147 can be configured to register both sensor contacts at the same time.
What happens when users with different finger sizes contact the sensor?
The AD7147 internal logic contains an Adaptive Threshold algorithm that automatically adapts to the users finger size and readjusts the threshold levels accordingly.
Will the AD714x sensors work with a glove?
Some gloves will work while contacting the sensors but some gloves will not, depending on the thickness of the glove. ADI have tested gloves of 2 mm thickness and have found this to work successfully.
Can conductive material be placed directly on top of the sensors?
Conductive material interferes with the electric field generated by the AD7147, and so it is not recommended to place conductive material directly over the sensors.
Can the AD714x work under wet/humid conditions?
Like any capacitive sensor application, the sensors performance will be affected by water and humidity. The AD7147 internal environmental algorithms will automatically track and correct for humidity conditions. Water on the sensors (including saturated) will not cause a false activation. We recommend testing to ensure the sensor performance is satisfactory for the required application
How far away from the sensor can proximity be detected?
Proximity can be detected up to a maximum of 20mm from the surface of the sensor, depending on the size of the sensor. The AD714x evaluation board can be used to experiment with proximity detection.
AD7147
Is there a power-up configuration?
Yes, the AD7147 BANK 2 connection set up registers and some BANK 1 set up registers have to be configured at start-up.
Is auto calibration possible?
Yes, the AD714x internal logic contains two calibration algorithms for correcting drift errors due to environmental (e.g., temperature and humidity) conditions.
What is the IC temperature range?
The AD714x is specified at -40°C to +85°C.
What is the AD714x operating voltage range?
VCC = 2.6V to 3.6V
VDRIVE = 1.65V to 3.6V
Does the AD714x have an interrupt?
Yes, the AD714x interrupt can be configured to be asserted at the end of each CDC conversion cycle or only upon user contact with a sensor.
Should I be concerned about ESD?
The AD714x ESD Human Body Model is specified at 2.5kV.The AD714x sensors have been subjected to the ESD air discharge up to 15kV under both contact and non-contact conditions without any failures. Note: ESD events have to be carefully considered during development in order to prevent the ESD charge from coupling directly onto the AD714x sensor board circuits.
SOFTWARE
Is there software required for buttons?
Yes. A few lines of code are required to ensure that, if an error occurs, for example on power-up, the part is recalibrated. Sample C code is available online.
Does Analog Devices provide the open source C-Code for sliders and scroll wheels?
Yes, the code is provided free of charge and royalties once the customer signs the AD714x SW licence agreement.
What is the code space for a Slider?
Typically: 4990 bytes program memory, 254 bytes RAM
What is the code space for a Scroll Wheel?
Typically: 11k code memory , 667 bytes data memory