Touch sensitive control devices are now prevalent on many electronic devices such as mobile phones, MP3 players, personal digital assistants as well as white goods such as cookers and freezers. This is because they are space saving in terms of an amount of “surface real estate” available to position user controls, robust in that there is a reduction in the amount of mechanical components required in their implementation and they can also be made to resist potentially harmful substances in an environment in which they are disposed. For the example of white goods, the presence of water and other aqueous substances is typically harmful to contact switches. Therefore, the touch sensitive switch can be disposed behind a protective layer preventing damage from being caused by the aqueous substances. Furthermore, a touch sensitive control can be disposed in front of a display screen such as for example an LCD display screen with the effect that a user can select a particular function by touching the screen at a position at which a particular menu option has been displayed.
There are various forms of touch sensitive controls which use a capacitive sensor to sense the presence of a body such as a user's finger. A touch sensitive capacitive sensor for example is disclosed in WO-97/23738. In WO-97/23738 a single coupling plate is provided and disposed to form a touch sensitive switch. The touch sensitive plate is referred to as a key. In accordance with this example, the key is charged using a drive circuit for a drive part of a measurement cycle and then this charge is measured by transferring the induced charge from the key by a charge detection circuit during a measurement part of the cycle. The charging and transferring parts of the cycle can vary widely and can be selected in accordance with the application concerned. The sensor can detect the presence of an object near the key as a result of a change in an amount of the charge induced onto the key, even in the presence of interfering substances.
Another form of touch sensitive control is disclosed in WO-00/44018. In this example a pair of electrodes are provided which act as a key so that the presence of a body such as a user's finger is detected as a result of a change in an amount of charge which is transferred between the two electrodes. With this arrangement, one of the pair of electrodes (labelled X) is driven with a drive circuit and the other of the pair of electrodes (labelled Y) is connected to a charge measurement circuit which detects an amount of charge present on the Y plate when driven by the X plate. As disclosed in WO-00/440018 several pairs of electrodes can be arranged to form a matrix of sensing areas which can provide an efficient implementation of a touch sensitive two-dimensional position sensor. Such two dimensional capacitive sensors are typically used with devices which include touch sensitive screens or touch sensitive keyboards/keypads which are used in the example of consumer electronic devices and domestic appliances. As indicated above, such two dimensional capacitive touch sensors can be used in conjunction with liquid crystal displays or cathode ray tubes to form such touch sensitive screens.
Although touch sensitive capacitive sensors, such as those described above and disclosed in the above-mentioned disclosures, have been successfully deployed in many applications, some applications can present a challenging environment for detecting a change in charge as a result of the presence of a body. For example, noise which may be present for a particular application can cause a disruption in accurately measuring an amount of charge transferred from a capacitively charged key for the various examples set out above.