QST108CB0 datasheet(27/51 Pages) STMICROELECTRONICS

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QST108

Design guidelines

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Design guidelines

5.1

CS sense capacitor

The CS sense capacitors accumulate the charge from the key electrodes and determine

sensitivity. Higher values of CS make the corresponding sensing channel more sensitive.

The values of CS can differ for each channel, permitting differences in sensitivity from key to

key or to balance unequal sensitivities. Unequal sensitivities can occur due to key size and

placement differences and stray wiring capacitances. More stray capacitance on a sense

trace will desensitize the corresponding key. Increasing the CS for that key will compensate

for the loss of sensitivity.

The CS capacitors can be virtually any plastic film or low- to medium-K ceramic capacitor.

The normal CS range is 1nF to 50nF depending on the sensitivity required: larger values of

CS require better quality to ensure reliable sensing. In certain circumstances the normal CS

range may be exceeded. Acceptable capacitor types for most uses include PPS film,

polypropylene film, and NP0 and X5R / X7R ceramics. Lower grades than X5R or X7R are

not recommended.

5.2

Sensitivity tuning

Sensitivity can be altered to suit various applications and situations on a channel-by-

channel basis. The easiest and most direct way to impact sensitivity is to alter the value of

each CS: more CS yields higher sensitivity. Each channel has its own CS value and can

therefore be independently adjusted.

5.2.1

Increasing sensitivity

Sensitivity can also be increased by using larger electrode areas, reducing panel thickness,

or using a panel material with a higher dielectric constant.

5.2.2

Decreasing sensitivity

In some cases the circuit may be too sensitive. Gain can be lowered further by a number of

strategies:

●

making the electrode smaller

●

making the electrode into a sparse mesh using a high space-to-conductor ratio

●

decreasing the CS capacitors

5.2.3

Key balance

A number of factors can cause sensitivity imbalances. Notably, SNS wiring to electrodes can

have differing stray amounts of capacitance to ground. Increasing load capacitance will

cause a decrease in gain. Key size differences, and proximity to other metal surfaces can

also impact gain.

The keys may thus require “balancing” to achieve similar sensitivity levels. This can be best

accomplished by trimming the values of the CS capacitors to achieve equilibrium. The RS

resistors have no effect on sensitivity and should not be altered. Load capacitances to

ground can also be added to overly sensitive channels to reduce their gain.

These should be in the order of a few picofarads.

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