3.6 ESD PROTECTION

In cases where the electrode is placed behind a dielectric

panel, the IC will be protected from direct static discharge.

However even with a panel, transients can still flow into the

electrodes via induction, or in extreme cases via dielectric

breakdown. Porous materials may allow a spark to tunnel

right through the material. Testing is required to reveal any

problems. The device does have diode protection on its SNS

pins which absorb and protect the device from most induced

discharges, up to 20mA; the usefulness of the internal

clamping will depending on the dielectric properties, panel

thickness, and rise time of the ESD transients.

In extreme cases ESD dissipation can be aided further with

added series resistors in line with the electrodes as shown in

Figure 1-1. Because the charge time is 1.2 µs, the circuit can

tolerate large values of series-R, up to 20k ohms in cases

where electrode Cx load is below 10pF. Extra diode

protection at the electrodes can also be used, but this often

leads to additional RFI problems as the diodes will rectify RF

signals into DC which will disturb the measurement.

Directly placing semiconductor transient protection devices

or MOV's on the sense leads is not advised; these devices

have extremely large amounts of nonlinear parasitic C which

will swamp the capacitance of the electrode.

Series-R’s should be low enough to permit at least 6 RC

time-constants to occur during the charge and transfer

phases, where R is the added series-R and C is the load Cx.

If the device is connected to an external control circuit via a

cable or long twisted pair, it is possible for ground-bounce to

cause damage to the Out pins and/or interfere with key

sensing. Noise current injection into the power supply is best

dealt with by shunting the noise aside to chassis ground with

capacitors, and limited using resistors or ferrites.

3.7 RFI PROTECTION

PCB layout, grounding, and the structure of the input circuitry

have a great bearing on the success of a design that can

withstand strong RF interference.

The circuit is remarkably immune to RFI provided that certain

design rules be adhered to:

1. Use SMT components to minimize lead lengths.

2. Always use a ground plane under and around the circuit

and along the sense lines, that is as unbroken as

possible except for relief under and beside the sense

lines to reduce total Cx. Relieved rear ground planes

should be ‘mended’ by bridging over them at 1cm

intervals with 0.5mm ‘rungs’ like a ladder.

3. Ground planes should be connected only to a common

point near the Vss pins of the IC.

4. Route sense traces away from other traces or wires that

are connected to other circuits.

5. Sense electrodes should be kept away from other

circuits and grounds which are not directly connected to

the sensor’s own circuit ground; other grounds will

appear to float at high frequencies and couple RF

currents into the sense lines.

6. Keep the 6 Cs sampling capacitors and all series-R

components close to the IC.

7. Use a 0.1µF minimum ceramic bypass cap very close to

the QT160/161 supply pins.

8. Use series-R’s in the sense lines, of as large a value as

the circuit can tolerate.

9. Bypass input power to chassis ground and again at

circuit ground to reduce line-injected noise effects.

Ferrites over the power wiring may be required to

attenuate line injected noise.

Achieving RF immunity mostly requires diligence and a good

working knowledge of grounding, shielding, and layout

techniques.

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QT160/161 1.07/0904