at42qt113b ATMEL Corporation, at42qt113b Datasheet - Page 14

• Manufacturer Part Number: at42qt113b
• Manufacturer: ATMEL Corporation

4.4
4.5
14
ESD Protection
EMC Issues
AT42QT113B
The QT113B includes internal diode protection on its pins to absorb and protect the device from
most induced discharges, up to 20 mA. The electrode should always be insulated against direct
ESD; a glass or plastic panel is usually enough as a barrier to ESD. Glass breakdown voltages
are typically over 10 kV per mm thickness.
ESD protection can be enhanced by adding a series resistor Rseries (see
in line with the electrode, of value between 1 k and 50 k. The optimal value depends on the
amount of load capacitance Cx; a high value of Cx means Rseries has to be low. The pulse
waveform on the electrode should be observed on an oscilloscope, and the pulse should look
very flat just before the falling edge. If the pulse voltage never flattens, the gain of the sensor is
reduced and there can be sensing instabilities.
Rseries and Cs should both be placed very close to the chip.
The use of semiconductor transient protection devices, Zeners, or MOV's on the sense lead is
not advised; these devices have extremely large amounts of parasitic capacitance which will
swamp the QT113B and render it unstable or diminish gain.
External AC fields (EMI) due to RF transmitters or electrical noise sources can cause false
detections or unexplained shifts in sensitivity.
The influence of external fields on the sensor is reduced by means of the Rseries described in
Section 4.4 on page
low-pass filter for incoming RF signals; the roll-off frequency of this network is defined by:
If for example Cs = 22 nF, and R
than any credible external noise source (except for mains frequencies). However, Rseries and
Cs must both be placed very close to the body of the IC so that the lead lengths between them
and the IC do not form an unfiltered antenna at very high frequencies.
14. The Cs capacitor and Rseries (see
series
= 10 k, the rolloff frequency to EMI is 723Hz, vastly lower
F
R
=
------------------------------
2R
series
1
Cs
Figure 1-1 on page
Figure 1-1 on page
3) form a natural
9525B–AT42–09/10
3)