any adjacent trace or ground plane next to or under either
SNS trace will cause an increase in Cx load and desensitize
the device.
For proper operation a 100nF (0.1uF) ceramic bypass
capacitor must be used directly between Vdd and Vss;
the bypass cap should be placed very close to the
device’s power pins.
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 QT113 and render it
unstable or diminish gain.
3.6 EMC ISSUES
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 above in Section 3.5. The Cs
capacitor and Rseries (see Figure 1-1) form a natural
low-pass filter for incoming RF signals; the roll-off frequency
of this network is defined by -
1
F
R
=
2✜R
series
C
s
If for example Cs = 22nF, and Rseries = 10K ohms, the rolloff
frequency to EMI is 723Hz, vastly lower 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.
3.5 ESD PROTECTION
The QT113 includes internal diode protection on its pins to
absorb and protect the device from most induced discharges,
up to 20mA. 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 10kV per mm thickness.
ESD protection can be enhanced by adding a series resistor
Rseries (see Figure 1-1) in line with the electrode, of value
between 1K and 50K ohms. 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 instabilties.
Rseries and Cs should both be placed very close to the
chip.
lQ
7
R1.05/0405
QUANTUM [ QUANTUM RESEARCH GROUP ]
