Application Note P4097
© TeraTron GmbH
Page 10 of 21
V1.1, 07.06.01
150nF
CF
10nF
DEMOD_IN
VSS
from Ant.
CAGND
GND
100nF
1.5nF
DVSS
CDC
Figure 5.: Ground signal layout
The buffer capacitors should be located close between the DV
DD
and DV
SS
pins. Any voltage drop due
to copper resistance or track inductance will be seen via the antenna on the DEMOD_IN pin and will
be demodulated. This should be regarded for the larger electrolytic capacitor as well as for the
ceramic capacitor. Using two sets of capacitors makes it easier to place and route them on the circuit
board. One set is buffering the high current supply DV
DD
and DV
SS
, whereas the other capacitors (or
just the ceramic one) buffers the analog supply V
DD
and V
SS
.
The signal DEMOD_IN is very susceptible against capacitive coupling of noisy tracks. The capacitive
voltage divider should be located close to the input pin and connected to a proper analog ground. A
ripple voltage or a voltage drop relative to V
SS
is coupled into the DEMOD_IN pin due to the capacitor
ratio much stronger than the useful signal coming from the antenna.
The CDEC capacitor which connects the sampler with the filter is susceptible against capacitive
coupling of noise. The capacitor should be located close to the chip and the tracks should be short
and not close to other traces with fast changing voltage levels.
The antenna connection, if an external antenna is used, should be bypassed with two small ceramic
capacitors to ground close to the connector. This suppresses high frequency voltages to ground which
are picked up by the wiring harness and reduces the radiation out of the circuit into the wiring harness.
Other connector pins which are connected to a wiring harness should be treated similar. The ground
plane to which these capacitors are connected should be routed carefully to achieve a low impedance
for high frequencies.
Unused pins except the CF pin in external clock mode can be left open as the inputs are internally
pulled to ground and outputs are push-pull types without tri-state mode and therfore defined voltage
levels.
3.8 EMI Filter
In applications where the functionality even under the influence of strong electromagnetic fields is
required, additional filter circuitry for connecting the antenna coil with the P4097 is recommended. The
filter shown below suppresses high frequent voltages which could be picked up by the antenna cable
or the antenna itself. Because of the –60dB level of the useful transponder signal in relation to the
125kHz carrier frequency the communication is by nature susceptible against electromagnetic
interference.
