AN912 Datasheet, PDF (7/16 Pages) Microchip Technology – Designing LF Talkback for a Magnetic Base Station

DATA SLICER
The data slicer is essentially a comparator with some
input hysteresis voltage to reduce the influence of
noise. The overall system gain of the decoupler and the
low-pass filter, at 2.5 kHz, is about 29 dB or a factor of
28, and the system should be able to detect the 2 mV
data signal. The headroom between the hysteresis and
data signal was chosen to be about 9 dB or a factor of
2.8. This means that the minimum input voltage to
overcome the data slicer hysteresis is about 700 µV.
This translates to 20 mV of hysteresis for the data
slicer. Most comparators have some deliberate hyster-
esis to improve noise stability and this amount should
be extracted from the required hysteresis when calcu-
lating the amount of required feedback. Figure 9 shows
a typical hysteresis circuit and Equation 5 can be used
to calculate the amount of hysteresis for a single-ended
circuit.
EQUATION 5:
VHYST =
R1
VDD
R2
FIGURE 9:
Input
VREF
-
+
R1
R2
Output
For example, if a comparator with 10 mV of offset and
hysteresis is used, then an additional 10 mV of hyster-
esis should be added. The resistor R2 is calculated to
be 5 MΩ for a VDD of 5 VDC and R1 = 10 kΩ.
AN912
AN EXAMPLE SYSTEM
A complete circuit with layout, based on the foregoing
design study, is shown in this section. The circuit
diagram is shown in Figure 11. The top and bottom
layout for the printed circuit board is shown in
Figure 12. The PIC16F648A was chosen for the
application, it has two comparators, a USART,
EEPROM and 4k of Flash program memory. The
PIC16F648A can be substituted with its smaller
program memory equivalents, the PIC16F627A or
PIC16F628A. The filter examples have been converted
to operate from a single 5 VDC supply. The 2.5 VDC
virtual ground is provided by the voltage divider
consisting of R23 and R24, shown in Figure 11. The
Reference voltage does not have to be actively
buffered, it is lightly loaded. A 0.1 µF decoupling
capacitor C10 is sufficient for noise reduction.
A TC4422 FET driver, U1, drives the resonant tank
consisting of L1 and C2. The tank generates a strong
magnetic field and the voltage at the test pin TP1 can
reach 320V peak-to-peak. The main antenna, L1, is an
air-cored inductor with a 25 mm radius and 41 turns of
26-gage wire, and has a 162 µH inductance. The
inductor L2 and capacitors C3 and C4 are not popu-
lated and are added to the printed circuit board to test
alternative antennas. The peak detector consists of D1,
C5, R1, and R2, and is connected to the decoupling
stage via C6. The RC time constant of the decoupling
tank is set by C6 and R4 to 177 µs, which is substan-
tially longer than the minimum filter requirement of
64 µs. Resistor R3 is used to change the decoupler's
time constant to 11 µs by changing RB7 from a high-
impedance input to an output.
The decoupler buffer, U2:A, has a gain of 6 dB and a
low pass cutoff frequency at 9.8 kHz, set by R5 and C8.
The R22 resistor is used to ensure the proper DC bias
of the stage, but does not have a significant effect on
the overall sensitivity. The output of the decoupler is
connected to the input of the low pass Bessel filter and
one of the PIC16F648's comparators. The remaining
op amp, U2:B, is used for the Bessel filter. U2 is a dual
MCP6002 op amp that has a GBWP of 1 MHz. The
filter components should have better tolerances than
the high voltage components and 1% resistors. The 5%
NP0 capacitors are recommended.
 2004 Microchip Technology Inc.
DS00912A-page 7