MCP2030_13 Datasheet, PDF (31/66 Pages) Microchip Technology – Three-Channel Analog Front-End Device

5.0 FUNCTIONAL DESCRIPTION
AND THEORY OF DEVICE
OPERATION
The MCP2030 contains three analog input channels for
signal detection and LF talk-back. This section
provides the function description of the device.
Each analog input channel has internal tuning
capacitors, sensitivity control circuits, an input signal
strength limiter and an LF talk-back modulation
transistor. An Automatic Gain Control (AGC) loop is
used for all three input channel gains. The output of
each channel is OR’d and fed into a demodulator. The
digital output is passed to the LFDATA pin. Figure 5-1
shows the block diagram of the device and Figure 5-2
shows the input signal path.
There are a total of eight Configuration registers. Six of
them are used for device operation options, one for
column parity bits and one for status indication of
device operation. Each register has 9 bits including one
row parity bit. These registers are readable and
writable by SPI commands except for the STATUS
register, which is read-only.
The device’s features are dynamically controllable by
programming the Configuration registers.
5.1 RF Limiter
The RF Limiter limits LC pin input voltage by de-Q’ing
the external LC resonant antenna circuit. The limiter
begins de-Q’ing the external LC antenna when the
input voltage exceeds V_{DE_Q}, progressively de-Q’ing
harder to reduce the antenna input voltage.
The signal levels from all 3 channels are combined
such that the limiter attenuates all 3 channels
uniformly, in respect to the channel with the strongest
signal.
5.2 Modulation Circuit
The modulation circuit consists of a modulation
transistor (FET), internal tuning capacitors and external
LC antenna components. The modulation transistor
and the internal tuning capacitors are connected
between the LC input pin and LCCOM pin. Each LC
input has its own modulation transistor.
When the modulation transistor turns on, its low Turn-
on Resistance (R_M) clamps the induced LC antenna
voltage. The coil voltage is minimized when the
modulation transistor turns-on and maximized when
the modulation transistor turns-off. The modulation
transistor’s low turn-on resistance (R_M) results in a high
modulation depth.
The LF talk-back is achieved by turning on and off the
modulation transistor.
MCP2030
The modulation data comes from the external micro-
controller section via the digital SPI as “Clamp On”,
“Clamp Off” commands. Only those inputs that are
enabled will execute the Clamp command. A basic
block diagram of the modulation circuit is shown in
Figure 5-1 and Figure 5-2.
The modulation FET is also shorted momentarily after
Soft Reset and Inactivity timer time-out.
5.3 Tuning Capacitor
Each channel has internal tuning capacitors for external
antenna tuning. The capacitor values are programmed
by the Configuration registers up to 63 pF, 1 pF per step.
Note:
The user can control the tuning capaci-
tor by programming the Configuration
registers. See Register 5-2 through
Register 5-4 for details.
5.4 Variable Attenuator
The variable attenuator is used to attenuate, via AGC
control, the input signal voltage to avoid saturating the
amplifiers and demodulators.
Note:
The variable attenuator function is
accomplished by the device itself. The
user cannot control its function.
5.5 Sensitivity Control
The sensitivity of each channel can be reduced by the
channel’s Configuration register sensitivity setting.
This is used to desensitize the channel from optimum.
Note:
The user can desensitize the channel
sensitivity by programming the
Configuration registers. See Register 5-5
and Register 5-6 for details.
5.6 AGC Control
The AGC controls the variable attenuator to limit the
internal signal voltage to avoid saturation of internal
amplifiers and demodulators (Refer to Section 5.4
“Variable Attenuator”).
The signal levels from all 3 channels are combined
such that the AGC attenuates all 3 channels uniformly
in respect to the channel with the strongest signal.
Note:
The AGC control function is accomplished
by the device itself. The user cannot
control its function.
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DS21981B-page 31