MCP2035-ISLASHST datasheet(33/68 Pages) MICROCHIP

33 / 68 page

 2012 Microchip Technology Inc.

DS22304A-page 33

MCP2035

5.0

FUNCTIONAL DESCRIPTION

AND THEORY OF DEVICE

OPERATION

The MCP2035 contains an analog input channel for

signal detection and LF talk-back. This section

provides the functional description of the device.

The input channel has internal tuning capacitors,

sensitivity control circuits, an input signal strength

limiter and an LF talk-back modulation transistor. An

AGC loop is used for input channel gains. The output of

the input channel is 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 nine bits including

one row parity bit. These registers are readable and

writable by SPI commands, except for the STATUS

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 VDE_Q, progressively de-Q’ing

harder to reduce the antenna input voltage.

5.2

Modulation Circuit

for LF Talk-Back

The LF talk-back is achieved by turning on and off the

modulation transistor. 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 LCX input pin and LCCOM

pin. Each LC input has its own modulation transistor.

When the modulation transistor turns on, its low

Turn-on Resistance (RM) 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 (RM)

results in a high modulation depth.

The modulation data comes from the external micro-

controller section via the digital SPI as “Clamp On”,

“Clamp Off” commands. 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

The input tuning capacitor values are programmed by

the Configuration registers up to 63 pF, 1 pF per step.

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.

5.5

Sensitivity Control

The sensitivity of the input channel can be reduced by

the Configuration register sensitivity setting. This is

used to desensitize the channel from optimum.

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”

5.7

Fixed Gain Amplifiers 1 and 2

FGA1 and FGA2 provide a maximum two-stage gain

of 40 dB.

Note:

The LF-Talk back is only used when it

needs to communicate back to the Base

Station using the same Base Station’s low

frequency (128 kHz) carrier frequency. A

typically LF-Talk back range is up to a few

inches. For the BodyCom applications, it

uses HF (~8 MHz) for the return signal.

Note:

The

user

can

control

the

tuning

capacitor

programming

the

Configuration registers. See Register 5-2

for details.

Note:

The variable attenuator function is

accomplished by the device itself. The

user cannot control its function.

Note:

The user can desensitize the channel

sensitivity

programming

the

Configuration registers. See Register 5-5

for details.

Note:

The AGC control function is accomplished

by the device itself. The user cannot

control its function.

Note:

The user cannot control the gain of these

two amplifiers.

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