MC33794 Просмотр технического описания (PDF) - Motorola => Freescale

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MC33794

Electric Field Imaging Device

Motorola => Freescale 

SYSTEM/APPLICATION INFORMATION

INTRODUCTION

The 33794 is intended for use in detecting objects using an

electric field. The IC generates a low radio frequency sine wave.

The frequency is set by an external resistor and is optimized for

120 kHz. The sine wave has very low harmonic content to

reduce potential interference at higher harmonically related

frequencies. The internal generator produces a nominal 5.0 V

peak-to-peak output that is passed through an internal resistor

of about 22 kΩ. An internal multiplexer routes the signal to one

of 11 pins under control of the ABCD input pins. A receiver

multiplexer simultaneously connected to the selected electrode

routes its signal to a detector, which converts the sine wave to

a DC level. This DC level is filtered by an external capacitor and

is multiplied and offset to increase sensitivity. All of the

unselected electrode outputs are grounded by the device. The

current flowing between the selected electrode and the other

grounded electrodes plus other grounded objects around the

electrode causes a voltage drop across the internal resistance.

Objects brought into or out of the electric field change the

current and resulting voltage at the IC pin, which in turn reduces

the voltage at LP_CAP and LEVEL.

A shield driver is included to minimize the effect of

capacitance caused by using coaxial cables to connect to

remote electrodes. By driving the coax shield with this signal,

the shield voltage follows that of the center conductor,

significantly reducing the effective capacitance of the coax and

maintaining sensitivity to the capacitance at the electrode.

The 33794 is made to work with and support a

microcontroller. It provides two voltage regulators, a power-on-

reset/out-of-range voltage detector, watchdog circuit, lamp

driver and sense circuit, and a physical layer ISO-9141

communications interface.

BLOCK DIAGRAM COMPONENTS

Refer to Figure 1, 33794 Internal Block Diagram, page 2, for

a graphic representation of the block diagram information in this

section.

OSC

This section generates a high purity sine wave. The center

frequency is controlled by a resistor attached to R_OSC. The

normal operating frequency is around 120 kHz. A square wave

version of the frequency output is available at CLK. Timing for

the power-on reset and watchdog (POR/WD) circuit are derived

from this oscillator’s frequency.

MUX OUT

This circuit directs the output of the sine wave to one of nine

possible electrode outputs or two reference pins. All unused

pins are automatically grounded. The selected output is

controlled by the ABCD inputs.

MUX IN

This circuit connects the selected electrode, reference, or

one of two internal nodes to an amplifier/detector. The selection

is controlled by the ABCD inputs and follows the driven

electrode/reference when one is selected.

RECT

The rectifier circuit detects the level from MUX IN by

offsetting the midpoint of the sine wave to zero volts and

inverting the waveform when it is below the midpoint. It is

important to avoid DC loading of the signal, which would cause

a shift in the midpoint voltage of the signal from the MUX IN.

LPF

The rectified sine wave is filtered by a low pass function in

the LPF formed by an internal resistance and an external

capacitance attached to LP_CAP. The nominal value of the

internal resistance is 50 kΩ. The value of the external capacitor

is selected to provide filtering of noise while still allowing the

desired settling time for the detector output. A 10 nF capacitor

would allow 99% settling in less than 5.0 ms.

GAIN and OFFSET

This circuit multiplies the detected and filtered signal by a

gain and offsets the result by a DC level. This results in an

output range that covers 1.0 V to 4.0 V for capacitive loading of

the field in the range of 10 pF to 100 pF. This allows higher

sensitivity for a digital-to-analog converter with a 0 V-to-5.0 V

input range.

ATTN

This circuit passes the undetected signal to SIGNAL for

external use.

LAMP CKT

This section controls the operation of the LAMP_OUT pin.

When LAMP_CTRL is asserted, LAMP_OUT is pulled to

LAMP_GND. If one side of an indicator lamp or LED (with

appropriate current setting resistor) is connected to a positive

voltage source and the other is connected to LAMP_OUT, and

LAMP_GND is connected to ground, the lamp will light. This

circuit provides current limiting to prevent damage to itself in the

case of a shorted lamp or during a high-surge condition typical

of an incandescent lamp burnout.

MOTOROLA ANALOG INTEGRATED CIRCUIT DEVICE DATA

33794

15

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