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

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MC33794

Electric Field Imaging Device

Motorola => Freescale 

SOICW-EP PIN FUNCTION DESCRIPTION (continued)

Pin

Pin Name

Formal Name

Definition

10

DIS_SHIELD

Shield Driver

This pin is used to turn off the shield signal. The purpose of doing this is to be able

to detect that the shield signal is not working or the connection to the coax shields is

broken. If either of these conditions exists, there will be little or no change in the

capacitance measured when the DIS_SHIELD is asserted. If the SHIELD output is

working and properly connected, the capacitance of the coax will not be cancelled

when this pin is asserted and the measured capacitance will appear to change by

approximately the capacitance between the center conductor and the shield in the

coax.

14 – 11

A, B, C, D

Selector Inputs

These input pins control which electrode or reference is active. Selection values are

shown in Table 1, Electrode Selection, page 14.

15

SIGNAL

Undetected Signal

This is the undetected signal being applied to the detector. It has a DC level with the

low radio frequency signal superimposed on it. Care must be taken to minimize DC

loading of this signal. A shift of DC will change the center point of the signal and

adversely affect the detection of the signal.

16

LEVEL

Detected Level

This is the detected, amplified, and offset representation of the signal voltage on the

selected electrode. Filtering of the rectified signal is performed by a capacitor

attached to LP_CAP.

17

PWR_MON

Power Monitor

This is connected through a voltage divider to VPWR. It allows reduction of the

voltage so it will fall within the range of the ADC on the MCU.

18

LP_CAP

Low-Pass Filter Capacitor A capacitor on this pin forms a low pass filter with the internal series resistance from

the detector to this pin. This pin can be used to determine the detected level before

amplification or offset is applied. A 10 nF capacitor connected to this pin will smooth

the rectified signal. More capacitance will increase the response time unnecessarily.

19

R_OSC

Oscillator Resistor

A resistor from this pin to circuit ground determines the operating frequency of the

oscillator. The 33794 is optimized for operation around 120 kHz.

24

CLK

Clock

This pin provides a square wave output at the same frequency as the internal

oscillator. The edges of the square wave coincide with the peaks (positive and

negative) of the sine wave.

25

VDD_MON

VDD Monitor

This is connected through a voltage divider to VDD. It allows reduction of the voltage

so it will fall within the range of the ADC on the MCU.

26

VDD

VDD Capacitor

A capacitor is connected to this pin to filter the internal analog regulated supply. This

supply is derived from VPWR.

27

VPWR

Positive Power Supply

12 V power applied to this pin will be converted to the regulated voltages needed to

operate the part. It is also converted to 5.0 V (VCC) and 8.5 V (VDD) to power the

MCU and external devices.

28

VCC

5.0 V Regulator Output This output pin requires a 47 µF capacitor and provides a regulated 5.0 V for the

MCU and for internal needs of the 33794.

29

AGND

Analog Ground

This pin is connected to the ground return of the analog circuitry. This ground should

be kept free of transient electrical noise like that from logic switching. Its path to the

electrical current return point should be kept separate from the return for GND.

30

SHIELD

Shield Driver

This pin connects to cable shields to cancel cable capacitance.

32

GND

Ground

This pin and metal backing is the IC power return and thermal radiator/conductor.

35

TEST

Test Mode Control

This pin is normally connected to circuit ground. There are special operating modes

associated with this pin when it is not at ground.

MOTOROLA ANALOG INTEGRATED CIRCUIT DEVICE DATA

33794

7

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