MC33794 查看數據表（PDF） - Motorola => Freescale

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MC33794

Electric Field Imaging Device

Motorola => Freescale 

ISO-9141

This circuit connects to an ISO-9141 bus to allow remote

communications. ISO_IN is data from the bus to the MCU and

ISO_OUT is data to drive onto the bus from the MCU.

POR/WD

This circuit is a combined power-on reset and watchdog

timer. The RST output is held low until a certain amount of time

after the VCC output has remained above a minimum operating

threshold. If VCC falls below the level at any time, RST is pulled

low again and held until the required time after VCC has returned

high. An overvoltage circuit is also included, which will force a

reset if VCC rises above a maximum voltage. The watchdog

function also can force RST low if too long an interval is allowed

to pass between positive transitions on WD_IN.

VCC REG

This circuit converts an unregulated voltage from VIN to a

regulated 5.0 V source, which is used internally and available

for other components requiring a regulated voltage source.

VDD REG

This is a regulator for analog devices that require more than

5.0 V. This is used by the device and some current is available

to operate op-amps and other devices. By having this higher

voltage available, some applications can avoid the need for a

rail-to-rail output amplifier and still achieve the 0 V-to-5.0 V

output for a digital-to-analog converter input. VDDMON is a

divided output from VDD, which allows a 0 V-to-5.0 V ADC to

measure VDD.

CONTROL LOGIC

This contains the logic that decodes and controls the MUXes

and some of the test modes.

APPLICATION INFORMATION

The 33794 is intended to be used where an object’s size and

proximity are to be determined. This is done by placing

electrodes in the area where the object will be. The proximity of

an object to an electrode can be determined by the increase in

effective capacitance as the object gets closer to the electrode

and modifies the electric field between the electrode and

surrounding electrically common objects. The shape and size of

an object can be determined by using multiple electrodes over

an area and observing the capacitance change on each of the

electrodes. Those that don’t change have nothing near them,

and those that do change have part of the object near them.

The voltage measured is an inverse function of the

capacitance between the electrode being measured and the

surrounding electrodes and other objects in the electric field

surrounding the electrode. Increasing capacitance results in

decreasing voltage. The value of series resistance (22 kΩ) was

chosen to provide a nearly linear relationship at 120 kHz over a

range of 10 pF to 100 pF.

The measured value will change with any change in

frequency, series resistance, driving voltage, or detector

sensitivity. These can change with temperature and time. The

proper use of REF_A and REF_B will allow much of the

changes to be compensated for.

A typical measurement algorithm would start by measuring

the voltage for two known value capacitors (attached to REF_A

and REF_B). The value of these capacitors would be chosen to

be near the minimum and maximum values of capacitance

expected to be seen at the electrodes. These reference

voltages and the known capacitance values are then used with

the electrode measurement voltage to determine the

capacitance seen by the electrode. This method can be used to

detect short- and long-term changes due to objects in the

electric field and significantly reduce the effect of temperature-

and time-induced changes.

The 33794 does not contain an ADC. It is intended to be

used with an MCU that contains one. Offset and gain have been

added to the 33794 to maximize the sensitivity over the range

of 0 pF to 100 pF. An 8-bit ADC can resolve around 0.4 pF of

change and a 10-bit converter around 0.1 pF. Higher resolution

results in more distant detection of smaller objects.

DC loading on the electrodes should be avoided. The signal

is generated with a DC offset that is more than half the peak-to-

peak level. This keeps the signal positive above ground at all

times. The detector uses this voltage level as the midpoint for

detection. All signals below this level are inverted and added to

all signals above this level. Loading of the DC level will cause

some of the positive half of the signal to be inverted and added

and will change the measurement.

If it is not possible to assure that the electrodes will always

have a high DC resistance to ground or a voltage source, a

series capacitor of about 10 nF should be connected between

the IC electrode pins and the electrodes.

33794

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MOTOROLA ANALOG INTEGRATED CIRCUIT DEVICE DATA

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