3055 Datasheet, PDF (6/8 Pages) Allegro MicroSystems – MULTIPLEXED TWO-WIRE HALL-EFFECT SENSOR ICs

3055
MULTIPLEXED
TWO-WIRE HALL EFFECT
SENSOR IC
FIGURE 3
BUS INTERCONNECTION
ADDRESS
RESET
ANALOG OUT
UGN3055U AND UGS3055U
M^(PU^OSL^ITIT^VEI^{) B}P^USL^SE^UPX^PLE^Y D TWO-WIRE HALL EF-
FECT SENSOR IC
01
02
28
29
30
BUS RETURN
Dwg. EH-005
Bus Configuration
A maximum of 30 sensors may be connected
across the same two wire bus as shown in
Figure 3. It is recommended that the sensors
use a dedicated digital ground wire to mini-
mize the effects of changing ground potential
(as in the case of chassis ground in the
automotive industry).
The bus was not designed to require two-wire
twisted-pair wiring to the sensors; however,
in areas of extreme EMI (electro-magnetic
interference), it may be advisable to install a
small bypass capacitor (0.01 µF for example)
between the supply and ground terminals of
each sensor instead of using the more
expensive wiring.
Bus Driver
It is recommended that the bus be controlled
by microprocessor-based hardware for the
following reasons:
• Sensor address information may be stored
in ROM in the form of a look up table.
• Bus faults can be pinpointed by the
microprocessor by comparing the diagnos-
tic response to the expected response in
the ROM look up table.
• The microprocessor, along with an A/D
converter, can also be used to self cali-
brate the quiescent currents in the bus and
hence be able to easily detect a signal
response.
• The microprocessor can also be used to filter out random line noise
by digitally filtering the bus responses.
• The microprocessor can easily keep track of the signal responses,
initiate the appropriate action; e.g., light a lamp, sound an alarm,
and also pinpoint the location of the signal.
Optimally, the microprocessor is used to control bus-driving circuitry
that will accept TTL level inputs to drive the bus and will return an
analog voltage representation of the bus current.
Interface Schematic
The bus driver is easily designed using a few operational amplifiers,
resistors, and transistors. Figure 4 shows a schematic of a recom-
mended bus driver circuit that is capable of providing 6 V to 9 V
transitions, resetting the bus, and providing an analog measurement of
the current for use by the A/D input of the microprocessor.
In Figure 4, the Address pin provides a TTL-compatible input that is
used to control the Bus supply. A HIGH (5 V) input switches Q1 ON
and sets the bus voltage to 6 V through the resistor divider R4, R5,
and the Zener Z1. A LOW input switches OFF Q2 and sets the bus
voltage to 9 V. This voltage is fed into the positive input of the opera-
tional amplifier OP1 and is buffered and made available at Bus Supply
(or sensor supply). Bus reset control is also available in the form of a
TTL-compatible input. When this input, which is marked Reset, is
HIGH, Q2 is switched ON and the positive input of the op amp is set to
the saturation voltage of the transistor (approximately 0 V). This resets
the bus.
A linear reading of the bus current is made possible by amplifying the
voltage generated across R6 (which is I_BUS * R6). The amplifier, OP2,
is a standard differential amplifier of gain R9/R7 (provided that R7 =
R8, R9 = R10). The gain of the total transimpedance amplifier is given
by:
V_OUT = I_BUS * R6 * R9/R7
This voltage is available at the terminal marked Analog Out.
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