AN1857 Freescale Semiconductor / Motorola, AN1857 Datasheet - Page 19

• Manufacturer Part Number: AN1857
• Description: A 3-Phase ac Induction Motor Control System Based on the MC68HC908MR32
• Manufacturer: Freescale Semiconductor / Motorola

AN1857
MOTOROLA
The analog isolation block is based on Agilent Technologies’ HCNR201
high-linearity optocoupler. The HCNR201 consists of an LED and two
photodiodes. The LED and one of the photodiodes (PD1) is on the input
side of the optoisolation barrier, and the other photodiode (PD2) is on the
output side. The package is constructed so that each photodiode
receives approximately the same amount of light from the LED.
Feedback amplifier, A1, is configured with PD1 to monitor the light
output of the LED and automatically adjust LED current to compensate
for any non-linearity. The output photodiode then converts a stable,
linear light output of the LED into a current, which is then converted back
into a voltage by amplifier A2.
Circuit operation may not be immediately obvious from inspecting
Figure
amplifier A1 adjusts LED forward current (I
(I
Analysis of the input circuit reveals that increasing the input voltage
increases the voltage at the inverting input terminal of A1. Amplifier A1
amplifies that increase, causing I
that PD1 is connected, I
back toward ground. A1 will continue to increase I
input voltage stabilizes near its ground reference voltage. Assuming that
no current flows into the inputs of A1, all of the current flowing through
R1 will flow through PD1. Since the inverting input of A1 is at
approximately 0 volts, the current through R1, and therefore I
equal to V
I
value of R1 and is independent of the optocoupler’s characteristics. Also
note that I
relationship between the input voltage and the photodiode current.
The physical construction of the optocoupler’s package determines the
relative amounts of light that fall on the two photodiodes and, therefore,
the ratio of the photodiode currents. This results in a current, I
is very nearly equal to I
resistance amplifier that converts I
V
expression that relates the output voltage to the input voltage,
V
matches the input.
PD1
PD1
Out
Out
Freescale Semiconductor, Inc.
/V
= –V
) is equal to V
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= I
In
10, particularly the input part of the circuit. Stated briefly,
PD2
= (R3/R1). Therefore, with R1 = R3, the output signal closely
In
PD1
In
/R1. Note that I
* R3. Combining input and output equations results in an
/R1. Essentially, amplifier A1 adjusts I
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is directly proportional to V
In
/R1.
PD1.
PD1
PD1
Amplifier A2 and resistor R3 form a trans-
will pull the inverting input of the op-amp
depends only on the input voltage and the
F
PD2
and I
back into a voltage, V
PD1
In
F
, giving a very linear
) such that the current in PD1
to increase. Given the way
F
F
until its inverting
such that
System Hardware
Application Note
Out
PD1,
PD2,
, where
is
that
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