Avago Technologies ACPL-224-500E - Manual (Page 48)

AV02-4387EN 45 Avago Technologies
Isolated Current and Voltage Sens-
ing
Avago Technologies provides several
analog isolation amplifier prod-
ucts for isolated precision current
and voltage sensing for monitor-
ing motor performance. Avago
also offers a digital output isolation
amplifier for direct connection to
a micro-controller. Compared to
Hall-Effect sensors, Avago’s isolation
amplifiers have excellent gain and
offset characteristics, especially very
low offset drift over a wide tempera-
ture range. In addition, they provide
a compact, low-cost, and reliable so-
lution for motor drive designers that
need to accurately measure motor
voltage and current. Some of the key
performance features include:
• 12­bit linearity (HCPL­7860)
• over current detection (HCPL­
788J)
• ±200 mV input range
• ­40°C to +85°C operating tem-
perature range
• 15 kV/ µs isolation transient im-
munity
• Safety Standards Certifications:
UL, CSA, IEC/EN/DIN EN 60747-5-
2
Analog Signal Isolation for Speed
and Position Measurement
Avago Technologies offers a low
cost, high accuracy, isolated ampli-
fier solution for the measurement
of speed and position in a motor.
With the use of Avago Technologies’
HCNR201 and low cost operational
amplifiers, circuit designers have
the flexibility of designing analog
isolation amplifiers that have low
input bias currents, high bandwidth,
stable gain, and very high linearity.
Common Mode Rejection
The detector shield in the opto-
coupler prevents CMR failure due
to capacitive coupling from the
input side of the optocoupler,
through the package, to the detec-
tor IC. However, this shield does not
eliminate the capacitive coupling
between the LED and the optocou-
pler output pins and output ground
as shown in Figure 2. This capacitive
coupling causes
Figure 2. AC Equivalent for a Digital Optocoupler
perturbations in the LED current
during common mode transients
and becomes the major source of
CMR failures for a shielded optocou-
pler. The main design objective of a
high CMR LED drive circuit becomes
keeping the LED in the proper state
(on or off) during common mode
transients. The recommended
application circuits for Avago’s gate
drive applications can achieve 15
kV/ µs CMR while minimizing com-
ponent complexity.
Another cause of CMR failure for a
shielded optocoupler is direct cou-
pling to the optocoupler output
pins through C
LED01
and C
LED02
in
Figure 2. Many factors influence the
effect and magnitude of the direct
coupling including: the use of an
internal or external output pull-up
resistor, the position of the LED
current setting resistor, the connec-
tion of the unused input package
pins, and the value of the capaci-
tor at the optocoupler output (CL).
The recommended gate drive and
IPM interface application circuits for
Avago optocouplers have been op-
timized to reduce the likelihood of
CMR failure.
Motor Control Applications
1
2
SHIELD
8
7
6
5
3
4
20 k Ω
C LEDP
C LEDN
C LED02
C LED01