Avago Technologies ACPL-224-500E - Manual (Page 40)

AV02-4387EN 37 Avago Technologies
Analog Isolation Applications
Figure 1. HCNR-200/1 High Linearity Analog
Isolator
Introduction
Optoisolators transfer analog and
digital signals from one circuit
section or module to another in the
presence of a large potential dif-
ference or induced electrical noise
between the ground or common
points of these modules. Examples
of analog isolation applications
are interfaces to: A/D converters,
sensing circuits such as thermocou-
ples and transducers, patient moni-
toring equipment, motor speed
and position measurement circuits,
audio and video amplifiers, and
power supply feedback.
Basic Building Blocks for Analog
Isolation
Avago Technologies’ HCNR200/1 and
HCPL4562 constitute basic optical cou-
pling building blocks for high linearity
isolation applications. Figures 1 and 2
show the respective optical coupling
mechanisms for these two optoisola-
tors. Both these isolators use high-per-
formance AlGaAs LEDs and photodiode
combinations with higher speed and
linearity compared to conventional op-
toisolators. The HCNR200/1 LED illumi-
nates two closely matched photodiodes,
one on the input side, and another on
the output side. With a suitable appli-
cations circuit for the HCNR200/1, the
nonlinearity and drift characteristics of
the LED can be virtually eliminated. The
output photodiode produces a photo-
current that is linearly related to the light
output of the LED. The close matching of
the photodiodes and advanced design
of the package ensure the high linear-
ity and stable gain characteristics of the
optoisolator.
Figure 2. HCPL-4562 High Bandwidth Analog
Isolator
Figure 3. HCPL-4562 Base Photo Current vs.
Input Current
The HCNR200/1 optoisolator can be used
as a basic analog isolation building block
for a wide variety of applications that
require good stability, linearity, band-
width and low cost. The HCNR200/1 is
very flexible and, by appropriate design
of the application circuit, is capable of
operating in many different modes, in-
cluding unipolar, bipolar, ac/dc, invert-
ing and non-inverting.
The HCPL-4562 and HCNW4562 are
recommended for very high band-
width (up to 15 MHz) AC analog
designs. If the output transistor
is biased in the active region, the
current transfer ratio relationship for
the HCPL-4562 can be represented
as:
I
C
= K ( I
F
/ I
FQ
) n
where I
C
is the collector current; I
F
is the LED input current, I
FQ
is LED
input current at which K is meas-
ured; K is the collector current when
I
F
= I
FQ
; and n is the slope of I
C
vs. I
F
on logarithmic scale.
The exponent n varies with I
F
, but
over some limited range of DI
F
, n can
be regarded as a constant. For ac-
signal applications, the HCPL-4562
can be biased at an appropriate
quiescent current where the ratio of
the incremental photodiode current
to incremental LED current is nearly
constant. Figure 3 shows the linear-
ity characteristics of the HCPL-4562.
8
7
6
1
3
HCNR200/1
5
2
4
LED
NC
NC
PD1 PD2
K1 K2
8 V
CC
7 V
B
6 V
O
5 GND
NC
ANODE
CATHODE
NC
1
2
3
4
80
70
60
50
40
30
20
10
0
0 2 0 18 16 14 12 10 8 6 4 2
I F - INPUT CURRENT - mA
I PB
- BASE PHOTO CURRENT -
µA
T A = 25 °C
V PB > 5 V
HCPL-4562