NONLINEARITY
DEFINITIONS
ISOLATION-MODE VOLTAGE, VISO
Nonlinearity is specified to be the peak deviation from a best
straightline expressed as a percent of peak-to-peak full scale
output (i.e. ±10mV at 20Vp-p ≈ 0.05%).
The isolation-mode voltage is the voltage which appears
across the isolation barrier, i.e., between the input common
and the output common. (See Figure 1.)
Two isolation voltages are given in the electrical specifica-
tions: “rated continuous” and “test voltage”. Since it is
impractical on a production basis to test a “continuous”
voltage (infinite test time is implied), it is a generally
accepted practice to test at a significantly higher voltage for
some reasonable length of time. For the 3650 and 3652, the
“test voltage” is equal to 1000V plus two times the “rated
continuous” voltage. Thus, for a continuous rating of 2000V,
each unit is tested at 5000V.
THEORY OF OPERATION
Prior to the introduction of the 3650 family optical isolation
had not been practical in linear circuits. A single LED and
photodiode combination, while useful in a wide range of
digital isolation applications, has fundamental limitations—
primarily nonlinearity and instability as a function of time
and temperature.
The 3650 and 3652 use a unique technique to overcome the
limitations of the single LED and photodiode isolator.
Figure 2 is an elementary equivalent circuit for the 3650,
which can be used to understand the basic operation without
considering the cluttering details of offset adjustment and
biasing for bipolar operation.
COMMON-MODE VOLTAGE, VCM
The common-mode voltage is the voltage midway between
the two inputs of the amplifier measured with respect to
input common. It is the algebraic average of the voltage
applied at the amplifiers’ input terminals. In the circuit in
Figure 1, (V+ + V–)/2 = VCM. (NOTE: Many applications
involve a large system “common-mode voltage.” Usually in
such cases the term defined here as “VCM” is negligible and
the system “common-mode voltage” is applied to the ampli-
fier as “VISO” in Figure 1.)
Isolation Barrier
RK
CR3
+V
CR1
CR2
I1
I2
–
+
+VCC
λ1
λ2
RG
–
+
I2
A2
ISOLATION-MODE REJECTION
A1
IIN
VIN
I3
The isolation-mode rejection is defined by the equation in
Figure 1. The isolation-mode rejection is not infinite be-
cause there is some leakage across the isolation barrier due
to the isolation resistance and capacitance.
VOUT
–V
–VCC
Output Common
Input Common
RK
VOUT = VIN
RG
Isolation Barrier
RG1
FIGURE 2. Simplified Equivalent Circuit of Linear Isolator.
+
V+
Two matched photodiodes are used—one in the input (CR3)
and one in the output stage (CR2)—to greatly reduce
nonlinearities and time-temperature instabilities. Amplifier
A1, LED CR1, and photodiode CR3 are used in a negative
feedback configuration such that I1 = IIN RG (where RG is the
user supplied gain setting resistor). Since CR2 and CR3 are
closely matched, and since they receive equal amounts of
light from the LED CR1 (i.e., λ1 = λ2), I2 = I1 = IIN. Amplifier
A2 is connected as a current-to-voltage converter with VOUT
= I2 RK where RK is an internal 1MΩ scaling resistor. Thus
the overall transfer function is:
RIN
VD
RG2
+
V–
VOUT
–
–
IL
VCM
C
C
(Input)
(Output)
VISO
System
Ground
106
106
RG1 + RG2 + RIN
VCM
VISO
VOUT = VIN
, (RG in Ωs)
VOUT
=
VD
+
+
RG
CMRR
IMRR
This improved isolator circuit overcomes the primary
limitations of the single LED and photodiode combination.
The transfer function is now virtually independent of any
degradation in the LED output as long as the two photo-
diodes and optics are closely matched(1). Linearity is now a
FIGURE 1. Illustration of Isolation-Mode and Common-
Mode Specifications.
NOTE: (1) The only effect of decreased LED output is a slight decrease in full
scale swing capability. See Typical Performance Curves.
®
6
3650/52
BB [ BURR-BROWN CORPORATION ]
