Application Notes—Analog Optical Isolators
APPLICATION NOTE #1 Audio Applications
The LDR output element of AOIs is almost purely resistive in nature.
This property makes the AOI a very useful device for the control of AC
signals. Further, because AOIs also possess very low noise and low
harmonic distortion characteristics, they are ideal for use as variable
resistors, capable of being remotely adjusted in a wide range of audio
applications and control circuits.
The focus of this note is on the use of AOIs in audio applications.
However, many of the approaches used are equally applicable to
higher frequency AC amplification and control circuits.
resistor, the feedback may approach an open circuit condition at
maximum gain. In this open loop state, the circuit becomes unstable
and may latch up. The parallel resistor R
3
sets the maximum gain of
the amplifier and stabilizes the DC output voltage. Resistor R
2
is in
series with the AOI output and sets the minimum gain of the circuit. For
op-amps with unity gain compensation, R
2
is set equal to R
3
so the
circuit gain does not drop below one. The maximum voltage on the cell
(LDR) is e
out
. If minimizing distortion is a consideration, e
out
should be
kept below 1.0V.
Op-Amp Input Resistor Control
When the AOI is used as the input resistor of an op-amp, Figure 1d, a
fixed resistor in series will limit the maximum gain as well as prevent
overload of the previous stage.
Control Circuits
Voltage Divider Circuits
The output element of the AOI is a two terminal variable resistor and
may be used in a voltage divider circuit as shown in Figures 1a and 1b.
Non-Inverting Op-Amp Circuits
The AOI can also be used in non-inverting op-amp circuits. Gain is
controlled potentiometrically and, again, resistors should be used to
limit the maximum gain. The circuit of Figure 1e requires a resistor in
series with the AOI, while the circuit of Figure 1f requires one in
parallel.
Shunt Input Control
Figure 1a shows the AOI as the shunt element. With I
F
= 0, the
photocell has a very high resistance so e
out
= e
in
. When I
F
is injected
into the LED, the AOI output resistance decreases pulling down the
output voltage. Since the cell cannot be driven to zero resistance, the
value of R
1
must be selected to give the desired maximum attenuation.
A VTL5C4 with a maximum “on” resistance of 200 ohms at I
F
= 10 mA
requires an R
1
of 6100 ohms for 30 db voltage attenuation (producing
a 1000:1 power ratio). The actual attenuation ratio will be greater since
the 10 mA “on” resistance is typically 125 ohms.
When the maximum I
F
is less than 10 mA, the series resistance must
be greater to get the same attenuation ratio. If R
1
is made large, the
insertion loss (db attenuation at I
F
= 0) will be higher when the output is
loaded. The maximum voltage across the photocell in this circuit is
equal to the input voltage assuming no insertion loss. An input voltage
as high as 5 – 10V will produce noticeable distortion but that will drop
as I
F
is increased. To minimize distortion, the voltage across the cell
should be kept below 1.0V at the normal operating point.
General Considerations
The circuit application and AOI characteristics will influence the choice
of circuit to use. In Figure 1a to 1f, gain vs. I
F
curves are given for each
circuit, as well as input impedance and gain formulas. Once the proper
circuit function is selected, AOI response speed must be considered.
Because an LDR (photocell) turns “on” fast and “off” slowly, circuits of
Figure 1d and 1e will increase in gain rapidly but be slower in the
decreasing gain. The circuits of Figure 1c and 1f respond faster when
the gain is reduced. All other design considerations are the same as
they would be for any op-amp circuit. In all the amplifier configurations,
a gain ratio of 1000:1 or higher can be achieved.
Series Input Control
With an AOI as the series element as shown in Figure 1b, e
out
= 0 at I
F
= 0. The maximum voltage across the cell is e
in
, but decreases as I
F
increases.
Op-Amp Feedback Resistor Control
The AOI may also be used as the input or feedback resistor of an
operational amplifier. When used in the feedback loop, Figure 1c, a
fixed resistor should be used in parallel. With no parallel limiting
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