VT33CT datasheet(62/76 Pages) PERKINELMER

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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.

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.

Shunt Input Control

Figure 1a shows the AOI as the shunt element. With IF = 0, the

photocell has a very high resistance so eout = ein. When IF 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 R1 must be selected to give the desired maximum attenuation.

A VTL5C4 with a maximum “on” resistance of 200 ohms at IF = 10 mA

requires an R1 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 IF is less than 10 mA, the series resistance must

be greater to get the same attenuation ratio. If R1 is made large, the

insertion loss (db attenuation at IF = 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 IF is increased. To minimize distortion, the voltage across the cell

should be kept below 1.0V at the normal operating point.

Series Input Control

With an AOI as the series element as shown in Figure 1b, eout =0 at IF

= 0. The maximum voltage across the cell is ein, but decreases as IF

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

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 R3 sets the maximum gain of

the amplifier and stabilizes the DC output voltage. Resistor R2 is in

series with the AOI output and sets the minimum gain of the circuit. For

op-amps with unity gain compensation, R2 is set equal to R3 so the

circuit gain does not drop below one. The maximum voltage on the cell

(LDR) is eout. If minimizing distortion is a consideration, eout 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.

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.

General Considerations

The circuit application and AOI characteristics will influence the choice

of circuit to use. In Figure 1a to 1f, gain vs. IF 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.

VT33CT Datasheet(PDF) 62 Page - PerkinElmer Optoelectronics

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