VT33CT datasheet(71/76 Pages) PERKINELMER

Part #	VT33CT

Description	Photoconductive Cells and Analog Optoisolators (Vactrols)

PDF	76 Pages

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Manufacturer	PERKINELMER [PerkinElmer Optoelectronics]

Direct Link	http://www.perkinelmer.com

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71 / 76 page

Application Notes—Analog Optical Isolators

The control loop consists of op-amp A2 and resistors R1 and R4. This

circuit sets the LED current so that:

If we set:

R3 = R4

and:

R1 = R2

then:

eout / ein = VREF / VC

or:

eout = ein (VREF / VC)

where

VC = control voltage

Note that R1 and R2 are the two halves of the cell. These two resistors

match within 10% and track over a wide range within 5% so that the

gain is closely set by VC when VREF is fixed.

The limits of operation are:

0 < VC < VREF

and the signal must never be so large that amplifier A1 saturates when

the gain is at maximum.

This circuit performs a dividing operation with ein and VC as the

numerator and denominator respectively. The accuracy is limited by

the tracking ability of the two sides of the photocell. The error due to

matching can be eliminated by trimming R4.

Figure 10. Electrically Programmable Gain

REF

-----------

()

----------------------

out

--------

REF

-----------

Gain

VT33CT Datasheet(PDF) 71 Page - PerkinElmer Optoelectronics