HCNR201 Datasheet HCNR201, HCNR201-550E, HCNR200-350E | P16-P18

Theory of Operation

Figure 1 illustrates how the HCNR200/201 high‑linearity

opto coup ler is congured. The basic optocoupler con‑

sists of an LED and two photodiodes. The LED and one of

the photodiodes (PD1) is on the input leadframe and the

other photodiode (PD2) is on the output leadframe. The

package of the optocoupler is constructed so that each

photo diode receives approxi mately the same amount of

light from the LED.

An external feedback amplier can be used with PD1 to

monitor the light output of the LED and automatically

adjust the LED current to compensate for any non‑linear‑

ities or changes in light output of the LED. The feedback

amplier acts to stabilize and linearize the light output

of the LED. The output photodiode then converts the

stable, linear light output of the LED into a current, which

can then be converted back into a voltage by another

amplier.

Figure 12a illustrates the basic circuit topology for

implement ing a simple isolation amplier using the

HCNR200/201 optocoupler. Besides the optocoupler,

two external op‑amps and two resistors are required.

This simple circuit is actually a bit too simple to function

properly in an actual circuit, but it is quite useful for ex‑

plaining how the basic isolation amplier circuit works (a

few more components and a circuit change are required

to make a practical circuit, like the one shown in Figure

12b).

The operation of the basic circuit may not be immedi‑

ately obvious just from inspecting Figure 12a, particu‑

larly the input part of the circuit. Stated briey, amplier

A1 adjusts the LED current (I

), and therefore the current

in PD1 (I

PD1

), to maintain its “+” input terminal at 0 V. For

example, increasing the input voltage would tend to in‑

crease the voltage of the “+” input terminal of A1 above

0 V. A1 amplies that increase, causing I

to increase, as

well as I

PD1

. Because of the way that PD1 is connected,

PD1

will pull the “+” terminal of the op‑amp back toward

ground. A1 will continue to increase I

until its “+” termi‑

nal is back at 0 V. Assuming that A1 is a perfect op‑amp,

no current ows into the inputs of A1; therefore, all of the

current owing through R1 will ow through PD1. Since

the “+” input of A1 is at 0 V, the current through R1, and

there fore I

PD1

as well, is equal to V

/R1.

Essentially, amplier A1 adjusts I

so that

PD1

= V

/R1.

Notice that I

PD1

depends ONLY on the input voltage and

the value of R1 and is independent of the light output

characteris tics of the LED. As the light output of the

LED changes with temperature, ampli er A1 adjusts I

to compensate and maintain a constant current in PD1.

Also notice that I

PD1

is exactly proportional to V

, giving

a very linear relationship between the input voltage and

the photodiode current.

The relationship between the input optical power and

the output current of a photodiode is very linear. There‑

fore, by stabiliz ing and linearizing I

PD1

, the light output of

the LED is also stabilized and linearized. And since light

from the LED falls on both of the photodiodes, I

PD2

will be

stabilized as well.

The physical construction of the package determines the

relative amounts of light that fall on the two photodiodes

and, therefore, the ratio of the photodiode currents. This

results in very stable operation over time and tempera‑

ture. The photodiode current ratio can be expressed as a

constant, K, where

K = I

PD2

PD1

Amplier A2 and resistor R2 form a trans‑resistance am‑

plier that converts I

PD2

back into a voltage, V

OUT

, where

OUT

= I

PD2

*R2.

Combining the above three equations yields an overall

expression relating the output voltage to the input volt‑

age,

OUT

= K*(R2/R1).

Therefore the relationship between V

and V

OUT

is con‑

stant, linear, and independent of the light output

characteris tics of the LED. The gain of the basic isola tion

amplier circuit can be adjusted simply by adjusting the

ratio of R2 to R1. The parameter K (called K

in the electri‑

cal specications) can be thought of as the gain of the

optocoupler and is specied in the data sheet.

Remember, the circuit in Figure 12a is simplied in order

to explain the basic circuit opera tion. A practical circuit,

more like Figure 12b, will require a few additional compo‑

nents to stabilize the input part of the circuit, to limit the

LED current, or to optimize circuit performance. Example

applica tion circuits will be discussed later in the data

sheet.

to worry about. How ever, the second circuit requires two

optocouplers, separate gain adjustments for the posi‑

tive and negative portions of the signal, and can exhibit

crossover distor tion near zero volts. The correct circuit to

choose for an applica tion would depend on the require‑

ments of that particular application. As with the basic

isolation amplier circuit in Figure 12a, the circuits in Fig‑

ure 14 are simplied and would require a few additional

compo nents to function properly. Two example circuits

that operate with bipolar input signals are discussed in

the next section.

As a nal example of circuit design exibility, the simpli‑

ed schematics in Figure 15 illus trate how to implement

4‑20 mA analog current‑loop transmitter and receiver

circuits using the HCNR200/201 optocoupler. An impor‑

tant feature of these circuits is that the loop side of the

circuit is powered entirely by the loop current, eliminat‑

ing the need for an isolated power supply.

The input and output circuits in Figure 15a are the same

as the negative input and positive output circuits shown

in Figures 13c and 13b, except for the addition of R3 and

zener diode D1 on the input side of the circuit. D1 regu‑

lates the supply voltage for the input amplier, while R3

forms a current divider with R1 to scale the loop current

down from 20 mA to an appropriate level for the input

circuit (<50 µA).

As in the simpler circuits, the input amplier adjusts the

LED current so that both of its input terminals are at the

same voltage. The loop current is then divided

between R1 and R3. I

PD1

is equal to the current in R1 and

is given by the following equation:

PD1

= I

LOOP

*R3/(R1+R3).

Combining the above equation with the equations used

for Figure 12a yields an overall expression relating the

output voltage to the loop current,

OUT

LOOP

= K*(R2*R3)/(R1+R3).

Again, you can see that the relationship is constant, lin‑

ear, and independent of the charac teristics of the LED.

The 4‑20 mA transmitter circuit in Figure 15b is a little dif‑

ferent from the previous circuits, partic ularly the output

circuit. The output circuit does not directly generate an

output voltage which is sensed by R2, it instead uses Q1

to generate an output current which ows through R3.

This output current generates a voltage across R3, which

is then sensed by R2. An analysis similar to the one above

yields the following expression relating output current

to input voltage:

LOOP

= K*(R2+R3)/(R1*R3).

Circuit Design Flexibility

Circuit design with the HCNR200/201 is very exible

because the LED and both photodiodes are acces sible

to the designer. This allows the designer to make perf‑

ormance trade‑os that would otherwise be dicult to

make with commercially avail able isolation ampliers

(e.g., band width vs. accuracy vs. cost). Analog isola tion

circuits can be designed for applications that have either

unipolar (e.g., 0‑10 V) or bipolar (e.g., ±10 V) signals, with

positive or negative input or output voltages. Several

simplied circuit topologies illustrating the design ex‑

ibility of the HCNR200/201 are discussed below.

The circuit in Figure 12a is congured to be non‑invert‑

ing with positive input and output voltages. By simply

changing the polarity of one or both of the photodiodes,

the LED, or the op‑amp inputs, it is possible to imple ment

other circuit congu ra tions as well. Figure 13 illustrates

how to change the basic circuit to accommodate both

positive and negative input and output voltages. The in‑

put and output circuits can be matched to achieve any

combina tion of positive and negative voltages, allowing

for both inverting and non‑inverting circuits.

All of the congurations described above are unipolar

(single polar ity); the circuits cannot accom mo date a sig‑

nal that might swing both positive and negative. It is pos‑

sible, however, to use the HCNR200/201 optocoupler to

implement a bipolar isolation amplier. Two topologies

that allow for bipolar operation are shown in Figure 14.

The circuit in Figure 14a uses two current sources to

oset the signal so that it appears to be unipolar to the

optocoupler. Current source I

OS1

provides enough oset

to ensure that I

PD1

is always positive. The second current

source, I

OS2

, provides an oset of opposite polarity to ob‑

tain a net circuit oset of zero. Current sources I

OS1

and

OS2

can be implemented simply as resistors connected to

suitable voltage sources.

The circuit in Figure 14b uses two optocouplers to obtain

bipolar operation. The rst optocoupler handles the pos‑

itive voltage excursions, while the second optocoupler

handles the negative ones. The output photo diodes are

connected in an antiparallel conguration so that they

produce output signals of opposite polarity.

The rst circuit has the obvious advantage of requiring

only one optocoupler; however, the oset performance

of the circuit is dependent on the matching of I

OS1

and

OS2

and is also dependent on the gain of the optocoupler.

Changes in the gain of the opto coupler will directly af‑

fect the oset of the circuit.

The oset performance of the second circuit, on the

other hand, is much more stable; it is inde pendent of

optocoupler gain and has no matched current sources

The preceding circuits were pre sented to illustrate the

exibility in designing analog isolation circuits using the

HCNR200/201. The next section presents several com‑

plete schematics to illustrate practical applications of the

HCNR200/201.

Example Application Circuits

The circuit shown in Figure 16 is a high‑speed low‑cost

circuit designed for use in the feedback path of switch‑

mode power supplies. This application requires good

bandwidth, low cost and stable gain, but does not re‑

quire very high accuracy. This circuit is a good example

of how a designer can trade o accuracy to achieve

improve ments in bandwidth and cost. The circuit has a

bandwidth of about 1.5 MHz with stable gain character‑

istics and requires few external components.

Although it may not appear so at rst glance, the circuit

in Figure 16 is essentially the same as the circuit in Fig‑

ure 12a. Amplier A1 is comprised of Q1, Q2, R3 and R4,

while amplier A2 is comprised of Q3, Q4, R5, R6 and R7.

The circuit operates in the same manner as well; the only

dierence is the performance of ampliers A1 and A2.

The lower gains, higher input currents and higher oset

voltages aect the accuracy of the circuit, but not the

way it operates. Because the basic circuit operation has

not changed, the circuit still has good gain stability. The

use of discrete transistors instead of op‑amps allowed

the design to trade o accuracy to achieve good band‑

width and gain stability at low cost.

To get into a little more detail about the circuit, R1 is se‑

lected to achieve an LED current of about 7‑10 mA at the

nominal input operating voltage according to the fol‑

lowing equation:

= (V

/R1)/K1,

where K

(i.e., I

PD1

) of the optocoupler is typically about

0.5%. R2 is then selected to achieve the desired output

volt age according to the equation,

OUT

= R2/R1.

The purpose of R4 and R6 is to improve the dynamic re‑

sponse (i.e., stability) of the input and output circuits by

lowering the local loop gains. R3 and R5 are selected to

provide enough current to drive the bases of Q2 and Q4.

And R7 is selected so that Q4 operates at about the same

collector current as Q2.

The next circuit, shown in Figure 17, is designed to achieve

the highest possible accuracy at a reasonable cost. The

high accuracy and wide dynamic range of the circuit is

achieved by using low‑cost precision op‑amps with very

low input bias currents and oset voltages and is limited

by the performance of the opto coupler. The circuit is de‑

signed to operate with input and output voltages from

1 mV to 10 V.

The circuit operates in the same way as the others. The

only major dierences are the two compensa tion capaci‑

tors and additional LED drive circuitry. In the high‑speed

circuit discussed above, the input and output circuits are

stabilized by reducing the local loop gains of the input

and output circuits. Because reducing the loop gains

would decrease the accuracy of the circuit, two compen‑

sation capacitors, C1 and C2, are instead used to improve

circuit stability. These capacitors also limit the bandwidth

of the circuit to about 10 kHz and can be used to reduce

the output noise of the circuit by reducing its bandwidth

even further.

The additional LED drive circuitry (Q1 and R3 through

R6) helps to maintain the accuracy and band width of the

circuit over the entire range of input voltages. Without

these components, the transcon duc t ance of the LED

driver would decrease at low input voltages and LED

currents. This would reduce the loop gain of the input

circuit, reducing circuit accuracy and bandwidth. D1 pre‑

vents excessive reverse voltage from being applied to

the LED when the LED turns o completely.

No oset adjustment of the circuit is necessary; the gain

can be adjusted to unity by simply adjusting the 50 kohm

poten tiometer that is part of R2. Any OP‑97 type of op‑

amp can be used in the circuit, such as the LT1097 from

Linear Technology or the AD705 from Analog Devices,

both of which oer pA bias currents, µV oset voltages

and are low cost. The input terminals of the op‑amps and

the photodiodes are connected in the circuit using Kelvin

connections to help ensure the accuracy of the circuit.

The next two circuits illustrate how the HCNR200/201 can

be used with bipolar input signals. The isolation amplier

in Figure 18 is a practical implemen tation of the circuit

shown in Figure 14b. It uses two opto couplers, OC1 and

OC2; OC1 handles the positive portions of the input sig‑

nal and OC2 handles the negative portions.

Diodes D1 and D2 help reduce crossover distortion by

keeping both ampliers active during both positive and

negative portions of the input signal. For example, when

the input signal positive, optocoupler OC1 is active while

OC2 is turned o. However, the amplier control ling OC2

is kept active by D2, allowing it to turn on OC2 more rap‑

idly when the input signal goes negative, thereby reduc‑

ing crossover distortion.

Balance control R1 adjusts the relative gain for the posi‑

tive and negative portions of the input signal, gain con‑

trol R7 adjusts the overall gain of the isolation amplier,

and capac i tors C1‑C3 provide compensa tion to stabilize

the ampliers.

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P19

HCNR201

Mfr. #:

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Description:

High Linearity Optocouplers 1 Ch 60mW 25mA

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HCNR201

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