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Isolation Circuitsfor General PurposeAnalog Applications

Introduction To help you choose and designwith Hewlett-Packard isolationcomponents, this Designer’s Guidecontains analog isolation circuitsfor general purpose industrialapplications including a low-costcircuit for motor speed andposition measurement. There aretwo other Hewlett-Packard docu-

Contents:

Overview of Analog Isolation Applications....................................2

Application Circuits:

• Low-cost Isolation Amplifier for Motor and Speed Measurement........ 4

• Isolation Amplifier for Bipolar Signals..................................................... 5

• Isolated 4 - 20 mA Analog Current Loop Transmitter/Receiver ............ 6

• 15 MHz AC-Coupled Isolation Amplifier ..................................................7

• Isolated 15-bit A/D Converter .................................................................... 9

ments that are available to helpyour inverter circuit design:

• Isolati on Cir cui ts for In vert er

Gate Dr i ve

(Publication Number5965-8071E)

• Isolati on Ci rcui ts for Curr ent

and Voltage Sensi ng

(Publication Number5965-8207E)

Information regarding Hewlett-Packard optoisolators (includingdata sheets and application notes)is available on the World Wide Webat:

www.hp.com/go/isolator



2

Optoisolators transfer analog anddigital signals from one circuit

section or module to another in thepresence of a large potentialdifference or induced electricalnoise between the ground orcommon points of these modules.Examples of analog isolationapplications are interfaces to: A/Dconverters, sensing circuits such asthermocouples and transducers,patient monitoring equipment,motor speed and position measure-ment circuits, audio and videoamplifiers, and power supply

feedback.

Hewlett-Packard offers twocategories of analog optoisolators.

The first category of optoisolators(such as the HCNR201 and HCPL-4562) are basic building blockswith highly linear LED and photo-detector/amplifier combination.

The second category of opto-isolators (such as the HCPL-7820and HCPL-7860/7870) have extrafunctionality and can be used toreplace conventional isolationamplifiers and analog-to-digitalconverters.

Basic Building Blocks forAnalog IsolationHP’s HCNR200/1 and HCPL4562constitute basic optical couplingbuilding blocks for high linearityisolation applications. Figures 1and 2 show the respective opticalcoupling mechanisms for these

two optoisolators. Both theseisolators use high-performanceAlGaAs LEDs and photodiodecombinations with higher speedand linearity compared toconventional optoisolators. TheHCNR200/1 LED illuminates twoclosely matched photodiodes, oneon the input side, and another onthe output side. With a suitableapplications circuit for the

Figure 1. HCNR200/1 High LinearityAnalog Isolator.

Figure 2. HCPL-4562 High BandwidthAnalog Isolator.

HCNR200/1, the nonlinearity

and drift characteristics of theLED can be virtually eliminated.

The output photodiode produces aphotocurrent that is linearlyrelated to the light output of theLED. The close matching of thephotodiodes and advanced designof the package ensure the highlinearity and stable gaincharacteristics of the optoisolator.

The HCNR200/1 optoisolator canbe used as a basic analog isolation

building block for a wide varietyof applications that require goodstability, linearity, bandwidth andlow cost. The HCNR200/1 is veryflexible and, by appropriate designof the application circuit, iscapable of operating in manydifferent modes, including unipo-lar, bipolar, ac/dc, inverting andnon-inverting.

The HCPL-4562 and HCNW4562 arerecommended for very high

bandwidth (up to 15 MHz) ACanalog designs. If the outputtransistor is biased in the activeregion, the current transfer ratiorelationship for the HCPL-4562 canbe represented as:

IC = K (IF/IFQ) n

where IC is the collector current;IF is the LED input current, IFQ isLED input current at which K ismeasured; K is the collectorcurrent when IF = IFQ; and n is theslope of IC vs. IF on logarithmicscale.

The exponent n varies with IF, butover some limited range of ∆IF, n can be regarded as a con-stant. For ac-signal applications,the HCPL-4562 can be biased at anappropriate quiescent currentwhere the ratio of the incrementalphotodiode current to incremental

LED current is nearly constant.Figure 3 shows the linearitycharacteristics of the HCPL-4562.

Figure 3. HCPL-4562 Base PhotoCurrent vs. Input Current.

Overview of Analog Isolation Applications

7

1

2

3

4 5

6

8NC

ANODE

CATHODE

NC

VCC

VB

VO

GND

I P B

– B A

S E P H O T O C

U R R E N T – µ A

00

IF – INPUT CURRENT – mA

204

70

122 8 10 16

80

30

20

50

186 14

TA = 25 °CVPB > 5 V

HCPL-4562

60

40

10



3

Integrated High-CMRIsolation Amplifiers andA/D ConverterHP offers integrated high common-mode rejection isolation amplifiersand A/D converters for applica-tions where high accuracy, smallsize, low component count, andelectrical noise immunity aremajor issues. The HCPL-78xx

series of isolation amplifiers andA/D converters use sigma-deltaanalog-to-digital conversion

techniques for internally couplingthe signal. By deploying thesecircuit techniques, the nonlinearityand drift characteristics of theLED are eliminated. Figures 4 and5 conceptually describe HP’sisolation amplifier and A/D con-verters.

Analog Function Part Number Gain Non- Signal CMR - VDE InsulationTolerance linearity Bandwidth kV/µs Rating* -

(±%) (%) (kHz) (VCM = VIORM

1,500 V) (V peak)

Building block for a HCNR200/1 5 0.01 1,000 Not 1,414generic Iso-Amp (HCNR201) Specified (Option 050)

Building block for HCPL-4562 Not 0.25 17,000 630 V (Option 060)AC-coupled Iso-Amp HCNW4562 Specified 0.15 13,000 1,414

Integrated High-CMR HCPL-7800 1 0.35 85 10,000 848Iso-Amp HCPL-7800A/B

HCPL-7820/25 3 0.15 200 20,000 848HCPL-7840 5 0.2 100 10,000 848HCPL-J784 5 0.2 100 10,000 891

Isolated A/D HCPL-7860 and 1 0.15 22 15,000 848Converter HCPL-7870

Analog Optoisolator Selection Guide

Note: Refer to the technical data sheets for detailed specifications.

Figure 5. HCPL-7860/7870 Isolated A/D Converter.

Figure 4. High CMR Analog IsolationAmplifier.

VDD1

GND

CLAT

CCLK

CDAT

MCLK1

MDAT1

MCLK2

MDAT2

VIN+

VIN–

GND1

HCPL-7860

HCPL-7870

3-WIRE

SERIAL

INTERFACE

DIGITAL

OUTPUT

DIGITAL INTERFACE ICOPTICALLY

COUPLED

MODULATOR

VDD2

VDD

CHAN

SCLK

SDAT

CS

THR1

OVR1

RESET

MCLK

MDAT

GND2

ISOLATED

+5 V

NON-ISOLATED+5 V

ANALOG

INPUT

C1

0.1 µF

C2

0.1 µFC3

10 µF

+

1

2

SHIELD

8

+

–

+

–

7

6

5

3

4

IDD1

VDD1

VIN+

VIN–

GND1

VDD2

VOUT+

VOUT–

GND2

IDD2



4

Low-cost Isolation Amplifier for Motor Speed and PositionMeasurement

Description This is a high-speed, low-costisolation amplifier that can beused for the measurement of motorspeed and position. The analogsignal coming from the motor isassumed to be 0 to 10 V, or 4 to 20mA. This circuit can be used inapplications where high band-width, low-cost, and stable gain arerequired, but where accuracy is notcritical. This circuit is a goodexample of how a designer cantrade off accuracy to achieveimprovements in bandwidth andcost. The circuit has a bandwidthof about 1.5 MHz with stable gain

Performance of Circuit• 1.5 MHz bandwidth• Stable gain• Low-cost support circuit• Circuit couples only positive voltage signals

Benefits• Low cost solution for coupling positive voltage analog signals• Simple way for isolating motor speed and position analog signals

characteristics and requires fewexternal components.

The input amplifier is comprised of Q1, Q2, R3 and R4, while theoutput amplifier is comprised of Q3, Q4, R5, R6 and R7. The use of

discrete transistors instead of op-amps allows the designer to trade-off accuracy to achieve goodbandwidth and gain stability at lowcost. R1 is selected to achieve anLED current of about 7-10 mA atthe nominal input operatingvoltage according to the followingequation:

IF = (VIN / R1) / K1,

where K1 (i.e., IPD1/IF ) of theoptocoupler is typically about 0.5%.R2 is then selected to achieve thedesired output voltageaccording to the equation,

VOUT /VIN = R2 / R1.

The purpose of R4 and R6 is toimprove the dynamic response(i.e., stability) of the input andoutput circuits by lowering thelocal loop gains. R3 and R5 areselected to provide enough currentto drive the bases of Q2 and Q4.And R7 is selected so that Q4operates at about the same collec-tor current as Q2.

VIN

Q2 2N3904

Q1 2N3906

LED

ISOLATIONBARRIER

VCC1 +5 V

PD1

HCNR200/1

4-20 mA

R1

68 kΩ

R3

10 kΩ

R4

10 Ω

VOUT

Q4 2N3904

Q3 2N3906

VCC2 +5 V

PD2

R2

68 kΩ

R5

10 kΩ

R7

470

R6

10 Ω

M

A

0-10 V+

–



5

Description This circuit shows how theHCNR200/1 high linearity opto-coupler can be used for transmit-ting bipolar analog signals acrossan isolation boundary. This circuituses two optocouplers: OC1 and

OC2; OC1 handles the positiveportions of the input signal andOC2 handles the negative portions.

Diodes D1 and D2 help reducecross-over distortion by keepingboth amplifiers active during both

Isolation Amplifier for Bipolar Signals

Performance of Circuit• 0.01%nonlinearity• Low transfer gain variation: ±5%(K3 of HCNR201)• Low crossover distortion within the dc to 100 Hz frequency band

Benefits• Low cost solution for bipolar analog signals.• Worldwide insulation safety certification

positive and negative portions of the input signal. For example,when the input signal is positive,optocoupler OC1 is active whileOC2 is turned off. However, theamplifier controlling OC2 is keptactive by D2, allowing it to turn on

OC2 more rapidly when the inputsignal goes negative, therebyreducing crossover distortion.

Balance control R1 adjusts therelative gain for the positiveand negative portions of the input

C1 10 pF

C3 10 pF

VIN

R2

180 kΩ R4

680 Ω

R5

680R3

180 kΩ

R6

180 kΩ

R7

50 kΩ

D1

ISOLATION

BARRIER

GAIN

D2

OC1

PD1OC1

PD2

OC2

PD2OC2

PD1

OC2

LED

OC1

LED

+

+

–

–

C2 10 pF

R1

50 kΩ

BALANCE

+

– VOUT

HCNR200/1

signal, gain control R7 adjusts theoverall gain of the isolation ampli-fier, and capacitors C1-C3 providecompensation to stabilize theamplifiers.



6

Isolated 4 - 20 mA Analog Current Loop Transmitter/Receiver

Description The HCNR200/1 AnalogOptocoupler isolates both thetransmitter and receiver circuitfrom the 4 - 20 mA Analog CurrentLoop. One important feature of this circuit is that the loop side of the circuit is powered by the loop

current. No isolated power supplyis required.

The zener diode D1 on the inputside of the receiver circuitregulates the supply voltage forthe input amplifier, while R3forms a current divider with R1 toscale the loop current down from20 mA to an appropriate level forthe input circuit (<50 mA).

Performance of Circuit• HCNR200/1 nonlinearity: 0.1%• HCNR201 gain tolerance:±5%

Benefits• Low-cost, simple circuit• No isolated power supply needed on the 4 - 20 mA side of the circuit

In this simple circuit, the inputamplifier adjusts the LED currentso that both of its input terminalsare at the same voltage. Theloop current is then dividedbetween R1 and R3. IPD1is equal tothe current in R1 and is given bythe following equation:

IPD1= ILOOP • R3 / (R1+R3).

The ratio of the output voltage tothe loop current is,

VOUT /ILOOP =K • (R2•R3) / (R1+R3).

One can see that the relationship isconstant, linear, and independent

of the characteristics of the LED.

The 4 - 20 mA transmitter circuit isa little different from a standardisolated amplifier circuit, particu-larly the output circuit. Theoutput circuit does not directlygenerate an output voltage which

is sensed by R2, it instead uses Q1to generate an output currentwhich flows through R3. Thisoutput current generates a voltageacross R3, which is then sensedby R2. An analysis similar to theone above yields the followingexpression relating output currentto input voltage:

ILOOP / VIN = K • (R2+R3)/(R1•R3)

+

–

+

–

+

–

+

–

VIN

VCC

+IIN

–IIN

ISOLATION

BARRIER

VOUT

+IOUT

–IOUT

ISOLATION

BARRIER

HCNR200/201

R1

R3

PD1

LED

D1

R2

PD2

R2

Q1 PD2

R3

D1

(A) RECEIVER

(B) TRANSMITTER

LED

PD1

R1



7

Description This circuit, with the HCPL-4562Wideband Analog/VideoOptocoupler, is optimized forvideo signal coupling. The peakedresponse of the detector circuithelps extend the frequency rangeover which the gain is relatively

constant. The number of gainstages, the overall circuit topology,and the dc bias points are allchosen to maximize the band-width.

The application circuit incorpo-rates several features thathelp maximize the bandwidthperformance of the HCPL-4562.Most important of these features is

15 MHz AC-Coupled Isolation Amplifier

Performance of Circuit• Typical bandwidth: 15 MHz• Typical Gain variation: -1.1 dB at 5 MHz with reference at 0.1 MHz• Isolation Mode Rejection: 122 dB at 120 Hz

Benefits• Cost-effective, high performance video interface circuit

peaked response of the detectorcircuit that helps extend thefrequency range over which thevoltage gain is relatively constant.

The number of gain stages, theoverall circuit topology, and thechoice of DC bias points are allconsequences of the desire to

maximize bandwidth performance.

To use the circuit, first select R1to set VE for the desired LEDquiescent current by:

For a constant value VINp-p, thecircuit topology (adjusting the gainwith R4) preserves linearity bykeeping the modulation factor(MF) dependent only on VE.

1

2

8

7

HCPL-4562

6

5

3

447 µF

VCC1

100 Ω

500 Ω

51 Ω

6.8 kΩ

1.0 kΩ

VIN

R1

R2

R4 = POTENTIOMETER+51 Ω RESISTOR

VE

R3

Q1

C1

D1

1N4150

1

2

2

2

9.1 kΩR6

1.0 kΩ

15 kΩ

R8

R7

750 ΩR9

470 ΩR11

100 ΩR10

Q2

Q3

Q4

VOUT

VCC2

HIGH

IMPEDANCE

LOAD

GV = VOUT /VIN

Q1 - Q4 = 2N3904

IFQ

IFp-p ≅

IPBp-pVINp-p

IPBQ

IFp-p

VE

VIN /R4

Modulation

Factor (MF):

≅

(2)

(3)

(4)

=

=2 IFQ

iFp-p VINp-p

2 VE

conti nues

IFQ =VE

R4

≅GV VE R10

(∂IPB/∂IF) R7R9(1)



8

For a given GV, VE, and VCC, DCoutput voltage will vary only withhFEX.

For 9 V < VCC< 12 V, select the

value of R11 such that:

The voltage gain of the secondstage (Q3) is approximately equalto:

Increasing R'11 (R'11 includes theparallel combination of R11 and theload impedance) or reducing R9

(keeping R9/R10 ratio constant)will improve the bandwidth.

Finally, adjust R4to achieve thedesired voltage gain.

Definitions:

GV = Voltage Gain

IFQ = Quiescent LED forwardcurrent

iFp-p = Peak-to-peak small signalLED forward current

VINp-p = Peak-to-peak small signalinput voltage

iPBp-p = Peak-to-peak small signalbase photocurrent

IPBQ = Quiescent base photocur-rent

VBEX = Base-Emitter voltage of HCPL-4562/HCNW4562 transistor

IBXQ= Quiescent base current of HCPL-4562/HCNW4562 transistor

hFEX = Current Gain (IC/IB) of HCPL-4562/HCNW4562 transistor

VE = Voltage across emitter

degeneration resistor R4

f T4= Unity gain frequency of Q4

CCQ3= Effective capacitance fromcollector of Q3 to ground

VO = VCC – VBE4 –

VCC – 2 VBE

[VBEX = (IPBQ – IBXQ) R7]

IPBQ

Where:

and,

≅

IBXQ ≅

(5)R9R10

GV VE R10

R7R9

R6 hFEX

(6)

(7)

≅VO

R11≤ (8)

4.25 V

470Ω≤ 9.0 mAICQ4

(9)•

1

1

2π R'11 f T4

CCQ3

+1 + s R9

R9

R10

where typically

GV

VOUT

VIN(10)

= 0.0032

≅ ≅∂IPB

∂IF

∂IPB

∂IF

R7R9

R4R10



9

Description The HCPL-7860 Isolated Modulator

and the HCPL-7870 DigitalInterface IC together form anisolated programmable two-chipanalog-to-digital converter. Theisolated modulator allows directmeasurement of analog signalsthrough the resistor divider circuitR1 and R2 while the digital inter-face IC can be programmed tooptimize the conversion speed andresolution.

Isolated 15-bit A/D Converter

Performance of Isolated A/D Converter• Resolution due to linearity: 12 bits at tDELAY = 18µs

14 bits at tDELAY = 94µs

• Common-mode transient dV/dt = 15 kV/µs @ VCM = 1500 Vrejection:

• Signal-to-Noise: SNR = 62 dB (minimum)

• Regulatory Isolation Ratings: VISO = 3750 V (per UL 1577)VIORM = 848 V; VIOTM = 6000 V

• Input Offset Drift: 4µV / °C (typical)

• Reference Voltage Tolerance: ± 4% (± 1%within shipment tube)

Benefits• Integrated analog-to-digital converter means fewer components required.• High common-mode transient rejection ensures no corruption of data.• Low gain temperature-coefficient and offset voltage ensure high accuracy

measurements.

In operation, the HCPL-7860Isolated Modulator optocoupler

converts a low-bandwidth analoginput into a high-speed one-bitdata stream by means of a sigma-delta (Σ∆) oversampling modula-tor. The Digital Interface ICconverts the single-bit data streamfrom the Isolated Modulator intofifteen-bit output words andprovides a serial output interfacethat is compatible with SPI®,QSPI®, and Microwire® protocols,

allowing direct connection to amicrocontroller. The isolated A/D

converter provides fast over-rangedetection (i.e. for short-circuitdetection) and adjustable thresh-old detection (for over-currentdetection). The Digital Interface ICmay be programmed to one of fivedifferent conversion modesand three different pre-triggermodes. It also has programmableoffset calibration, for even higheraccuracy.

C1

0.1 µF

C2

0.1 µFC3

10 µF

ANALOG

INPUT

R2

R1

ISOLATED

+5 V

NON-ISOLATED

+5 V

HCPL-7860

OPTICALLY

COUPLED

MODULATOR

VDD1

VIN+

VIN–

GND1

CCLK

CLAT

CDAT

MCLK1

MDAT1

MCLK2

MDAT2

GND

VDD

CHAN

SCLK

SDAT

CS

THR1

OVR1

RESET

VDD2

MCLK

MDAT

GND2

HCPL-7870

+

3-WIRE

SERIAL

INTERFACE

DIGITAL

OUTPUT

DIGITAL INTERFACE IC



Technical References:1. Information regarding Hewlett-Packard optoisolators (includingdata sheets and application notes)

is available on the World Wide Webat:

www.hp.com/go/isolator

2.Desi gner ’s Gui de to Isolati on Cir cui ts for In vert er Gate Dri ve

(Publication Number 5965-8071E)

3.Desi gner ’s Gui de to Isolati on Cir cui ts for Cur r ent and Voltage

Sensi ng


4.Regulator y Gui de to Isolati on Ci rcu i ts


5. The data sheets in the followingtable are available through theComponents Sales Response

Center at Tel: 1-800-235-0312 orthrough a Faxback service at

Tel: 1-800-450-9455.

Hewlett-Packard Data Sheets

HP Part Number Title HP Publication HP Faxback IDNumber Number

HCNR200 High Linearity Analog Optocoupler 5965-3577E 12139

HCNR201

HCPL-4562 High Bandwidth, Analog/Video Optocoupler 5965-3597E 53869

HCNW4562

HCPL-7820 High CMR Isolation Amplifier 5965-3591E 80432

HCPL-7825

HCPL-7840 Analog Isolation Amplifier 5965-4784E 12130

HCPL-J784 High Insulation Analog Isolation Amplifier New*

HCPL-7800/A/B High CMR Isolation Amplifier 5965-3592E 55807

HCPL-7860/ Isolated 15-bit A/D Converter 5965-5255E 11997

7870/0870

* The “New” HP Optocouplers will soon beassigned Publication and Faxback IDNumbers. Contact your sales representativeto order the data sheet.

For technical assistance or the location of your nearest Hewlett-Packard sales office,distributor or representative call:

Americas/Canada: 1-800-235-0312 or

(408) 654-8675

Far East/Australasia: Call your local HP

sales office.

J apan: (81 3) 3335-8152

Europe: Call your local HP sales office.

Data Subject to Change

Copyright © 1997 Hewlett-Packard Co.

Printed in U.S.A. 5965-8223E (8/97)

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