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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
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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
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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
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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+
–
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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.
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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
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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)
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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
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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
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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
(Publication Number 5965-8207E)
4.Regulator y Gui de to Isolati on Ci rcu i ts
(Publication Number 5965-5853E)
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)