Isolation Circuits For General Purpose Analog Applications: Contents
Isolation Circuits For General Purpose Analog Applications: Contents
Isolation Circuits For General Purpose Analog Applications: Contents
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
Introduction
To help you choose and design with Hewlett-Packard isolation components, this Designers Guide contains analog isolation circuits for general purpose industrial applications including a low-cost circuit for motor speed and position measurement. There are two other Hewlett-Packard documents that are available to help your inverter circuit design: Isolation Circuits for Inverter Gate Drive (Publication Number 5965-8071E) Isolation Circuits for Current and Voltage Sensing (Publication Number 5965-8207E) Information regarding HewlettPackard optoisolators (including data sheets and application notes) is available on the World Wide Web at: www.hp.com/go/isolator
IC = K (IF/IFQ) n where IC is the collector current; IF is the LED input current, IFQ is LED input current at which K is measured; K is the collector current when IF = IFQ; and n is the slope of IC vs. IF on logarithmic scale. The exponent n varies with IF, but over some limited range of IF, n can be regarded as a constant. For ac-signal applications, the HCPL-4562 can be biased at an appropriate quiescent current where the ratio of the incremental photodiode current to incremental LED current is nearly constant. Figure 3 shows the linearity characteristics of the HCPL-4562.
NC 1 ANODE 2 CATHODE 3 NC 4
8 VCC 7 VB 6 VO 5 GND
HCNR200/1, the nonlinearity and drift characteristics of the LED can be virtually eliminated. The output photodiode produces a photocurrent that is linearly related to the light output of the LED. The close matching of the photodiodes and advanced design of the package ensure the high linearity and stable gain characteristics of the optoisolator. The HCNR200/1 optoisolator can be used as a basic analog isolation building block for a wide variety of applications that require good stability, linearity, bandwidth and low cost. The HCNR200/1 is very flexible and, by appropriate design of the application circuit, is capable of operating in many different modes, including unipolar, bipolar, ac/dc, inverting and non-inverting.
80 70 60 50 40 30 20 10 0 0 2 4 6
HCPL-4562
TA = 25 C VPB > 5 V
8 10 12 14 16 18 20
IF INPUT CURRENT mA
series of isolation amplifiers and A/D converters use sigma-delta analog-to-digital conversion techniques for internally coupling the signal. By deploying these circuit techniques, the nonlinearity and drift characteristics of the LED are eliminated. Figures 4 and 5 conceptually describe HPs isolation amplifier and A/D converters.
IDD1 VDD1 1
IDD2 8 VDD2
VIN+
VOUT+
VIN
VOUT
GND1
4 SHIELD
GND2
ISOLATED +5 V
OPTICALLY COUPLED MODULATOR VDD1 VIN+ VDD2 MCLK MDAT GND2 C2 0.1 F
DIGITAL INTERFACE IC CCLK CLAT CDAT MCLK1 MDAT1 MCLK2 MDAT2 GND VDD CHAN SCLK SDAT CS THR1 OVR1 RESET + C3 10 F 3-WIRE DIGITAL SERIAL INTERFACE OUTPUT
VIN GND1
HCPL-7860
HCPL-7870
Building block for a generic Iso-Amp Building block for AC-coupled Iso-Amp
HCNR200/1
0.01
1,000
HCPL-4562 HCNW4562
Integrated High-CMR HCPL-7800 Iso-Amp HCPL-7800A/B HCPL-7820/25 HCPL-7840 HCPL-J784 Isolated A/D Converter HCPL-7860 and HCPL-7870
0-10 V R1 68 k VIN
R3 10 k
LED
R2 68 k
M PD1
PD2
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
Description
This is a high-speed, low-cost isolation amplifier that can be used for the measurement of motor speed and position. The analog signal coming from the motor is assumed to be 0 to 10 V, or 4 to 20 mA. This circuit can be used in applications where high bandwidth, low-cost, and stable gain are required, but where accuracy is not critical. This circuit is a good example of how a designer can trade off accuracy to achieve improvements in bandwidth and cost. The circuit has a bandwidth of about 1.5 MHz with stable gain
where K1 (i.e., IPD1 /IF ) of the optocoupler is typically about 0.5%. R2 is then selected to achieve the The input amplifier is comprised of desired output voltage Q1, Q2, R3 and R4, while the according to the equation, output amplifier is comprised of Q3, Q4, R5, R6 and R7. The use of VOUT /VIN = R2 / R1. discrete transistors instead of opamps allows the designer to trade- The purpose of R4 and R6 is to off accuracy to achieve good improve the dynamic response bandwidth and gain stability at low (i.e., stability) of the input and cost. R1 is selected to achieve an output circuits by lowering the LED current of about 7-10 mA at local loop gains. R3 and R5 are the nominal input operating selected to provide enough current voltage according to the following to drive the bases of Q2 and Q4. equation: And R7 is selected so that Q4 operates at about the same collecIF = (VIN / R1) / K1, tor current as Q2.
C1 10 pF
R2 180 k
VIN
R1 50 k BALANCE
+ D2
HCNR200/1
C2 10 pF ISOLATION BARRIER
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
Description
This circuit shows how the HCNR200/1 high linearity optocoupler can be used for transmitting bipolar analog signals across an isolation boundary. This circuit uses two optocouplers: OC1 and OC2; OC1 handles the positive portions of the input signal and OC2 handles the negative portions. Diodes D1 and D2 help reduce cross-over distortion by keeping both amplifiers active during both
positive and negative portions of the input signal. For example, when the input signal is positive, optocoupler OC1 is active while OC2 is turned off. However, the amplifier controlling OC2 is kept active by D2, allowing it to turn on OC2 more rapidly when the input signal goes negative, thereby reducing crossover distortion. Balance control R1 adjusts the relative gain for the positive and negative portions of the input
signal, gain control R7 adjusts the overall gain of the isolation amplifier, and capacitors C1-C3 provide compensation to stabilize the amplifiers.
VOUT
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
Description
The HCNR200/1 Analog Optocoupler isolates both the transmitter and receiver circuit from the 4 - 20 mA Analog Current Loop. One important feature of this circuit is that the loop side of the circuit is powered by the loop current. No isolated power supply is required. The zener diode D1 on the input side of the receiver circuit regulates the supply voltage for the input amplifier, 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 mA).
In this simple circuit, the input amplifier 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. IPD1 is equal to the current in R1 and is given by the following equation: IPD1 = ILOOP R3 / (R1+R3). The ratio of the output voltage to the loop current is, VOUT /ILOOP = K (R2R3) / (R1+R3).
of the characteristics of the LED. The 4 - 20 mA transmitter circuit is a little different from a standard isolated amplifier circuit, particularly 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 flows 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:
One can see that the relationship is ILOOP / VIN = K (R2+R3)/(R1R3) constant, linear, and independent
Q2
Q4
5 R8 VE 1.0 k
Q3 R11 470 2
1N4150 D1
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
Description
This circuit, with the HCPL-4562 Wideband Analog/Video Optocoupler, is optimized for video signal coupling. The peaked response of the detector circuit helps extend the frequency range over which the gain is relatively constant. The number of gain stages, the overall circuit topology, and the dc bias points are all chosen to maximize the bandwidth. The application circuit incorporates several features that help maximize the bandwidth performance of the HCPL-4562. Most important of these features is
peaked response of the detector For a constant value VINp-p, the circuit topology (adjusting the gain circuit that helps extend the with R4) preserves linearity by frequency range over which the voltage gain is relatively constant. keeping the modulation factor The number of gain stages, the (MF) dependent only on VE. overall circuit topology, and the IFp-p VIN /R4 choice of DC bias points are all (2) consequences of the desire to maximize bandwidth performance. IF IPBp-p VIN p-p To use the circuit, first select R1 to set VE for the desired LED quiescent current by:
V GV VE R10 IFQ = E R4 (IPB/IF) R7R9 (1)
IFQ IPBQ =
p-p
VE
(3)
iFp-p 2 IFQ
VINp-p 2 VE
(4)
continues
For a given GV, VE, and VCC, DC output voltage will vary only with hFEX.
VO = VCC VBE4 Where: IPBQ and, IBXQ VCC 2 VBE R6 hFEX (7) GV VE R10 R7R9 (6) R9 R10 [VBEX = (IPBQ IBXQ) R7] (5)
Definitions: GV = Voltage Gain IFQ = Quiescent LED forward current iFp-p = Peak-to-peak small signal LED forward current VINp-p = Peak-to-peak small signal input voltage iPBp-p = Peak-to-peak small signal base photocurrent
For 9 V < VCC < 12 V, select the value of R11 such that:
VO 4.25 V 9.0 mA R11 470
ICQ4
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
The voltage gain of the second stage (Q3) is approximately equal to:
R9 R10 1 (9) 1 + s R9 CCQ3 + 1 2 R'11 fT4
Increasing R'11 (R'11 includes the parallel combination of R11 and the load impedance) or reducing R9 (keeping R9/R10 ratio constant) will improve the bandwidth. Finally, adjust R4 to achieve the desired voltage gain.
VOUT VIN IPB IF IPB IF R7R9 R4R10 = 0.0032
GV
(10)
where typically
ISOLATED +5 V R1 OPTICALLY COUPLED MODULATOR VDD1 R2 C1 0.1 F VIN+ VIN GND1 VDD2 MCLK MDAT GND2 C2 0.1 F
DIGITAL INTERFACE IC CCLK CLAT CDAT MCLK1 MDAT1 MCLK2 MDAT2 GND VDD CHAN SCLK SDAT CS THR1 OVR1 RESET HCPL-7870 + C3 10 F 3-WIRE SERIAL INTERFACE DIGITAL OUTPUT
ANALOG INPUT
HCPL-7860
SNR = 62 dB (minimum) VISO = 3750 V (per UL 1577) VIORM = 848 V; VIOTM = 6000 V 4 V / C (typical) 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.
Description
The HCPL-7860 Isolated Modulator and the HCPL-7870 Digital Interface IC together form an isolated programmable two-chip analog-to-digital converter. The isolated modulator allows direct measurement of analog signals through the resistor divider circuit R1 and R2 while the digital interface IC can be programmed to optimize the conversion speed and resolution.
In operation, the HCPL-7860 Isolated Modulator optocoupler converts a low-bandwidth analog input into a high-speed one-bit data stream by means of a sigmadelta () oversampling modulator. The Digital Interface IC converts the single-bit data stream from the Isolated Modulator into fifteen-bit output words and provides a serial output interface that is compatible with SPI, QSPI, and Microwire protocols,
allowing direct connection to a microcontroller. The isolated A/D converter provides fast over-range detection (i.e. for short-circuit detection) and adjustable threshold detection (for over-current detection). The Digital Interface IC may be programmed to one of five different conversion modes and three different pre-trigger modes. It also has programmable offset calibration, for even higher accuracy.
Technical References:
1. Information regarding HewlettPackard optoisolators (including data sheets and application notes) is available on the World Wide Web at: www.hp.com/go/isolator 2. Designers Guide to Isolation Circuits for Inverter Gate Drive (Publication Number 5965-8071E) 3. Designers Guide to Isolation Circuits for Current and Voltage Sensing (Publication Number 5965-8207E) 4. Regulatory Guide to Isolation Circuits (Publication Number 5965-5853E) 5. The data sheets in the following table are available through the Components Sales Response Center at Tel: 1-800-235-0312 or through a Faxback service at Tel: 1-800-450-9455.
* The New HP Optocouplers will soon be assigned Publication and Faxback ID Numbers. Contact your sales representative to 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. Japan: (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)