Precision Analog Optocoupler

Precision Analog Optocoupler
Todd P. Meyrath 1
Atom Optics Laboratory
Center for Nonlinear Dynamics
University of Texas at Austin
c○ 2004
July 2002
revised February 14, 2005
In this note, we give a simple design for a precision analog optocoupler which may
be useful for ground loop removal or voltage isolation. The circuit here is intended
for precision low frequency (under 10 kHz) applications and uses a HCNR201 as the
primary optocoupling device. It is based on that shown in Figure 17 of HCNR201
datasheet but includes a few extra options. The circuit given in Fig 17 has a voltage
range from 1 mV to over 10 V and has high accuracy and stability. For many applications
this is sufficient. However, there is a drawback in that the circuit can not
actually pass a 0 V signal, the ≈ 1 mV actually is a limit since the photodiode always
must be slightly reverse biased. Even a slowly rising signal below ≈ 1 mV produces
a non-adiabatic spike in the output (which the author found out the hard way on a
critical application). The various versions of bipolar drivers given on the data sheet
suffer from cross-over transition problems when using two optocouplers. Here we use
a simple solution so the circuit can be made bipolar and can have 0 V signals without
cross-over problems. This is to shift the input-output range down by adding to the
input voltage before the optocoupler and then subtracting it back off at the output.
This allows one to shift the range by anything between 0 V and 5 V. To have 0 V to
10 V one need only add and subtract a few millivolts, or for −5 V to 5 V add and
subtract 5 V. Naturally this does not increase the voltage range of the device which
is still 10 V. As drawn on the schematic, the given voltage dividers give a voltage
adjustment range between 0 V and 1.5 V, the voltage dividers may be adjusted for
desired range of operation. For the full 0 V to 5 V range, one must only replace R5
and R8 by 0 Ω shorts.
The board layout includes several options which may be disabled when not desired.
The table here gives the options and components to be omitted when options are not
used. R22 and R23 are optional gain resistors which will generally not be used.
Option to remove Add Jumper Omit parts
Input Adder W1 D1, C19, R5-R7, R11, U1, R23
Output Subtracter W2 D2, C18, R8-R10, R12, U4, R22
Output Line Driver W3 R13, U5, U6, R20, C13-C17, R21
1 Please send comments, questions, corrections, insults to meyrath@physics.utexas.edu
1

The optional output line driver can supply currents of up to 1/4 A. This can be
used when the output is supplied to common 50 Ω loads. It includes an output integrator
involving R21 and C15, which may be disabled by omitting C15 and replacing
R21 by a 0 Ω short. The output integrator can improve output stability when driving
some loads, especially reactive loads.
The device provides a linear response with a transfer slope near unity which is
partially limited by the matching of R1 and R2+R3. It is advisable to use high quality
resistors. If trimming is not desired, one may omit R3 and short the pads with a 0 Ω
resistor and use a precision pair for R1 and R2.
Layout available at http://george.ph.utexas.edu/~ meyrath/papers
Revisions: Version 2.1 Added protection diodes D4 to D8. Version 2.0 Added
sum and subtraction options, output line driver option.
2

Figure 1: Photo of completed PCB. This implementation includes all of the options.
Only the required heatsink on the BUF chip is missing.
3

Parts
Qu. Label Part # Manufacturer/Description
1 R1 200 kΩ 1260 pkg resistor.
1 R2 196 kΩ 1260 pkg resistor.
2 R4,R20 1 kΩ 1260 pkg resistor.
2 R5,R8 23.2 kΩ 1260 pkg resistor.
3 R3,R6,R9 Bourns / 3214W series 10 kΩ trimpot.
2 R7,R10 0 Ω 1260 pkg resistor.
1 R13,R21 100 Ω 1260 pkg resistor.
3 R11,R12,R16 2 kΩ 1260 pkg resistor.
1 R17 7.5 kΩ 1260 pkg resistor.
1 R18 300 Ω 1260 pkg resistor.
1 R19 33 kΩ 1260 pkg resistor.
0-3 W1-W3 0 Ω (jumper) 1260 pkg resistor.
1 C1 68 pF cap, 1206 chip pkg.
1 C2 33 pF cap, 1206 chip pkg.
13 C3-C15 100 nF cap, 1206 chip pkg.
12 C16-C27 T491A106M016AS Kermet / 10 µF solid tantalum surface mnt.
2 U1,U4 INA128UA Texas Inst. / inst. amp, 8-SOIC pkg.
3 U2,U3,U5 OPA227UA Texas Inst. / precision opamp, 8-SOIC pkg.
1 U6 BUF634P † Texas Inst. / 1/4 Amp high speed buffer, 8-DIP pkg.
2 U7,U8 LM78L12ACM National Semi. / +12V reg, 8-SOIC pkg.
2 U9,U10 LM79L12ACM National Semi. / -12V reg, 8-SOIC pkg.
1 OP1 HCNR201 Agilent Tech. / Analog optocoupler, 8-DIP wide pkg.
1 Q1 2N3906 PNP Transistor, TO-92 pkg.
2 D1,D2 LM336M-5.0 National Semi. / 5.0V refernce, 8-SOIC pkg.
1 D3 1N4150 Diode, DO-35 pkg.
5 D4-D8 20V Zener Diode, DO-35 pkg.
2 J1,J2 227222-1 AMP-Tyco Elec. / Vertical PCB mnt BNC receptacle.
2 J3,J4 70543-0002 Molex / 3 pin vertical header power conn.
1 50-57-9403 Molex / 3 pin mate housing.
16-02-0102 Molex / female crimp pins.
Quantity is per board, label is on the PCB, part # is manufacturer number.
Most parts obtained from www.mouser.com, www.digikey.com, or www.alliedelec.com.
† Note, it is absolutely required to glue a small heat sink the buffer package.
4

PRECISION ANALOG OPTOCOUPLER CIRCUIT
Todd Meyrath
UT-Austin
July 2002
updated Dec 2004
Analog In
BNC
6
U3
C19
10µF
D4
+
C2, 33pF
OPA227
3
-12V
2
8
4
R4
1kΩ
R6
10kΩ
D1
R7
0Ω
R11
2kΩ
R5
23.2kΩ
6
3
2
5
OC1
HCNR201
1
INA128
U1
W1
0Ω
R23
5
R2
196kΩ
8
6
D1, D2: LM336M-5.0
D3: 1N4150
R3
10kΩ
R1
200kΩ
+12V
C18
10µF
+
3
4
OC1
HCNR201
R12
2kΩ R8
23.2kΩ
8
4 D2
R9
10kΩ
R10
0Ω
W2
2
3
-
OPA227
+
C1, 68pF
U2
R16
2kΩ
R17
7.5kΩ
6
R19
33kΩ
R13
100Ω
D3
-12V
3
2
+12V
R18
300Ω
Q1
2N3906
0Ω 0Ω
R22
3
2
1
8
INA128
5
U4
6
U5
2
1
W3
OC1
HCNR201
+15V
OPA227 BUF634
6
3
R20
4
1kΩ
C15
100nF
Additional Connections:
+15V
External Power Connections
-15V
C20,21+
10µF
C24,25
10µF
+
U6
8
7
+
+-15V
C17
10µF
OPA227,
INA128
C16
10µF
6
R21
100Ω
LM78L12M
U7, U8
C14
100nF
-
2-3, 6-7
+
7
4
-12V
2-3, 6-7
LM79L12M
U9, U10
1
5
1
+
+
C13
100nF
+12V
Power Supply Connections, as on each side
D5,D6
D7,D8
C3-C7
0.1µF
C8-C12
0.1µF
C22,23
10µF
C26,27
10µF
+12V
-12V
Analog
Output
BNC