Download Single-Supply Operation of Isolation Amplifiers

APPLICATION BULLETIN
®
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SINGLE-SUPPLY OPERATION OF ISOLATION AMPLIFIERS
By Rod Burt and R. Mark Stitt (602) 746-7445
For simplicity, many systems are designed to operate from a
single external power supply. In battery powered systems
such as aircraft and automotive, it’s often a requirement.
Isolation amplifiers such as the ISO120 and ISO122 can be
easily modified for input side single-supply operation with
the addition of an INA105 difference amplifier. With ISO
amps, it’s the isolated input side power supply which most
often needs to be single supply. The output side of the ISO
amp uses a split ±15V power supply, allowing a full ±10V
output swing.
The most common application is for a single ended input
referred to ground as shown. For a differential input, pin 2
can be connected to a second input instead of ground. This
provides a 0V to 5V differential input with common-mode to
either rail.
To understand how the circuit works, consider the operation
of the INA105 difference amplifier. The difference amplifier
forces its output (pin 6) relative to its reference (pin 1) to be
equal to the differential input (pin 3 – pin 2). The difference
amplifier reference pin and the ISO amp common are held at
approximately 5.1V by the 10kΩ resistor and the zener
diode. This pseudo ground establishes an arbitrary acceptable operating point for the ISO amp. The difference amplifier then translates its input, relative to true ground, up to the
5.1V pseudo ground. In other words, a 0V to 5V input
between pins 3 and 2 of the INA105 is seen as a 0V to 5V
signal at the ISO amp input.
Isolated power is often at a premium and both the ISO120/
122 and the INA105 operate on relatively low power. Common zener diodes, on the other hand, may require several mA
for proper operation. The 1N4689 zener diode specified is a
low level type designed for applications requiring low operating currents. It has a sharp breakdown voltage specified at
a low 50µA.
The difference amplifier has advantages as compared to
traditional single-supply amplifiers. The inputs of a difference amplifier can swing to both the positive and negative
power-supply rails. In fact, in the application shown in
Figures 1 and 2, the input range of the circuit extends
approximately 2V below ground (the negative power supply
rail). This is because the resistors internal to the INA105
divide the input level in half as seen by the op amp.
The technique is illustrated in Figures 1 and 2 using the
ISO120 and ISO122. These ISO amps are specified for
operation from dual supplies as low as ±4.5V and can be
operated with a total single power supply voltage as low as
9V. The circuit shown is designed for operation from a single
+15V power supply. This allows a 0V to +5V input range.
VS1 (+15V)
7
INA105
Difference Amp
2
5
R1
10kΩ
+VS2 (+15V)
R2
15 In
6
1
9
Signal Source
VIN
+
RS
R3
3
R4
ISO
122
(1)
1
RC
Gnd
16
4
Reference
IN4689
5.1V
7
8
VOUT = VIN
10
2
–VS1
Com 2
–VS2 (–15V)
NOTE: (1) Select to match RS .
FIGURE 1. Single Supply Operation of the ISO122 Isolation Amplifier.
16
©
SBOA004
1990 Burr-Brown Corporation
1
AB-009A
Printed in U.S.A. August, 1991
VS1 (+15V)
7
INA105
Difference Amp
2
5
R1
10kΩ
3
23 In
ISO 15 11
120
10
22 Osc
21 Com
12
9
24 Gnd
16
4
–VS1
Com 2
6
Signal Source
VIN
+
RS
4
(1)
R4
R3
3
+VS2 (+15V)
R2
1
RC
Reference
IN4689
5.1V
VOUT = VIN
–VS2 (–15V)
NOTE: (1) Select to match RS .
FIGURE 2. Single Supply Operation of the ISO120 Isolation Amplifier.
The accuracy of the INA105 difference amplifier relies on
careful resistor ratio matching (R3/R4 = R1/R2). Any source
impedance of the signal (RS) adds to the difference resistor
(R3). For low source impedances, the error is acceptable. For
better accuracy at higher source impedances, a compensating
resistor (RC) can be added to restore the ratio matching. The
resistors in the INA105 are 25kΩ. For 0.1% gain accuracy,
no compensating resistor is required with source impedances
up to 25Ω. For source impedances up to 2.5kΩ, use a
compensating resistor which matches RS within 1%. If the
source impedance is not known exactly, a trim pot can be
used to adjust gain accuracy.
For operation with source impedances greater than 2.5kΩ, a
unity-gain-connected single-supply op amp can be added to
buffer the input as shown in Figure 3. Although the input
range of the OPA1013 single-supply op amp includes the
negative rail, its output can not quite swing all the way to the
rail. The negative swing limit of this circuit is therefore
≈100mV—still adequate in many applications.
For an instrumentation amplifier (IA) front end, the other
half of the OPA1013 can be connected to the inverting input
of the INA105 (pin 2) as shown in Figure 4.
For a true single-supply ISO amp with high impedance
differential inputs, the circuit shown in Figure 5 can be used.
In this circuit, the inputs—and therefore the outputs—of the
OPA1013s are level-shifted up a VBE with a matched pair of
PNP input transistors. The transistors are biased as emitter
followers by a pair of 100µA current sources contained in the
REF200 dual current source.
VS1 (+15V)
7
INA105
Difference Amp
2
5
R1
10kΩ
+VS2 (+15V)
R2
6
15 In
1
9
1/2
OPA1013
VIN
R3
3
ISO
122
R4
1
Gnd
16
4
Reference
IN4689
5.1V
7
8
10
2
–VS1
Com 2
–VS2 (–15V)
FIGURE 3. Single Supply (almost—see text), High Input Impedance Isolation Amplifier.
2
VOUT = VIN
The circuits shown in this bulletin were designed for 0V to
5V operation from a single +15V power supply. With reduced range, operation from a lower voltage is feasible. For
higher input range the circuit can be operated from a higher
supply voltage. Table 1 shows the ranges obtainable for
selected power supplies.
VS
(V)
INPUT RANGE
FIGURES 1, 2
(V) (1)
INPUT RANGE
FIGURES 3, 4
(V) (1)
INPUT RANGE
FIGURE 5
(V) (1)
20+
15
12
–2 to +10
–2 to +5
–2 to +2
0.1 to +10
0.1 to +5
0.1 to +2
–0.3 to +10
–0.3 to +5
–0.3 to +2
Note: (1) Since the amplifier is unity gain, the input range is also the output
range. The output can go to –2V since the output section of the ISO amp
operates from dual supplies.
TABLE 1. Single-Supply ISO Amp Input Range vs Power
Supply.
VS1 (+15V)
7
1/2
OPA1013
INA105
Difference Amp
2
5
VN
R1
10kΩ
+VS2 (+15V)
R2
6
15 In
1
9
1/2
OPA1013
R4
R3
3
ISO
122
1
Gnd
16
VP
4
7
10
2
–VS1
Reference
VOUT = VP – V N
8
Com 2
IN4689
5.1V
–VS2 (–15V)
FIGURE 4. Single Supply (almost—see text), Isolation Amplifier with High-Impedance Differential Inputs..
VS1 (+15V)
7
100µA
REF200
1/2
OPA1013
INA105
Difference Amp
2
5
R1
10kΩ
+VS2 (+15V)
R2
6
15 In
1
9
VN
PMI
MAT03
1/2
OPA1013
3
1
Gnd
16
4
VP
R4
R3
ISO
122
Reference
IN4689
5.1V
FIGURE 5. Single Supply Isolation Amplifier with High-Impedance Differential Inputs.
3
7
8
VOUT = VP – V N
10
2
–VS1
Com 2
–VS2 (–15V)
power supply, the input common mode range is approximately +125V, –50V. With a +12V supply, the input common mode range is approximately ±50V. Differential input
range remains as shown in Table I for Figures 1 and 2.
For a single-supply ISO amp with higher common-modevoltage differential inputs, an INA117 high common-mode
voltage difference amplifier can be substituted for the INA105
difference amplifier as shown in Figure 6. With a +15V
VS1 (+15V)
7
V2
2
10kΩ
V3
6
380kΩ
3
21.1kΩ
20kΩ
INA117
4
8
+VS2 (+15V)
380kΩ
380kΩ
1
5
Reference
IN4689
5.1V
3
23 In
15
11
ISO
10
21 120
22 Com
12
9
24 Gnd
16
4
–VS1
Com 2
VOUT = V3 –V2
–VS2 (–15V)
FIGURE 6. Single Supply Isolation Amplifier with High Common-Mode Range Differential Inputs.
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4
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