Download Programmable-Gain Instrumentation Amplifiers

APPLICATION BULLETIN
®
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PROGRAMMABLE-GAIN INSTRUMENTATION AMPLIFIERS
By David Jones (602) 746-7696 and R. Mark Stitt
The INA115 is a precision instrumentation amplifier (IA).
Its gain-sense and amplifier output connections are available
so that gains can be accurately set using external resistors.
By adding switches or a multiplexer, you can make a
precision programmable-gain IA (PGIA). Using the circuit
topology shown in this bulletin, the switches are in series
with op amp inputs so their resistance does not add error.
If you need an IA with more gain steps, you can cascade two
or more PGIAs. For example, if you cascade two PGA205s
you will get gains of 1, 2, 4, 8, 16, 32, and
64V/V.
For other gains or gain-steps, you may want to make your
own PGIA using an INA115. The circuits and equations
below make it easy to design a programmable-gain IA with
any gains and any number of gain steps.
Burr-Brown offers fixed and programmable-gain IAs with
amplifiers similar to the INA115. If you need a fixed gain of
100V/V, use the INA131. If you need a digitally programmable IA with decade gains of 1, 10, 100, and 1000V/V, or
binary gains of 1, 2, 4, and 8V/V, use the PGA204 or
PGA205.
MODEL
GAINS (V/V)
INA131
PGA204
PGA205
100
1, 10, 100, 1000
1, 2, 4, 8
TABLE I. Fixed and Programmable-Gain IAs.
R0
1 VO1
R1
VIN–
G1
R2
Rm
2
G2
INA115
4
Amp
B
Gm
Gn
NC
3
12
Amp
C
Rn
NC
Rm
R2
14
11
VOUT
Ref
10
Gn
Gm
15
G2
G1
VIN+
5
Amp
A
R1
8 VO2
R0
FIGURE 1. Programmable-Gain Instrumentation Amplifier with n Gain Steps and Lowest Gain > 1V/V.
©
SBOA024
1992 Burr-Brown Corporation
www.BDTIC.com/TI
AB-042
Printed in U.S.A. September, 1992
If you want a PGIA with n gains and the first gain is not
unity (1V/V), use the following relationships and the circuit
shown in Figure 1.
If you want to use the 25kΩ feedback resistors in the
INA115 for the R0s, you can use the circuit shown in
Figure 2. Keep in mind that the gain accuracy and gain
drift will be limited by the internal feedback resistors. The
25kΩ feedback resistors have a tolerance of ±0.5% with a
temperature coefficient of resistance drift (TCR) of up to
100ppm/°C. In the INA131, PGA204 and PGA205, resistor matching and TCR tracking of the resistors on the die
give typical gain error and drift of 0.01% and
5ppm/°C.
RESISTOR VALUES FOR PGIA WITH n GAINS
AND LOWEST GAIN > 1V/V
R0 = Your choice (e.g. 25kΩ)
R0 = 25kΩ when using the internal feedback resistors in the
INA115 (see Figure 2)
R0 (G1 – G2)
R1 = G (1 – G )
2
1
G1 R0 (G2 – G3)
R2 = G G (1 – G )
2
3
1
G1 R0 (Gm – Gm+1)
Rm = G G (1 – G )
m
m+1
1
GAINS
(V/V)
R0
(Ω)
R1
(Ω)
R2
(Ω)
R3
(Ω)
R4
(Ω)
2, 4, 8, 16
10, 20, 50, 100
3dB, 6dB, 9dB, 12db
10, 100, 1k, 10k
10, 100, 200, 500
100, 200, 400, 800
25k
25k
25k
100k
49.9k
100k
12.4k
1.4k
17.8k
10k
4.99k
511
6.19k
825
12.4k
1k
280
255
3.09k
280
8.87k
100
165
127
6.19k
562
43.2k
22.1
221
255
NOTE: Nearest Standard 1% Resistor Values.
2 G1 R0
Rn = G (G – 1)
n
1
TABLE II. Examples of Resistor Values for Selected
Gains—Figure 1 and 2 Circuits.
Where:
Gm = Intermediate gain (V/V)
Gn = Highest gain (V/V)
R0, R1, ... Rn = Resistor value per circuit diagram (Ω)
NC
1 VO1
R1
VIN–
G1
R2
2
G2
INA115
4
Amp
B
Gm
Rm
Gn
3
12
R0
Amp
C
Rn
14
R0
11
VOUT
Ref
10
Rm
R2
Gn
Gm
15
G2
G1
VIN+
5
Amp
A
R1
8 VO2
NC
FIGURE 2. Programmable-Gain Instrumentation Amplifier with n Gain Steps and Lowest Gain > 1V/V. This circuit uses
the 25kΩ feedback resistor in the INA115 for R0 of Figure 1.
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2
If you want a PGIA with n gains and the first gain is unity
(1V/V), use the following relationships and the circuit shown
in Figure 3.
If you want to use the 25kΩ feedback resistors in the
INA115 for the R1s, you can use the circuit shown in
Figure 4. Keep in mind the gain accuracy and gain drift
limitations discussed previously.
RESISTOR VALUES FOR PGIA WITH n GAINS
AND LOWEST GAIN = 1V/V
R0 = 0
An actual example of a four-gain digitally programmable IA
is shown in Figure 5. It uses a four-channel differential
multiplexer (MUX) for gain switching.
R1 = Your choice (e.g. 25kΩ)
R1 = 25kΩ when using the internal feedback resistors in the
INA115 (see Figure 4)
R2 =
R1 (G2 – G3)
G3 (1 – G2)
R3 =
G2 R1 (G3 – G4)
G3 G4(1 – G2)
GAINS
(V/V)
R0
(Ω)
R1
(Ω)
1, 2, 4, 8
1, 8, 64, 512
1, 2, 5, 10
1, 10, 100, 1k
1, 10, 100, 200
0dB, 3dB, 6dB, 9dB
0
0
0
0
0
0
25k
25k
25k
49.9k
20k
24.9k
R2
(Ω)
R3
(Ω)
12.4k 6.19k
3.09
392
15k 4.99k
4.99k 499
2k
110
17.8k 12.4k
R4
(Ω)
12.4k
113
10k
110
221
60.4k
NOTE: Nearest Standard 1% Resistor Values.
Rm =
G2 R1 (Gm – Gm+1)
Gm Gm+1(1 – G2)
Rn =
2 G2 R1
Gn (G2 – 1)
TABLE III. Examples of Resistor Values for Selected Gains—
Figure 3 and 4 Circuits.
Where:
Gm = Intermediate gain (V/V)
Gn = Highest gain (V/V)
R0, R1, ... Rn = Resistor value per circuit diagram (Ω)
1 VO1
R1
VIN–
G1
R2
Rm
2
G2
INA115
4
Amp
B
Gm
Gn
NC
3
12
Amp
C
Rn
NC
Rm
R2
11
Ref
14
10
Gn
Gm
15
G2
G1
VIN+
5
Amp
A
R1
8 VO2
FIGURE 3. Programmable-Gain Instrumentation Amplifier with n Gain Steps and Lowest Gain = 1V/V.
3
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VOUT
1 VO1
VIN–
G1
R2
INA115
4
Amp
B
2
G2
Gm
Rm
Gn
3
12
R1
11
Amp
C
Rn
R1
14
VOUT
Ref
10
Rm
R2
Gn
Gm
15
G2
G1
VIN+
Amp
A
5
8 VO2
FIGURE 4. Programmable-Gain Instrumentation Amplifier with n Gain Steps and Lowest Gain = 1V/V. This circuit uses
the 25kΩ feedback resistor in the INA115 for R1 of Figure 3.
R1
49.9kΩ
R2
4.99kΩ
4
5
R3
499Ω
6
7
1
R4
110Ω
16
10
R3
499Ω
11
12
13
R2
4.99kΩ
HI-509
In1A
In2A
In3A
+V
En
OutA
14
2
VCC
VDD
8
In–
2
In4A
A0
3
Gnd
1
GS1
VO1
RG1
FB
12
VO
INA115
14
15
In4B
OutB
–
VIN
15
A1
In3B
4
9
In+
5
RG2
GS2
+
VIN
VO2
11
VO
Ref
10
8
In2B
In1B
–V
3
VEE
R1
49.9kΩ
A1
A0
GAIN
L
L
H
H
L
H
L
H
1
10
100
1000
FIGURE 5. Programmable-Gain Instrumentation Amplifier Example with Four Gain Steps and Lowest Gain = 1V/V.
The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes
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without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant
any BURR-BROWN product for use in life support devices and/or systems.
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