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The LT1167: Single Resistor Sets the Gain of the Best
Instrumentation Amplifier – Design Note 182
Alexander Strong and Kevin R. Hoskins
Introduction
Linear Technology’s next generation LT®1167 instrumentation amplifier uses a single resistor to set gains from 1 to
10,000. The single gain-set resistor eliminates expensive
resistor arrays and improves VOS and CMRR performance.
Careful attention to circuit design and layout, combined
with laser trimming, greatly enhances the CMRR, PSRR,
gain error and nonlinearity, maximizing application versatility. The CMRR is guaranteed to be greater than 90dB
when the LT1167’s gain is set at 1. Total input offset voltage
(VOS) is less than 60µV at a gain of 10. For gains in the range
of 1 to 100, gain error is less than 0.05%, making the gainset resistor tolerance the dominant source of gain error.
The LT1167’s gain nonlinearity is unsurpassed when compared to other monolithic solutions. It is specified at less
than 40ppm when operating at a gain of 1000 while driving
a 2kΩ load. The LT1167 is so robust that it can drive 600Ω
loads without a significant linearity penalty. These parametric improvements result in an overall gain error that
remains unchanged over the entire input common mode
range and is not degraded by supply perturbations or
varying load conditions. The LT1167 can operate over a
wide ±2.3V to ±18V supply voltage range with only 0.9mA
supply current. The LT1167 is offered in 8-pin PDIP and SO
packages, saving significant board space compared to
multi-op amp designs.
As shown in Figure 1, the LT1167’s gain is set by the value
of one external resistor. A single 0.1% precision resistor
sets the gain from 1 to 10, resulting in better than 0.14%
accuracy. At very high gains (≥1000), the error is less than
0.2% when using 0.1% precision resistors.
Low Input Bias Current and Noise Voltage
The LT1167 combines the pA input bias current of FET
input amplifiers with the low input noise voltage characteristic of bipolar amplifiers. Using superbeta input transistors, the LT1167’s input bias current is only 350pA maximum
at room temperature. The LT1167’s low input bias current,
unlike that of JFET input op amps, does not double for every
10°C. The bias current is guaranteed to be less then 800pA
at 85°C. The low noise voltage of 7.5nV√Hz at 1kHz is
achieved by idling a large portion of the 0.9mA supply
current in the input stage.
Input Protection
The inputs of the LT1167 feature low leakage internal protection diodes connected between each input and the supply pins. Their leakage is so low that they do not compromise
the low 350pA input bias current. These diodes are rated at
20mA when input voltages exceed the supply rails. Precision and indestructibility are combined when an external
20k resistor is placed in series with each input. There is little
offset voltage penalty because the 320pA offset current
from the LT1167 multiplied by the 20k input resistors
contributes less than 7µV additional offset. With the 20k
resistors, the LT1167 can handle both ±400VDC input
faults and ESD spikes over 4kV. This passes the IEC 10004-2 level 2 specification.
, LTC and LT are registered trademarks of Linear Technology Corporation.
V+
+
RG
499Ω
LT1167C
LT1167 MONOLITHIC
INSTRUMENTATION AMPLIFIER, G = 100
SUPPLY CURRENT = 1.3mA MAX
–
DN182 F01
Figure 1. Combining Precision Trimmed Internal Resistors with a
Single External Resistor Sets the LT1167 Gain with High Accuracy
6/98/182
www.BDTIC.com/Linear
5V
10µF
10µF
+
+
0.1µF
0.1µF
5V
0.1µF
3
3
R1
VREF
7
+
1k
1
LT1167
8
5
–
2
2
AIN
LTC1400
1
VCC
DOUT 5
CLK 6
CONV 7
CX
4
GND
0.1µF
SERIAL
INTERFACE
VSS
4
8
10µF
R1 = ∞
CX = 0.047µF – 5V
R1 = 5.6kΩ CX = 0.030µF
0.1µF
+
– 5V AV = 1
AV = 10
ADC’s small nominal input capacitance, preserving signal
integrity. Figure 3 shows two FFTs of the amplifier/ADC’s
output. Figures 3a and3b show the results of operating the
LT1167 at unity gain and a gain of ten, respectively. In both
cases, the typical SINAD is 70.6dB.
Current Source
Figure 4 shows a simple, accurate, low power programmable
current source. The differential voltage across pins 2 and
3 is mirrored across RG. The voltage across RG is amplified
and applied across R1, defining the output current. For example, opening RG and setting R1 equal to 1M sets the output current range from 30pA to 10µA for an input voltage
of 0V to 10V. The bottom of the range is limited by circuit
noise. The circuit can be operated as a current source or
sink by applying a positive or negative differential voltage,
respectively. The 50µA bias current flowing from the REF
pin (Pin 5) is buffered by the LT1464 JFET operational amplifier, increasing the resolution of the current source to 3pA.
VS
DN182 F02
3
+
VIN
Figure 2. The LT1167’s Dynamic Performance Allows It
to Convert Differential Signals into Single-Ended Signals
for the 12-Bit LTC1400
RG
VIN
2
6
LT1167
1
–
7
+
8
+
R1
VX
–
5
–
4
IL
–
ADC Signal Conditioning
In many industrial systems, differential inputs are used to
eliminate ground loops and reject noise on long lines. The
LT1167 is shown in Figure 2 changing a differential signal
into a single-ended signal. The single-ended signal is then
filtered with a passive 1st order RC lowpass filter and
applied to the LTC®1400 12-bit analog-to-digital converter
(ADC). The LT1167’s output stage can easily drive the
– VS
–40
AMPLITUDE (dB)
LT1464
SUPPLY VOLTAGE = ±5V
fIN = 3kHz
fSAMPLE = 400ksps
SINAD = 70.6dB
LT1167 GAIN = 1
–20
[( ) ( )]
V IN+ − V IN− G
VX
=
R1
kΩ +1
49
.
4
G=
RG
IL=
–60
10 – 3
–100
0
25
50
75 100 125 150 175 200
FREQUENCY (kHz)
DN182 F03a
(a)
0
SUPPLY VOLTAGE = ±5V
fIN = 5.5kHz
fSAMPLE = 400ksps
SINAD = 70.6dB
LT1167 GAIN = 10
–20
–40
LOAD CURRENT (A)
10 – 4
–120
AMPLITUDE (dB)
R1
LOAD
DN182 F4a
(a)
10 – 2
–80
V + = 15V
V – = – 15V
RG = ∞
10 – 5
R1 =
10 – 6 1k
10 – 7 10k
10 – 8 100k
10 – 9 1M
10 – 10 10M
10 –11
10 – 4
10 – 3
10 – 2
10 –1
1
INPUT VOLTAGE (VIN+ – VIN–) (V)
(b)
–60
10
DN182 F04b
Figure 4. (a) A Simple, Accurate, 3pA Resolution, Low
Power Programmable Current Source and (b) Current
Source’s Output is Linear over 8 Decades
–80
–100
–120
+
0
0
25
50
75 100 125 150 175 200
FREQUENCY (kHz)
DN182 F03b
(b)
Figure 3. These Plots Show the Results of Operating the
LT1167 at Unity Gain (a) and a Gain of 10 (b), Respectively.
Each Indicates a Typical SINAD of 70.6dB
Linear Technology Corporation
For literature on our Instrumentation Amps,
call 1-800-4-LINEAR. For applications help,
call (408) 432-1900, Ext. 2593
dn182f LT/TP 0698 370K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408)432-1900 ● FAX: (408) 434-0507 ● www.linear-tech.com
 LINEAR TECHNOLOGY CORPORATION 1998
www.BDTIC.com/Linear