DN182 - The LT1167: Single Resistor Sets the Gain of the Best Instrumentation Amplifier

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 gain-set 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 1000-4-2
level 2 specification.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
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
06/98/182_conv
5V
10μF
10μF
+
+
0.1μF
0.1μF
5V
0.1μF
3
+
1k
1
LT1167
8
5
–
2
2
AIN
LTC1400
1
VCC
DOUT 5
CLK 6
CX
4
GND
0.1μF
VSS
4
– 5V AV = 1
AV = 10
SERIAL
INTERFACE
CONV 7
8
R1 = ∞
CX = 0.047μF – 5V
R1 = 5.6kΩ CX = 0.030μF
0.1μF
10μF
DN182 F02
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.
Figure 2. The LT1167’s Dynamic Performance Allows It to
Convert Differential Signals into Single-Ended Signals for
the 12-Bit LTC1400
VS
SUPPLY VOLTAGE = ±5V
fIN = 3kHz
fSAMPLE = 400ksps
SINAD = 70.6dB
LT1167 GAIN = 1
–20
–40
2
6
LT1167
1
VIN–
7
+
8
RG
0
AMPLITUDE (dB)
3
VIN+
–
+
R1
VX
–
5
4
– VS
IL
LT1464
–60
+
R1
VREF
7
+
3
output stage can easily drive the 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.
–
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
[( ) ( )]
V IN+ – V IN– G
R1
VX
=
R1
G = 49.4k Ω + 1
RG
IL=
–80
–100
LOAD
DN182 F4a
(a)
–120
0
25
50
75 100 125 150 175 200
FREQUENCY (kHz)
DN182 F03a
10 – 2
10 – 3
(a)
SUPPLY VOLTAGE = ±5V
fIN = 5.5kHz
fSAMPLE = 400ksps
SINAD = 70.6dB
LT1167 GAIN = 10
AMPLITUDE (dB)
–20
–40
–60
10 – 5
R1 =
10 – 6 1k
10 – 7 10k
10 – 8 100k
10 – 9 1M
10 – 10 10M
–80
10 –11
10 – 4
–100
–120
10 – 4
LOAD CURRENT (A)
0
V + = 15V
V – = – 15V
RG = ∞
10 – 3
10 – 2
10 –1
1
INPUT VOLTAGE (VIN+ – VIN–) (V)
(b)
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
Data Sheet Download
www.linear.com
Linear Technology Corporation
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
For applications help,
call (408) 432-1900
dn182f_conv LT/TP 0698 370K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 1998
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