DN40 - Designing with a New Family of Instrumentation Amplifiers

Designing with a New Family of Instrumental Amplifiers
Design Note 40
Jim Williams
A new family of IC instrumentation amplifiers achieves
performance and cost advantages over other alternatives. Conceptually, an instrumentation amplifier is
simple. Figure 1 shows that the device has passive, fully
differential inputs, a single ended output and internally
set gain. Additionally, the output is delivered with respect
to the reference pin, which is usually grounded. Maintaining high performance with these features is difficult,
accounting for the cost-performance disadvantages
previously associated with instrumentation amplifiers.
Figure 2 summarizes specifications for the amplifier
family. The LTC®1100 has the extremely low offset, drift,
and bias current associated with chopper stabilization
techniques. The LT®1101 requires only 105μA of supply
current while retaining excellent DC characteristics. The
+
–
q NO FEEDBACK RESISTORS USED
q GAIN FIXED INTERNALLY (TYP 10 OR 100)
OR SOMETIMES RESITOR PROGRAMMABLE
q BALANCED, PASSIVE INPUTS
q
OUTPUT
DELIVERED WITH RESPECT TO
OUTPUT
DN040 F01
OUTPUT REFERENCE PIN
REFERENCE
Figure 1. Conceptual Instrumentation Amplifier
PARAMETER
Offset
Offset Drift
Bias Current
Noise (0.1Hz-10Hz)
Gain
Gain Error
Gain Drift
Gain Nonlinearity
CMRR
Power Supply
Supply Current
Slew Rate
Bandwidth
CHOPPER
STABILIZED MICROPOWER HIGH SPEED
LTC1100
LT1101
LT1102
10μV
100nV/°C
50pA
2μVp-p
100
0.03%
4ppm/°C
8ppm
104dB
Single or
Dual,
16V Max
2.2mA
1.5V/μs
8kHz
160μV
2μV/°C
8nA
0.9μV
10,100
0.03%
4ppm/°C
8ppm
100dB
Single or Dual,
44V Max
500μV
2.5μV/°C
50pA
2.8μV
10,100
0.05%
5ppm/°C
10ppm
100dB
Dual,
44V Max
105μA
0.07V/μs
33kHz
5mA
25V/μs
220kHz
Figure 2. Comparison of The New IC Instrumentation
Amplifiers
10/90/40_conv
FET input LT1102 features high speed while maintaining
precision. Gain error and drift are extremely low for all
units, and the single supply capability of the LTC1100
and LT1101 is noteworthy.
The classic application for these devices is bridge
measurement. Accuracy requires low drift, high common mode rejection and gain stability. Figure 3 shows
a typical arrangement with the table listing performance
features for different bridge transducers and amplifiers.
Bridge measurement is not the only use for these devices. They are also useful as general purpose circuit
components, in similar fashion to the ubiquitous op
amp. Figure 4 shows a voltage controlled current source
with load and control voltage referred to ground. This
simple, powerful circuit produces output current in
strict accordance with the sign and magnitude of the
control voltage. The circuit’s accuracy and stability are
almost entirely dependent upon resistor R. A1, biased
by VIN, drives current through R (in this case 10Ω) and
the load. A2, sensing differentially across R, closes a
loop back to A1. The load current is constant because
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of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
+VBIAS
+
OUT
–
BRIDGE TRANSDUCER AMPLIFER VBIAS COMMENTS
350 Strain Gage
(BLH #DHF –350)
LTC1100
10V Highest Accuracy,
30mA Supply
Current
1800Ω Semiconductor
(Motorola MPX2200AP)
LT1101
1.2V Lower Accuracy
& Cost. <800μA
Supply Current
Figure 3. Characteristics of Some Bridge TransducerAmplifier Combinations
A1
+
VIN
0q±10V
A = 5V/DIV
LT1006
–
0.05μF
B = 5mA/DIV
IK =
10k
A2
+
IK
R*
10Ω
LT1102
A = 100
–
VIN
R w 100
LOAD
HORIZ = 20μs/DIV
DN040 F05
DN040 F04
Figure 5. Dynamic Response of the Current Source
* = PRECISION FILM TYPE
Figure 4. Voltage Programmable Current Source
is Simple and Precise
15V
27k
15V
10k*
+
A1A
1/2 LT1078
LT1009
2.5V
274k*
–
250k*
0.1μF
15V
2k
50k
ZERO
+
A2
LT1101
A = 10
88.7Ω*
–
*1% FILM RESISTOR
Rp = ROSEMOUNT 118MFRTD
TRIM SEQUENCE:
SET SENSOR TO 0°C VALUE ADJUST ZERO
FOR 0V OUT. SET SENSOR TO 100°C VALUE.
ADJUST GAIN FOR 2.500V OUT. SET SENSOR
TO 400°C VALUE. ADJUST LINEARITY FOR
10.000V OUT. REPEAT AS REQUIRED.
Rp
100Ω AT
0°C RTD
–
A3
LT1101
+A = 10
+
5k
LINEARITY
A1B
1/2 LT1078
–
8.25k*
0V-10VOUT =
0°C-400°C ±0.05°C
2k
GAIN
13k*
10k*
DN040 F06
Figure 6. Linearized Platinum RTD Bridge. Feedback to Bridge from A3 Linearizes the Circuit
A1’s loop forces a fixed voltage across R. The 10k-0.5μF
combination sets rolloff, and the configuration is stable.
Figure 5 shows dynamic response. Trace A is the voltage control input while trace B is the output current.
Response is clean, with no slew residue or aberrations.
A final circuit, Figure 6, combines the current source and
a platinum RTD bridge to form a complete high accuracy
thermometer. A1A and A2 will be recognized as a form
of Figure 4’s current source. The ground referred RTD
sits in a bridge composed of the current drive and the
LT1009 biased resistor string. The current drive allows
the voltage across the RTD to vary directly with its
temperature induced resistance shift. The difference
between this potential and that of the opposing bridge
leg forms the bridge output. The RTD’s constant current drive forces the voltage across it to vary with its
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resistance, which has a nearly linear positive temperature coefficient. The non-linearity could cause several
degrees of error over the circuits 0°C-400°C operating
range. The bridges output is fed to instrumentation
amplifier A3, which provides differential gain while
simultaneously supplying non-linearity correction. The
correction is implemented by feeding a portion of A3’s
output back to A1’s input via the 10k-250k divider. This
causes the current supplied to Rp to slightly shift with
its operating point, compensating sensor non-linearity
to within ±0.05°C. A1B, providing additional scaled gain
furnishes the circuit output. To calibrate this circuit,
follow the procedure given in Figure 6.
Details of these and other instrumentation amplifier
circuits may be found in LRC Application Note 43,
“Bridge Circuits – Marrying Gain and Balances.”
For applications help,
call (408) 432-1900
dn40f_conv IM/GP 1090 • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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© LINEAR TECHNOLOGY CORPORATION 1990
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