Design Goals
Input ViDiff(Vi2
– Vi1) |
Output |
Supply |
ViDiff_Min |
ViDiff_Max |
VoMin |
VoMax |
Vcc |
Vee |
Vref |
–2.22 mV |
2.27 mV |
225 mV |
4.72 V |
5 V |
0 V |
2.5 V |
Strain Gauge Resistance Variation
(R10) |
Vcm
|
Gain |
115 Ω – 125 Ω |
2.39 V |
1001 V/V |
Design
Description
A strain gauge
is a sensor whose resistance varies with applied force. The change in resistance is
directly proportional to how much strain the sensor is experiencing due to the force
applied. To measure the variation in resistance, the strain gauge is placed in a
bridge configuration. This design uses a two op amp instrumentation circuit to
amplify a differential signal created by the change in resistance of a strain gauge.
By varying R10, a small differential voltage is created at the output of
the Wheatstone bridge which is fed to the two op amp instrumentation amplifier
input. Linear operation of an instrumentation amplifier depends upon the linear
operation of the primary building block: op amps. An op amp operates linearly when
the input and output signals are within the input common-mode and output-swing
ranges of the device, respectively. The supply voltages used to power the op amps
define these ranges.
Design Notes
- Resistors R5,
R6, and R7 of the Wheatstone bridge must match the
stain gauge nominal resistance and must be equal to avoid creating a bridge
offset voltage.
- Low tolerance resistors must be
used to minimize the offset and gain errors due to the bridge
resistors.
- Vex sets the
excitation voltage of the bridge and the common-mode voltage
Vcm.
- Vref biases the
output voltage of the instrumentation amplifier to mid-supply to allow
differential measurements in the positive and negative directions.
- R11 sets the gain of
the instrumentation amplifier circuit.
- R8 and R9
set the common-mode voltage of the instrumentation amplifier and limits the
current through the bridge. This current determines the differential signal
produced by the bridge. However, there are limitations on the current
through the bridge due to self-heating effects of the bridge resistors and
strain gauge.
- Make sure that R1 =
R3 and R2 = R4 and that ratios of
R2/R1 and R4/R3 are matched
to set the Vref gain to 1 V/V and maintain high DC CMRR of the
instrumentation amplifier.
- Linear operation is contingent
upon the input common-mode and the output swing ranges of the op amps used.
The linear output swing ranges are specified under the Aol test
conditions in the op amps data sheets.
- Using high-value resistors can
degrade the phase margin of the circuit and introduce additional noise in
the circuit.
Design Steps
- Select R5, R6
and R7 to match the stain gauge nominal resistance
- Choose R9 to set the
common mode voltage of the instrumentation amplifier at 2.39 V
- Calculate the gain required to
produce the desired output voltage swing
- Select R1,
R2, R3, and R4. To set the Vref gain
at 1 V/V and avoid degrading the instrumentation amplifier’s CMRR, R1
must equal R3 and R2 must equal R4.
- Calculate R11 to meet
the required gain
- Calculate the current through the
bridge
Design
Simulations
DC
Simulation Results
References
- Analog Engineer's Circuit Cookbooks
- SPICE Simulation File SBOMAU4
- TI Precision Designs TIPD170
- TI Precision Labs
- VCM vs.
VOUT plots for instrumentation amplifiers with two op
amps
Design Featured Op Amp
TLV9002 |
Vss
|
1.8 V to 5.5 V |
VinCM
|
Rail-to-rail |
Vout
|
Rail-to-Rail |
Vos
|
0.4 mV |
Iq
|
0.06 mA |
Ib
|
5 pA |
UGBW |
1 MHz |
SR |
2 V/µs |
#Channels |
1, 2, and 4 |
TLV9002 |
Design Alternate Op
Amp
OPA376 |
Vss
|
2.2 V to 5.5 V |
VinCM
|
(Vee – 0.1 V) to (Vcc – 1.3
V) |
Vout
|
Rail-to-Rail |
Vos
|
0.005 mV |
Iq
|
0.76 mA |
Ib
|
0.2 pA |
UGBW |
5.5 MHz |
SR |
2 V/µs |
#Channels |
1, 2, and 4 |
OPA376 |