Instrumentation Amplifier: Active Bridge

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Transcript Instrumentation Amplifier: Active Bridge

Instrumentation Amplifier: Active
Bridge
Vre
f
R1 =R +
ΔR
V2
-
R2 =R
RF
R5
+
R3 =R
T°
V1
-
V1
+
Vo
RL
R4 =R
+
R’5
V2
Instrumentation amplifier used to sense temperature changes
R’F
Instrumentation Amplifier: Active
Bridge
• Used to sense temperature changes
• Provide input to process control systems
• Due to extremely high input resistance of the
instrumentation amplifier, loading of the bridge is
essentially nonexistent
• R 1 = R2 = R3 = R 4 = R
• At 25 °C the bridge is in balance, and V1 = V2 =
Vref/2 (common-mode voltage at input of amp.)
• If CMRR is very large, Vo = 0 v
Instrumentation Amplifier: Active
Bridge
• Strain could be
determined if the
thermistor is replaced
with a strain gage
• Strain gage is resistor
whose value changes in
proportion to the strain
applied onto it
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Instrumentation Amplifier: Active
Bridge
• If the bridge environment is hostile (extreme heat,
pressure, etc.), the bridge is located at a distance
from the instrumentation amplifier
• Long connecting leads are used between bridge and
the amplifier
• Shielding of the leads is done to prevent stray
electromagnetic fields from inducing noise voltages
onto the signal lines
• Lines connected to the bridge output are also twisted
with each other leading to equal amplitudes of noise
on both lines producing a common-mode noise
signal
Common Noise Reduction Techniques
Vre
f
Shielding
Twisted Pair
Instrumentation
amplifier
-
+
Vo
Variable-Gain Instrumentation Amplifiers
• Gain of the instrumentation amplifier can be
adjusted by making minor circuit modifications
• Output voltage
Vo = (V2-V1) (RF/R2) [1+(2R1/RG)] where R1’=R1
• RG is chosen to provide desired voltage gain
• RG is replaced with a potentiometer if
continuously adjustable gain is desired
• For good CMRR, resistors RF-R’F and R2-R’2 must
be closely matched
Variable-Gain Instrumentation Amplifiers
V1
+
V1
RF
R2
R1
-
RG
R’1
+
R’2
V2
+
V2
R’F
Vo
Commercial Instrumentation
Amplifiers
• Instrumentation amplifiers can be constructed
using standard op amps and resistors
• For applications requiring very high performance,
commercially available dedicated
instrumentation amplifiers are a better choice
– e.g., LH0036 from National Semiconductor
• LH0036 features: CMRR = 100 dB, Rin = 300 MΩ,
adjustable gain, and guard drive
• Gain is set by placing a resistor of appropriate
value across pins 7 and 4
Commercial Instrumentation
Amplifiers
LH0036
National Semiconductor
Commercial Instrumentation
Amplifiers
• In LH0036 for gain adjustment, R3 = R4 = R5 = R6 and R1 = R2 =
25 kΩ
• Vo = (V2-V1) [1+(50 kΩ/RG)]
• Instrumentation amplifiers are normally used to process dc
voltages; therefore it may be desirable to limit bandwidth in
order to decrease the amplification of high-frequency noise
• Guard drive output is used to drive the input shielding to the
same potential as the common-mode voltage present at the
amplifier’s input; reducing the current leakage between input
wires and the shield
• Due to guard drive the potential difference between shield
and common-mode noise on signal lines is zero, eliminating
effects of stray capacitances
Active Guarding to Reduce Errors
Guard Drive
5
RG
Vin
VC
M
VC
M
-
2
4 LH0036
7
+
9
6
11
Vo
Amplifiers
• Voltage amplifiers
– Voltage-controlled voltage sources (VCVS)
– Av (unitless) (Vo/Vin)
• Current amplifiers
– Current-controlled current sources (ICIS)
– Ai (unitless) (io/iin)
• Transconductance amplifiers
– Voltage-controlled current sources (VCIS)
– gm (siemens) (io/Vin)
• Transresistance amplifiers
– Current-controlled voltage sources (ICVS)
– rm (Ohms) (Vo/iin)
Amplifiers
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Voltage-controlled current sources (VCIS)
• Inverting analysis
–
–
–
–
IL = - I1
I1 = Vin/R1  IL = - Vin/R1
Transconductance gm = -1/R1
IL = gm Vin
• Noninverting analysis
–
–
–
–
–
V1 = V 2
VR1 = Vin
I1 = Vin/R1 = IL
Transconductance gm = 1/R1
IL = gm Vin
Voltage-controlled current sources (VCIS)
Load
R1
-
IL
R2
Load
I1
V1
-
I1
Vin
+
V2
Vin
INVERTING
NONINVERTING
+
IL
Howland Current Source
• Floating load current sources (VCIS seen
before) perform quite well
• Often the load must be referred to ground:
Howland Current Source
• IL = - Vin/R (equal value resistors)
• gm = -1/R
• IL = gm Vin
Howland Current Source
R1
IF
R1
I1
Vin
+
R3
R4
IL
LOAD
Current-controlled voltage sources (ICVS)
•
•
•
•
•
•
For low-power applications
-IF = I1 = Iin
Vo= IFRF  Vo= -IinRF
Transresistance rm = RF
IL = rm Vin
Bias current compensation resistor RB = RF to
minimize output offset voltage
Current-controlled voltage sources (ICVS)
RF
IF
I1
Iin
+
Vo
RL
RB =RF
ICVS Photodiode Light Sensor
• Photodiodes and phototransistors are
modeled as current sources
• Circuit used in fiber optic data communication
systems
ICVS Photodiode Light Sensor
+V
RF
IF
Is
+
Vo
RL
RB =RF
Voltage Amplifier Variation
•
•
•
•
•
•
•
I1 = Vin/R1
I2 = - I1 = -Vin/R1
I2R2 = I3R3
I3 = I2R2/R3 = -(VinR2)/R1R3
I4 = I2 + I3
Vo = I4R4 + I2R2 = I4R4 + I3R3
Vo = I4R4 – I1R2 = (I2 + I3)R4 – VinR2/R1
= R4 {-(VinR2)/R1R3-(Vin/R1 )} – VinR2/R1
• Av = Vo/Vin = -{(R2R4/R1R3)+(R4/R1)+(R2/R1)
Voltage Amplifier Variation
R2
R4
I2
I4
I3
R3
R1
Vin
+
I1
RB
Vo
RL