Instrumentation Amplifier: Active Bridge

Vo

-

+

RFR5

V1V2

R’5 R’F

RL

-

+

-

+

V1

V2

R4 =R

R3 =R

R2 =R

R1 =R + ΔR

Vref

T°

Instrumentation amplifier used to sense temperature changes

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

• R1 = R2 = R3 = R4 = 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

Vo

-

+

Vref Twisted PairShielding

Instrumentation amplifier

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

Vo

-

+

RFR2V1

V2

R’2 R’F

RG

-

+

-

+

V1

V2

R1

R’1

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

National Semiconductor

LH0036

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

Vo

-

+

LH0036RG

VCM

Vin

VCM

2

9

5

6

47

11

Guard Drive

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

ktword.co.kr

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 = V2

– VR1 = Vin

– I1 = Vin/R1 = IL

– Transconductance gm = 1/R1

– IL = gm Vin

Voltage-controlled current sources (VCIS)

-

+

Load

R1IL

Vin

I1

-

+

LoadR2

IL

Vin

I1

V2

V1

INVERTING NONINVERTING

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

Vin

I1

R1

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

RB =RF

IF

Iin

I1

RL

Vo

ICVS Photodiode Light Sensor

• Photodiodes and phototransistors are modeled as current sources

• Circuit used in fiber optic data communication systems

ICVS Photodiode Light Sensor

-

+

RF

RB =RF

IF

Is

RL

Vo

+V

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

Vo

-

+

R3

R1

Vin

R2 R4

RLRB

I1

I2 I4I3