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Instrumentation Amplifier: Active Bridge VoVo -+-+ RFRF R5R5 V1V1 V2V2 R’5R’F RLRL -+-+ -+-+ V1V1 V2V2 R4 =R R3 =R R2 =R R1 =R + ΔR Vre f T° Instrumentation.

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Presentation on theme: "Instrumentation Amplifier: Active Bridge VoVo -+-+ RFRF R5R5 V1V1 V2V2 R’5R’F RLRL -+-+ -+-+ V1V1 V2V2 R4 =R R3 =R R2 =R R1 =R + ΔR Vre f T° Instrumentation."— Presentation transcript:

1 Instrumentation Amplifier: Active Bridge VoVo -+-+ RFRF R5R5 V1V1 V2V2 R’5R’F RLRL V1V1 V2V2 R4 =R R3 =R R2 =R R1 =R + ΔR Vre f T° Instrumentation amplifier used to sense temperature changes

2 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 = R 2 = R 3 = R 4 = R At 25 °C the bridge is in balance, and V 1 = V 2 = V ref /2 (common-mode voltage at input of amp.) If CMRR is very large, V o = 0 v

3 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

4 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

5 Common Noise Reduction Techniques VoVo -+-+ Vre f Twisted Pair Shielding Instrumentation amplifier

6 Variable-Gain Instrumentation Amplifiers Gain of the instrumentation amplifier can be adjusted by making minor circuit modifications Output voltage V o = (V 2 -V 1 ) (R F /R 2 ) [1+(2R 1 /R G )] where R 1 ’=R 1 R G is chosen to provide desired voltage gain R G is replaced with a potentiometer if continuously adjustable gain is desired For good CMRR, resistors R F -R’ F and R 2 -R’ 2 must be closely matched

7 Variable-Gain Instrumentation Amplifiers VoVo -+-+ RFRF R2R2 V1V1 V2V2 R’2 R’F RGRG V1V1 V2V2 R1R1 R’1

8 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, R in = 300 MΩ, adjustable gain, and guard drive Gain is set by placing a resistor of appropriate value across pins 7 and 4

9 Commercial Instrumentation Amplifiers National Semiconductor LH0036

10 Commercial Instrumentation Amplifiers In LH0036 for gain adjustment, R 3 = R 4 = R 5 = R 6 and R 1 = R 2 = 25 kΩ V o = (V 2 -V 1 ) [1+(50 kΩ/R G )] 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

11 Active Guarding to Reduce Errors VoVo -+-+ LH0036 RGRG VCMVCM Vin VCMVCM Guard Drive

12 Amplifiers Voltage amplifiers – Voltage-controlled voltage sources (VCVS) – A v (unitless) (V o /V in ) Current amplifiers – Current-controlled current sources (ICIS) – A i (unitless) (i o /i in ) Transconductance amplifiers – Voltage-controlled current sources (VCIS) – g m (siemens) (i o /V in ) Transresistance amplifiers – Current-controlled voltage sources (ICVS) – r m (Ohms) (V o /i in )

13 Amplifiers

14 Voltage-controlled current sources (VCIS) Inverting analysis – I L = - I 1 – I 1 = V in /R 1  I L = - V in /R 1 – Transconductance g m = -1/R 1 – I L = g m V in Noninverting analysis – V 1 = V 2 – V R1 = V in – I 1 = V in /R 1 = I L – Transconductance g m = 1/R 1 – I L = g m V in

15 Voltage-controlled current sources (VCIS) -+-+ Load R1R1 ILIL Vin I1I LoadR2R2 ILIL Vin I1I1 V2V2 V1V1 INVERTING NONINVERTING

16 Howland Current Source Floating load current sources (VCIS seen before) perform quite well Often the load must be referred to ground: Howland Current Source I L = - V in /R (equal value resistors) g m = -1/R I L = g m V in

17 Howland Current Source -+-+ R1R1 IFIF Vin I1I1 R1R1 R3R3R4R4 ILIL LOAD

18 Current-controlled voltage sources (ICVS) For low-power applications -I F = I 1 = I in V o = I F R F  V o = -I in R F Transresistance r m = R F I L = r m V in Bias current compensation resistor R B = R F to minimize output offset voltage

19 Current-controlled voltage sources (ICVS) -+-+ RFRF RB =RF IFIF Iin I1I1 RLRL VoVo

20 ICVS Photodiode Light Sensor Photodiodes and phototransistors are modeled as current sources Circuit used in fiber optic data communication systems

21 ICVS Photodiode Light Sensor -+-+ RFRF RB =RF IFIF IsIs RLRL VoVo +V

22 Voltage Amplifier Variation I 1 = V in /R 1 I 2 = - I 1 = -V in /R 1 I 2 R 2 = I 3 R 3 I 3 = I 2 R 2 /R 3 = -(V in R 2 )/R 1 R 3 I 4 = I 2 + I 3 V o = I 4 R 4 + I 2 R 2 = I 4 R 4 + I 3 R 3 V o = I 4 R 4 – I 1 R 2 = (I 2 + I 3 )R 4 – V in R 2 /R 1 = R 4 {-(V in R 2 )/R 1 R 3 -(V in /R 1 )} – V in R 2 /R 1 A v = V o /V in = -{(R 2 R 4 /R 1 R 3 )+(R 4 /R 1 )+(R 2 /R 1 )

23 Voltage Amplifier Variation VoVo -+-+ R3R3 R1R1 Vin R2R2R4R4 RLRL RBRB I1I1 I2I2 I4I4 I3I3

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