To amplify the low level output signal of a transducer so that it can drive the indicator or display is a measure function of an instrumentation amplifier. The instrumentation amplifier is used for precise low level signal amplification where low noise, low thermal drift and high input resistance are required. Besides this low power consumption, high CMRR and high slew rate are desirable for superior performance. There are many instrumentation amplifier ICs which makes the circuit extremely stable and accurate however these ICs are costly because they are very precise special purpose circuits in which most of the electrical parameters such as offsets, drifts and power consumption are minimized whereas input resistance, CMRR and slew rate are optimized. Thus the requirements of an instrumentation amplifier are more rigid than those for general purpose applications. However where the requirements are not very strict a general purpose op-amps can be employed in differential mode.The op-amp as an instrumentation amplifier must satisfy the following conditions.
- The differential input arrangement should be there i.e. two inputs are available.
- Single ended output is available.
- For different transducer outputs there is a facility of adjusting the gain of the amplifier so that same amplifier can be used for different transducers.
- Input resistance should be very high to avoid loading effect. If transducer gets loaded its characteristics get changed giving error in output.
- It should have high CMRR to reduce the unwanted common noise voltages.
Instrumentation amplifier with Transducer Bridge:
The resistive bridge is formed in which one of the arms contains a transducer. The resistance of the transducer changes due to changes in the physical parameter such temperature, pressure, level, light intensity etc. This change in resistance is converted into a proportional electrical signal. After suitable amplification such a signal is applied to a display or an indicator device which shows the correct value of the physical parameter.
The typical resistive bridge with transducer in one arm is called as transducer bridge. The bridge is excited with dc voltage VDC. The output of the bridge is voltage ‘Vab’. This output of the bridge is further amplified by instrumentation amplifier to drive the display unit. The output of the bridge is calculated for two condition namely unbalanced and balanced.
I) when bridge is balanced:
Initially all the arms are having resistance ‘R’. This is called balanced or reference condition or null condition. The output of the transducer bridge is given as the voltage difference between the two points i.e. ‘a’ & ‘b’. Thus in balanced condition the output of the transducer bridge is zero.
II) When bridge is unbalanced:
When the physical quantity is changed, the resistance of the transducer also changes by a factor ± ∆R and the new transducer resistance will be (R ± ∆R). All the arms of the bridge will not have the same resistances now. This is called unbalanced condition.
Let the transducer resistance is changed by ‘+∆R’ & hence effective transducer resistance is (R + ∆R). Thus in the unbalanced condition, the output of the bridge is proportional to the change in resistance ‘∆R’. This output is amplified by the instrumentation amplifier to get the final output. Thus the output is proportional to change in resistance ‘∆R’ which is proportional to change in the physical quantity.
Instrumentation Amplifier Quiz