In this tutorial I will discuss the Operational Amplifier as the Instrumentation Amplifier. In the previous post I have discussed about the operational amplifier as the comparator, inverting and non-inverting amplifier. The operational amplifier can also be used as the instrumentation amplifier which is the most common type of the amplifier used in the Data Acquisition Systems, Data processing System and Signal Conditioning circuits. In this post I will discuss the working of the Instrumentation Amplifier, its implementation by the Operational Amplifier, and the quirks that make it so useful in these fields. Integrated instrumentation amplifiers are high-quality op amps that contain internal precision feedback networks. They are ideal for measuring low-level signals in noisy environments without error, and amplifying small signals in the midst of high common-mode voltages. Integrated instrumentation amplifiers are well suited for direct connection to a wide variety of sensors such as strain gages, thermocouples, RTDs, current shunts, and load cells. They are commonly configured with three op amps – two differential inputs and one differential output amplifier.
After reading this post you will learn about the Operational Amplifier as the Instrumentation Amplifier, designing of the Instrumentation Amplifier to meet our customized requirements, applications of the Instrumentation Amplifier. So sit back keep reading and enjoy learning.
WHAT IS Instrumentation Amplifier:
Among the most common applications of the Operational Amplifier, the Instrumentation Amplifier is the most commonly known type of implementation using the Operational Amplifier. Instrumentation Amplifier is basically the Differential Amplifier with inputs connected to the Buffer Amplifiers. The Buffer Amplifiers are connected to the differential amplifier to give the high input impedance to the overall amplifier. The differential amplifier with resistors connected in the feedback network has low input impedance due to the resistors which although control the infinite gain but gives the effect of the low input impedance due to the alternate path generated because of the resistors feedback network so the effect of this low input impedance is catered using the buffer amplifiers connected to the inputs of the instrumentation amplifier. So the main purpose of the Operational Amplifier as the Instrumentation Amplifier is to amplify the differential signal along with the high input impedance. I have discussed in detail the advantages of having the input impedance in my previous posts. As a summary here the high input impedance is crucial for measuring the weak signals so that the input of the Amplify draw as low current as possible and thus not attenuate the signal. Also note here that the Output of the Instrumentation Amplifier is actually the output of the Operational Amplifier so it have low Output Impedance and thus the voltage drop will not occur at the Output no matter what current is taken out from the Operational Amplifier.
Operational Amplifier as Instrumentation Amplifier:
Let us now see the Operational Amplifier as the Instrumentation in some detail. The Instrumentation Amplifier can be implemented using three Operational Amplifiers in which two of the three Operational Amplifiers are used as the buffer amplifiers and one Operational Amplifier acts as the Differential Amplifier. The circuit for the Operational Amplifier based Instrumentation Amplifier is shown in the figure below:
The following figure shows the buffer amplifier connected to the differential amplifier to make the Instrumentation Amplifier. Note here that the Buffer Amplifier is used here along with the feedback network of resistors. I have mentioned in the discussion above that the buffer amplifier is used in the unity feedback configuration but it is always desirable to provide some gain in the buffer amplifier stage so the total burden of the Differential Amplifier portion is reduced and it can serve well for Electromagnetic Interference (EMI) removal from the signal.
The Buffer Amplifier can also be used in the Instrumentation Amplifier in the unity feedback configuration. That is the input signal is passed without any amplification. The Operational Amplifier as the Instrumentation Amplifier is most commonly used type of amplifier. You must have notice here that the Differential Amplifier in the Instrumentation Amplifier is a combination of both the Inverting and Non-Inverting Operational Amplifier configuration.
After understanding the Operational Amplifier as the Instrumentation let us now see the transfer function that defines the input / output relation of the Operational Amplifier as the Instrumentation Amplifier. Note here that the Buffer Amplifier will not have any effect on the amplification of the input signal or any other effect, the buffer amplifier only is used here to provide high input impedance to the input signal source and the relation between the input and output signal majorly depends on the Differential Amplifier portion of the Instrumentation Amplifier. The relation between the input and output of the Operational Amplifier as the
Application of Instrumentation Amplifier:
Instrumentation Amplifier is shown below:
The important consideration while designing the Instrumentation Amplifier is that the gain of both the Inverting and Non-Inverting sections of the Differential Amplifier should be exactly matched. This can be done by appropriately choosing the values of the resistors in the feedback network of the Inverting and Non-Inverting Amplifiers portion of the Differential Amplifier. As a rule of thumb remember the following analogy:
That is all for now I hope this post would be helpful for you. In the next post I will come up with more interesting applications of the Operational Amplifier. Till then stay connected. Keep reading and enjoy learning.