Instrumentation Amplifier | Isolation Amplifier | IEEE-488 GPIB Bus

 Instrumentation Amplifier

Instrumentation amplifier is a kind of differential amplifier with additional input buffer stages. 

The addition of input buffer stages makes it easy to match (impedance matching) the amplifier with the preceding stage.

Instrumentation amplifiers are commonly used in industrial test and measurement application.

The instrumentation amplifier also has some useful features like low offset voltage, high CMRR (Common mode rejection ratio), high input resistance, high gain etc.

The circuit diagram of a typical instrumentation amplifier using opamp is shown below.

The op-amps 1 & 2 are non-inverting amplifiers and together form an input stage of the instrumentation amplifier.

The op-amp 3 is a difference amplifier that forms the output stage of the instrumentation amplifier. 

Working of Instrumentation Amplifier:

The output stage of the instrumentation amplifier is a difference amplifier, whose output Vout is the amplified difference of the input signals applied to its input terminals. If the outputs of op-amp 1 and op-amp 2 are Vo1 and Vo2 respectively, then the output of the difference amplifier is given by,

            Vout = (R3/R2)(Vo1-Vo2)

The expressions for Vo1 and Vo2 can be found in terms of the input voltages and resistances. Consider the input stage of the instrumentation amplifier as shown in the figure below

The potential at node A is the input voltage V1. Hence the potential at node B is also V1, from the virtual short concept. Thus, the potential at node G is also V1.

The potential at node D is the input voltage V2. Hence the potential at node C is also V2, from the virtual short. Thus, the potential at node H is also V2.

Ideally the current to the input stage op-amps is zero. Therefore the current I through the resistors R1, Rgain and R1 remains the same.

Applying Ohm’s law between the nodes E and F,

            I = (Vo1-Vo2)/(R1+Rgain+R1) ——————— 1

            I = (Vo1-Vo2)/(2R1+Rgain)

Since no current is flowing to the input of the op-amps 1 & 2, the current I between the nodes G and H can be given as,

            I = (VG-VH)/Rgain = (V1-V2)/Rgain ——————- 2

Equating equations 1 and 2,

            (Vo1-Vo2)/(2R1+Rgain) = (V1-V2)/Rgain

            (Vo1-Vo2) = (2R1+Rgain)(V1-V2)/Rgain ———— 3

The output of the difference amplifier is given as, 

            Vout = (R3/R2) (Vo1-Vo2)

Therefore, (Vo1 – Vo2) = (R2/R3)Vout

Substituting (Vo1 – Vo2) value in the equation 3, we get 

            (R2/R3)Vout = (2R1+Rgain)(V1-V2)/Rgain

        i.e. Vout = (R3/R2){(2R1+Rgain)/Rgain}(V1-V2)

The above equation gives the output voltage of an instrumentation amplifier.

The overall gain of the amplifier is given by the term (R3/R2){(2R1+Rgain)/Rgain}.

The overall voltage gain of an instrumentation amplifier can be controlled by adjusting the value of resistor Rgain.

Advantages of Three Op-amp Instrumentation Amplifier

Gain of a three op-amp instrumentation amplifier circuit can be easily varied and controlled by adjusting the value of Rgain without changing the circuit structure.

The gain of the amplifier depends only on the external resistors used. Hence, it is easy to set the gain accurately by choosing the resistor values carefully.

The input impedance of the instrumentation amplifier is dependent on the non-inverting amplifier circuits in the input stage. The input impedance of a non-inverting amplifier is very high.

The output impedance of the instrumentation amplifier is the output impedance of the difference amplifier, which is very low.

The CMRR of the op-amp 3 is very high and almost all of the common mode signal will be rejected.

Isolation Amplifier

An isolation amplifier or a unity gain amplifier provides isolation from one fraction of the circuit to another fraction. So, the power cannot be drawn, used and wasted within the circuit.

The main function of this amplifier is to increase the signal. The same input signal of the op-amp is passed out exactly from the op-amp as an output signal.

These amplifiers are used to give an electrical safety battier as well as isolation.

What is an Isolation Amplifier?

An isolation amplifier can be defined as, an amplifier which doesn’t have any conductive contact among input as well as output sections.

Consequently, this amplifier gives ohmic isolation among the i/p & o/p terminals of the amplifier. This isolation must have less leakage as well as a high amount of dielectric breakdown voltage.

How to Achieve Isolation?

When the input impedance of an op-amp is extremely high then the isolation can be caused.

As this circuit includes high input impedance, then minute current can be drawn from the amplifier circuit

According to Ohms law, when the resistance is high, then the current will be drawn less from the power supply.

Therefore, an op-amp does not draw a significant quantity of current from the power source. So in practice, there is no current will be drawn as well as transferred from one portion to another portion of the circuit. Therefore, this amplifier works as an isolation device.

When the input impedance of an op-amp is low then it draws a vast amount of current. Ohms law states that, if load impedance has less resistance, then it draws huge current by the source of power so that high disturbances can be caused, and this is quite opposite to isolation.  Here, isolation amplifier works like a buffer and they do not strengthen signals although provide to isolate divisions of circuits.

IEEE-488 GPIB Bus

GPIB or General Purpose Interface Bus or IEEE 488 bus is still one of the most popular and versatile interface standards available today.

GPIB is widely used for enabling electronics test equipment to be controlled remotely, although it was also used in a many other applications including general computer communications.

It can be used to control a host of test instruments: everything from digital multimeters and signal generators of all sorts to switching matrices, spectrum analyzers, vibration meters . . . in fact any form of electronics test equipment. At one time it even became popular for linking computers to their printers and many low cost printers used GPIB.

Basic GPIB concept

GPIB or IEEE 488 bus is a very flexible system, allowing data to flow between any of the instruments on the bus, at a speed suitable for the slowest active instrument.

Up to fifteen instruments may be connected together with a maximum bus length not exceeding 20 m.

A further requirement for the bus is that there must also be no more than 2 m between two adjacent test instruments.

It is possible to purchase GPIB cards to incorporate into computers that do not have the interface fitted.

As GPIB cards are relatively cheap, this makes the inclusion of a GPIB card into the system a very cost effect method of installing it.

Have a unique address on the bus. Test instruments are allocated addresses in the range 0 to 30, and no two instruments on the same bus are allowed to have the same address. The addresses on the instruments can be changed and this may typically be done via the front panel, or by using switches often located on the rear panel.

Active extenders are available and these items allow longer buses: up to 31 devices theoretically possible, along with a greater overall length dependent upon the extender.

General Information about GPIB interface

GPIB interface, sometimes called the General Purpose Interface Bus (GPIB), is a general purpose digital interface system that can be used to transfer data between two or more devices. It is particularly well suited for interconnecting computers and instruments. Some of its key features are:

• Up to 15 devices may be connected to one bus
• Total bus length may be up to 20 m and the distance between devices may be up to 2 m
• Communication is digital (as opposed to analog) and messages are sent one byte (8 bits) at a time
• Message transactions are hardware  handshaked 
• Data rates may be up to 1 Mbyte/sec