Instrumentation Amplifier

The operational amplifier is mostly used for arnplification, as it has very high gain and wide bandwidth. The low level signal outputs of electrical transducers often need to be amplified before further processing. This is done by the use of instrumentation amplifier. The important features of instrumentation amplifiers are

1. Selectable gain with high gain accuracy and gain linearity

2. Differential input capability with high gain CMRR.

3. High stability of gain with low tempera co-efficient.

4. Low DC offset and drifts errors referred to input.

5. Low output impedance.

    The instrumentation amplifier is often a package consisting of op-amps wired up with accurate and stable resistive feedback to give a desired gain. The instrumentation amplifier can be used directly to amplify the signals by a fixed factor because of its closed-loop configuration.

The gain accuracy, gain stability and drift performance are normally specified by the vendors, hence there is no effort required in choosing the input and feedback configuration. Depending on the configuration employed, the instrumentation amplifier will often yield a high CMRR even when the source impedance exceeds 1 MQ and when the impedance is unbalanced for a large value of variations (1 to 10 K  In actual applications, an instrumentation amplifier is a specific combination of a suitable DC operational- amplifier wired up, with feedback.

The schematic diagram of an instrumentation amplifier constructed with DC op-amp and resistive network

Many of the input specifications of op-amps are directly to determine the input specifications of instrumentation amplifiers.

     Assume

The input amplifiers A1 and A2 act as the input buffers with unity gain for common mode signal ec and with gain of (1+2R2 /R1 ) for differential signals. The high input impedance is ensured by the non-inverting configuration in which they operate. The common- mode rejection is achieved by the following stage which is connected as a differential amplifier.

The optimum common-mode rejection can be obtained by adjusting R6 or R7 ensuring that R5 /R4 = R7/R6 The amplifier A3 can also be made to have some nominal gain for the whole amplifier by an appropriate selection of R4 R5 R6 and R7

    To reduce the pickup of noise voltages in the connections between the transducer and the amplifier, the leads to the transducer are kept as short as possible and the amplified signal is transmitted to the required distance. There are situations where the low level transducer must be transmitted through some length of wires. Noise currents are introduced in these lines. There are methods of shielding the connecting wires from external signal pickup. One effective method is called guarding; A signal source connected to a differential amplifier through wires that are in close proximity to a 220V. Power lead, which capacitive couples power line frequency voltages into the signal amplifier. If the amplifier is perfect, the voltage induced in each lead is the same and the capacitance and resistance paths to the ground are identical. Thus the power line interference is connected to the ground with identical currents from each side of the line.

If the leakage resistance or capacitance is different for c relative the currents t instrument ground are from only one side of the differential line. By the addition of a shield which is connected to one side of the signal and to the instrument case, the capacitive coupled signal from the power frequency line are coupled to the shield and are safely conducted to the instrument case and to the ground. The combination of the instrument case, its ground connection and the shield extending to the signal source represents a complete Shield around the entire measuring system.

    There are situations where noise environment is so severe that conventional amplifiers cannot survive the signal levels encountered. In these situations, an isolation amplifier is used to prevent the high noise signals from being conducted to the data acquisition equipment.

Isolation amplifier

The transducer is connected in a rather conventional fashion to an Instrumentation amplifier. The output of this amplifier is fed to a balanced modulator which provides a bipolar square wave with amplitude Proportional to the signal level. The high frequency square wave is called the carrier. The modulated square wave being an AC signal with no DC level can be coupled through a transformer to a balanced demodulator. The square-Wave signal generator transformer coupled to serve as the carrier for the demodulator which removes the carrier and restores the input level. After small amount of filtering, the output of the isolation amplifier is an accurate representation of the input voltage.