AD623 Instrumentation Amplifier IC

AD623 Instrumentation Amplifier IC

The AD623 is instrumentation amplifier IC which can work on single or dual supply. It has single gain set resistor programming. It has 8 pins. Its gain can be varied from 1 to 1000. With no external resistance, it can be configured as unity gain amplifier.

The various features of AD623 are,

1. It provides rail to rail output swing.

2. It is very easy to use.

3. Single and dual supply operation is possible.

4. Gain setting with only one external resistor is possible.

5. Gain variation from 1 to 1000 is possible.

6. Its accuracy is very high.

7. Its power consumption is low.

8. It has a wide input common mode range and amplifies signals with common mode voltages as low as 150 mV below ground.

9. It has a bandwidth of 800 kHz for gain G = 1.

10. It offers superior linearity, temperature stability and reliability in a minimum space.

11. Its a.c. CMRR increases with increasing gain which keeps the errors to minimum.

The Fig. 5.1.1 shows its pin diagram.

1. Function Block Diagram

The Fig. 5.1.2 shows the functional block diagram of IC AD623 instrumentation amplifier.

It uses basic 3 op-amp circuit approach. The input signal is applied to p-n-p transistors acting as voltage buffers and providing common mode input to the input amplifiers. The 50 kΩ resistor in the feedback of each amplifier assures the programmability of gain.

The differential output is given by,

 The differential voltage is then converted to single ended voltage using the output amplifier. This amplifier rejects any common mode signal at the output of the input amplifiers. The output voltage at pin 6 is measured with respect to the potential of pin 5. The gain is programmed by connecting external resistor R_G between the pins 1 and 8.

The various applications of AD623 are,

1. Industrial process control   

2. Light intensity meter

3. Thermocouple amplifiers   

4. Low power medical instrumentation

5. Temperature controllers and indicators

6. Low power data acquisition system

7. Difference amplifiers 

8. Transducer interfaces.

2. Load Cell Weight Measurement

Strain gauge load cells are the most common type of transducer used for the analog weight measurement. A load cell usually consists of four strain gauges in a Wheatstone bridge configuration.

The arrangement of load cell weight measurement is shown in the Fig. 5.1.3.

The elements are mounted on the base of the weight platform. When the weight is placed on the platform the strain gauges in the opposite arm elongate while the strain gauges in other two opposite arms get compressed.

This means R_T1 and R_T3 resistances decrease. While R_T2 and R_T4 resistances increase. Under no weight on the platform, the bridge is balanced and R_T1 = R_T2 = R_T3 = R_T4 = R and the bridge output is 0 V. Under unbalanced condition, the change in resistance is ΔR ohms. So increase of ΔR for R_T2 and R_T4 while decrease of ΔR for R_T1 and R_T3.

So the expressions for V_a and V_b are,

Hence the net output voltage is,

V_o = AΔR V_dc / R  ... (5.1.5)

The output voltage is proportional to the change in the resistance, which intum depends upon the applied weight. The meter connected at the output can be calibrated in kilograms.

Review Questions

1. Write a note on AD623 instrumentation amplifier IC.

2. Draw and explain the functional block diagram of AD623 instrumentation amplifier IC.

3. Explain the application of AD623 as load call weight measurement.