## Tuesday, June 28, 2011

### DEISGN OF INSTRUMENTATION AMPLIFIER

DEISGN OF INSTRUMENTATION AMPLIFIER

AIM :

To construct an Instrumentation amplifier using LF356 for a gain of 250, which amplifies the voltage of thermocouple circuit.

APPARATUS REQUIRED:

1. Instrumentation amplifier
2. Regulated power supply.
3. Thermocouple
4. CRO

DESIGN :

Consider fig(i)
At node 1:
Vo1-v1/r2 + (v2-v1 )/r1=0
R1*vo1 +r2 *v2-v1(r1+R2)=0
Vo1=v1(1+r2/r1)-v2*r2/r1--------------(1)

At node 2:
Vo2-v2/r2 + (v1-v2 )/r1=0
R1*vo2 +r2 * v1-v2(r1+R2)=0
Vo2=v2(1+r2/r1)-v1*r2/r1---------------(2)
Output voltage of the instrumentation amplifier,
Vout=( Vo2 – Vo1)*(r3/r)------------------(3)
Substituting for Vo2 & Vo1 from equations (1),(2)and (3)
Vout-[v2(1+r2/r1)-v1*r2/r1 v1(1+r2/r1)+v2*r2/r1)]*(r3/r)
As DV=v2-v1, therefore
Vout /  DV= R3 / R(1+2*R2 / R1)
This is gain equation for instrumentation amplifier.
R3 / R is assumed to be 10 by choosing R3 = 100 KW , R = 10 KW. The two R3’s are matched for the same value. The two R2’s are matched using 120 KW resistances and the R1 value is chosen to be 10 KW and thus the gain  obtained is 250.

theory:

In a number of industrial and consumer applications physical quantities such as temperature, pressure, light intensity are to be measures and controlled. These physical quantities are measured with the help of transducers has to be amplified so that it can drive the display system. This function is performed by an instrumentation amplifier

CIRCUIT DIAGRAM :

# PIN DIAGRAM OF LF356:

The important features of instrumentation amplifier are:
1. High Gain Accuracy
2. High CMRR
3. High Gain Stability With Low Temperature Coefficient
4. Low Dc Output
5. High Output Impedance

PROCEDURE:

1.      Connections are given as per the circuit diagram.
2.      The voltage from the bridge type transducer part (thermocouple) is amplified by the instrumentation amplifier.
3.      The amplified output voltage is noted for different temperature values.
4.      A graph is plotted between the temperature and the amplified voltage.

RESULT:

Thus the instrumentation amplifier was designed and the graph is plotted.