0.2 Operational amplifiers and signal conditioning  (Page 14/5)

Introduction:

In this lab, you will build several basic op-amp circuits to learn about the signal conditioning of sensorsignals.

Teaching objectives

• Understand how an op-amp works.
• Learn about different types of op-amp circuits and their uses.
• Gain practical experience in implementing op-amp circuits.
• Build a voltage divider, an inverter, a differential amplifier, a gain circuit, and low-pass filter circuits.
• Demonstrate the filtering capability of the low-pass filter and its use to prevent aliasing of sampled data.

Preliminary reading

Figliola and Beasley

Analog Signal Conditioning: 211-226 (Sections 6.6-6.8)

Component identification

In this lab, we will be using several types of electronic components: resistors, capacitors, trim potentiometers,and op-amps.

Resistors

Resistors come in a wide range of resistances. For op-amp circuits, values in the range of 10 k ohm to 250 k ohmwork best. For this lab, we will use primarily 10 k ohm and 100 k ohmresistors and maybe a few others. Resistors are color coded so that with some practice you can decipher resistance values from thecolor bands on the resistors you are using.

Capacitors

Capacitors also come in a wide range of values and are made using a variety of techniques. The capacitors that wewill use for signal conditioning are fairly small and have capacitances measured in Pico farads or microfarads. Somecapacitors are polarized and care must be taken to ensure that they are connected to a circuit with the proper orientation.

Trim potentiometers

A trim potentiometer is a variable resistor. It has three leads.

• Positive power supply
• Wiper
• Reference power

The resistance of the trim pot is measured between the wiper and one of the other two leads. A trim-pot isdepicted schematically in Figure 1. An adjustment screw moves the wiper along the length of a coil resistor. The resistance betweenlead 1 and the wiper increases or decreases as the wiper moves further or closer.

The output voltage (Vout ) is related to the input voltages by the following relationship:

Operational amplifier (op-amp)

For this lab, we will use the LM741 op-amp. The LM741 is a very common op-amp package, with one op-amp perintegrated circuit (IC). (Many op-amp packages have two or four op-amps per IC.) A schematic representation of the LM 741 is shownin Figure 3. The semi-circular depression on the chip marks the top of the chip and the pins are numbered counterclockwise around thechip. The highest number pin is always opposite pin 1.

LM741 Pinout:

• Null offset adjustment (not used)
• Inverting (-) input
• Non-inverting (+) input
• Connect to -12 V
• Null offset adjustment (not used)
• Output
• Connect to +12 V
• Not used

Data sheets for this and other integrated circuits (IC's) can be obtained from the National Semiconductor website (www.national.com).

The prototyping board

You will build circuits for this lab using a solderless prototyping board known as a proto-board or breadboard.Breadboards allow us to connect circuits by plugging in components and wires. No soldering is required. A drawing of a breadboard isshown in Figure 4 below. As indicated by the gray lines, horizontal rows of connectors in areas B and C are internally connected. Whena chip is plugged into the board as shown, these strips give you four connections to each of the IC pins. The connections in regionsA and D are intended as power and ground buses. As indicated by the gray lines, the columns in these regions are tied togethervertically.