Floyd, Digital Fundamentals, 10th ed

EET 2259 Unit 12Data Acquisition

Read Bishop, Chapter 8.

Lab #12 and Homework #12 due next week.

Floyd, Digital Fundamentals, 10th ed

Data Acquisition

The text’s Chapter 8 covers several kinds of material:

Data acquisition hardware (Sections 8.1 to 8.5): Not much on LabVIEW in these sections.

LabVIEW data acquisition without the DAQ Assistant (Sections 8.6 to 8.8): This is how you had to do it before LabVIEW version 7.0, and how some people still prefer to do it.

LabVIEW data acquisition using the DAQ Assistant (Sections 8.9 to 8.12): You’re already familiar with much of this.

Floyd, Digital Fundamentals, 10th ed

Data Acquisition Hardware Outline:

Components of a DAQ System (§8.1) Types of Signals (§8.2) Transducers and Signal Conditioning (§8.3) Signal Grounding and Measurements (§8.4) Analog-to-Digital Conversion (§8.5)

Each of these topics could occupy us for a week, so we’ll just hit the high points.

For lots more detail, see NI’s website: LabVIEW Measurements Manual Measurement Fundamentals Main Page

Floyd, Digital Fundamentals, 10th ed

Components of a DAQ System (§8.1)

Image from NI’s website

Floyd, Digital Fundamentals, 10th ed

Types of Signals (§8.2)

Image from LabVIEW Measurements Manual

Floyd, Digital Fundamentals, 10th ed

Transducers and Signal Conditioning (§8.3)

A transducer (or sensor) is a device that converts a physical quantity (such as temperature, pressure, light intensity, …) to an electrical signal (such as voltage or current).

Floyd, Digital Fundamentals, 10th ed

Some Common Transducers

Image from LabVIEW Measurements Manual

Floyd, Digital Fundamentals, 10th ed

Signal Conditioning In many cases the electrical signal

coming from the transducer must be conditioned in some way before it is converted into digital format for use by the computer.

Examples of signal conditioning include: Amplifying a weak signal Filtering out noise from the signal

Floyd, Digital Fundamentals, 10th ed

Signal Conditioning For Some Transducers

Image from LabVIEW Measurements Manual

Floyd, Digital Fundamentals, 10th ed

Signal Grounding and Measurements (§8.4)

Signal sources can be either: Grounded (also called referenced) Floating (also called non-referenced)

Floyd, Digital Fundamentals, 10th ed

Grounded Signal Source Examples:

Function generator or power supply whose negative terminal is tied to earth ground through the power cord.

Image from LabVIEW Measurements Manual

Floyd, Digital Fundamentals, 10th ed

Floating Signal Source Examples:

Our red trainer’s function generator or power supply, whose negative terminal is not tied to earth ground through the power cord.

Battery-powered devices Transformers Thermocouples

Image from LabVIEW Measurements Manual

Floyd, Digital Fundamentals, 10th ed

Measurement System Configuration Measurement systems can be

configured as either: Differential Referenced Single-Ended (RSE) Non-Referenced Single-Ended (NRSE)

The PCI-6221 cards inside our computers can be configured in any of these modes.

The myDAQ can only be configured in differential mode.

Floyd, Digital Fundamentals, 10th ed

Differential Measurement System

Image from LabVIEW Measurements Manual

Floyd, Digital Fundamentals, 10th ed

Referenced Single-Ended (RSE) Measurement System

Image from LabVIEW Measurements Manual

Floyd, Digital Fundamentals, 10th ed

Non-Referenced Single-Ended (NRSE) Measurement System

Image from LabVIEW Measurements Manual

Floyd, Digital Fundamentals, 10th ed

Matching Signal Grounding to Measurement Configuration

Image from LabVIEW Measurements Manual.

See also pages 11-14 of NI myDAQ User Guide and Specifications

.

See also NI tutorial onGround Loops and Returns.

Floyd, Digital Fundamentals, 10th ed

Analog-to-Digital Conversion (§8.5)

Before a computer can process analog information, we must first use an analog-to-digital converter (ADC) to transform the analog values into digital binary values.

Floyd, Digital Fundamentals, 10th ed

• The ADC periodically samples the analog signal, and converts each sampled value of the analog signal into a binary code.

• An ADC’s sampling rate tells how often the circuit “takes a snapshot” of the signal being digitized.

• It’s expressed either as number of samples per second or as a frequency.• Example: 1000 samples/sec or 1 kHz.

Sampling Rate

• The more bits that are used to represent the value of each smaple, the better the ADC’s resolution is. Better resolution results in a more accurate representation of the original signal.

• The following slides (from Floyd’s Digital Electronics) show an example of how using 4 bits results in much better resolution than using 2 bits.

Number of Bits and Resolution

Copyright ©2009 by Pearson Higher Education, Inc.Upper Saddle River, New Jersey 07458

All rights reserved.

Digital Fundamentals, Tenth EditionThomas L. Floyd

Figure 12.8 Light gray = original waveform. Blue = Reconstructed waveform using four quantization levels (2 bits).

Copyright ©2009 by Pearson Higher Education, Inc.Upper Saddle River, New Jersey 07458

All rights reserved.

Digital Fundamentals, Tenth EditionThomas L. Floyd

Figure 12.10 Light gray = original waveform. Blue = Reconstructed waveform using sixteen quantization levels (4 bits).

• There are several common ways of specifying an ADC’s resolution:• Number of bits, n• Number of output codes, = 2n

• Step size (which Bishop calls code width), = Vref / 2

n

Resolution

Resolution: Examples

Formula 4-bit ADC 10-bit ADCNumber of bits n 4Number of output codes

2n 16Step size (assuming 5 V reference voltage)

Vref / 2n 312.5 mV

• Let’s find the resolution and sampling rate for the ADCs inside our equipment.• NI myDAQ User Guide and Specifications• NI PCI-6221 Specifications

Specs for Our Equipment

LabVIEW Data Acquisition without the DAQ Assistant (§§8.6 – 8.8)

Before the DAQ Assistant was introduced, LabVIEW programmers had to use the functions on the Measurement I/O > NI-DAQmx palette to perform data acquisition.

Express VIs Express VIs (the DAQ Assistant is one

example) were introduced in LabVIEW 7.0.

They provide user-friendly ways to do things that you could also do using other LabVIEW functions and subVIs.

Seeing the Code Hidden Inside an Express VI

You can convert an Express VI to standard LabVIEW code by right-clicking and selecting “Open Front Panel.”

In the case of a DAQ Assistant, you can also right-click and select “Generate NI-DAQmx Code.”

LabVIEW Data Acquisition with the DAQ Assistant (§§8.9 – 8.12)

You already know much of what is discussed in these sections, since you’ve been using the DAQ Assistant since the start of this class.

Read these sections to review and strengthen your knowledge.