Measurement Computing USB-1208FS User manual

USB-1208FS
Analog and Digital I/O
User's Guide
Document Revision 6, March, 2010
© Copyright 2010, Measurement Computing Corporation

3
HM USB-1208FS.doc
Your new Measurement Computing product comes with a fantastic extra —
Management committed to your satisfaction!
Thank you for choosing a Measurement Computing product—and congratulations! You own the finest, and you can now enjoy
the protection of the most comprehensive warranties and unmatched phone tech support. It’s the embodiment of our mission:
To provide PC-based data acquisition hardware and software that will save time and save money.
Simple installations minimize the time between setting up your system and actually making measurements. We offer quick and
simple access to outstanding live FREE technical support to help integrate MCC products into a DAQ system.
Limited Lifetime Warranty: Most MCC products are covered by a limited lifetime warranty against defects in materials or
workmanship for the life of the product, to the original purchaser, unless otherwise noted. Any products found to be defective in
material or workmanship will be repaired, replaced with same or similar device, or refunded at MCC’s discretion. For specific
information, please refer to the terms and conditions of sale.
Harsh Environment Program: Any Measurement Computing product that is damaged due to misuse, or any reason, may be
eligible for replacement with the same or similar device for 50% of the current list price. I/O boards face some harsh
environments, some harsher than the boards are designed to withstand. Contact MCC to determine your product’s eligibility for
this program.
30 Day Money-Back Guarantee: Any Measurement Computing Corporation product may be returned within 30 days of
purchase for a full refund of the price paid for the product being returned. If you are not satisfied, or chose the wrong product by
mistake, you do not have to keep it.
These warranties are in lieu of all other warranties, expressed or implied, including any implied warranty of merchantability or
fitness for a particular application. The remedies provided herein are the buyer’s sole and exclusive remedies. Neither
Measurement Computing Corporation, nor its employees shall be liable for any direct or indirect, special, incidental or
consequential damage arising from the use of its products, even if Measurement Computing Corporation has been notified in
advance of the possibility of such damages.

USB-1208FS User's Guide
4
Trademark and Copyright Information
TracerDAQ, Universal Library, Measurement Computing Corporation, and the Measurement Computing logo are either
trademarks or registered trademarks of Measurement Computing Corporation.
Windows, Microsoft, and Visual Studio are either trademarks or registered trademarks of Microsoft Corporation
LabVIEW is a trademark of National Instruments.
CompactFlash is a registered trademark of SanDisk Corporation.
XBee and XBee-PRO are trademarks of MaxStream, Inc.
All other trademarks are the property of their respective owners.
Information furnished by Measurement Computing Corporation is believed to be accurate and reliable. However, no
responsibility is assumed by Measurement Computing Corporation neither for its use; nor for any infringements of patents or
other rights of third parties, which may result from its use. No license is granted by implication or otherwise under any patent or
copyrights of Measurement Computing Corporation.
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form by any
means, electronic, mechanical, by photocopying, recording, or otherwise without the prior written permission of Measurement
Computing Corporation.
Notice
Measurement Computing Corporation does not authorize any Measurement Computing Corporation product for use
in life support systems and/or devices without prior written consent from Measurement Computing Corporation.
Life support devices/systems are devices or systems which, a) are intended for surgical implantation into the body,
or b) support or sustain life and whose failure to perform can be reasonably expected to result in injury.
Measurement Computing Corporation products are not designed with the components required, and are not subject
to the testing required to ensure a level of reliability suitable for the treatment and diagnosis of people.

5
Table of Contents
Preface
About this User's Guide.......................................................................................................................7
What you will learn from this user's guide.........................................................................................................7
Conventions in this user's guide .........................................................................................................................7
Where to find more information.........................................................................................................................7
Chapter 1
Introducing the USB-1208FS................................................................................................................8
USB-1208FS block diagram...............................................................................................................................9
Software features................................................................................................................................................9
Connecting a USB-1208FS to your computer is easy.......................................................................................10
Chapter 2
Installing the USB-1208FS..................................................................................................................11
What comes with your USB-1208FS shipment? ..............................................................................................11
Hardware .........................................................................................................................................................................11
Additional documentation................................................................................................................................................11
Unpacking the USB-1208FS ............................................................................................................................11
Installing the software ......................................................................................................................................12
Installing the hardware .....................................................................................................................................12
Chapter 3
Functional Details...............................................................................................................................13
Theory of operation - analog input acquisition modes .....................................................................................13
Software paced mode.......................................................................................................................................................13
Continuous scan mode.....................................................................................................................................................13
External components ........................................................................................................................................13
USB connector.................................................................................................................................................................14
LED .................................................................................................................................................................................14
Screw terminal wiring......................................................................................................................................................14
Main connector and pin out .............................................................................................................................................15
Analog input terminals (CH0 IN - CH7 IN).....................................................................................................................15
Analog output terminals (D/A OUT 0 and D/A OUT 1)..................................................................................................18
Digital I/O terminals (Port A0 to A7, and Port B0 to B7)................................................................................................18
Power terminals ...............................................................................................................................................................19
Calibration terminal.........................................................................................................................................................19
Ground terminals .............................................................................................................................................................20
External trigger terminal..................................................................................................................................................20
SYNC terminal ................................................................................................................................................................20
Counter terminal..............................................................................................................................................................20
Accuracy...........................................................................................................................................................20
USB-1208FS channel gain queue feature.........................................................................................................23
Synchronizing multiple units............................................................................................................................23
Chapter 4
Specifications......................................................................................................................................25
Analog input.....................................................................................................................................................25
Analog output...................................................................................................................................................27
Digital input/output...........................................................................................................................................27
External trigger.................................................................................................................................................28
External clock input/output...............................................................................................................................28
Counter.............................................................................................................................................................29
Non-volatile memory........................................................................................................................................29

USB-1208FS User's Guide
6
Microcontroller.................................................................................................................................................29
Power................................................................................................................................................................29
General .............................................................................................................................................................30
Environmental ..................................................................................................................................................30
Mechanical .......................................................................................................................................................30
Main connector and pin out..............................................................................................................................30
4-channel differential mode.............................................................................................................................................31
8-channel single-ended mode...........................................................................................................................................31
Declaration of Conformity..................................................................................................................32

7
Preface
About this User's Guide
What you will learn from this user's guide
This user's guide explains how to install, configure, and use the USB-1208FS so that you get the most out of its
USB data acquisition features.
This user's guide also refers you to related documents available on our web site, and to technical support
resources.
Conventions in this user's guide
For more information on …
Text presented in a box signifies additional information and helpful hints related to the subject matter you are
reading.
Caution! Shaded caution statements present information to help you avoid injuring yourself and others,
damaging your hardware, or losing your data.
< : > Angle brackets that enclose numbers separated by a colon signify a range of numbers, such as those assigned
to registers, bit settings, etc.
bold text Bold text is used for the names of objects on the screen, such as buttons, text boxes, and check boxes. For
example:
1. Insert the disk or CD and click the OK button.
italic text Italic text is used for the names of manuals and help topic titles, and to emphasize a word or phrase. For
example:
The InstaCal installation procedure is explained in the Quick Start Guide.
Never touch the exposed pins or circuit connections on the board.
Where to find more information
For additional information relevant to the operation of your hardware, refer to the Documents subdirectory
where you installed the MCC DAQ software (C:\Program Files\Measurement Computing\DAQ by default), or
search for your device on our website at www.mccdaq.com.

8
Chapter 1
Introducing the USB-1208FS
This user's guide contains all of the information you need to connect the USB-1208FS to your computer and to
the signals you want to measure.
The USB-1208FS is a USB 2.0 full-speed device supported under popular Microsoft®Windows®operating
systems. It is designed for USB 1.1 ports, and was tested for full compatibility with both USB 1.1 and USB 2.0
ports.
The USB-1208FS features eight analog inputs, two 12-bit analog outputs, 16 digital I/O connections, and one
32-bit external event counter. The USB-1208FS is powered by the +5 volt USB supply from your computer. No
external power is required.
The analog inputs are software configurable for either eight 11-bit single-ended inputs, or four 12-bit
differential inputs. Sixteen digital I/O lines are independently selectable as input or output in two 8-bit ports.
A 32-bit counter can count TTL pulses. The counter increments when the TTL levels transition from low to
high (rising-edge).
A SYNC (synchronization) input / output line lets you pace the analog input acquisition of one USB module
from the clock output of another.
The USB-1208FS is shown in Figure 1. I/O connections are made to the screw terminals located along each side
of the USB-1208FS.
Figure 1. USB-1208FS

USB-1208FS User's Guide Introducing the USB-1208FS
9
USB-1208FS block diagram
USB-1208FS functions are illustrated in the block diagram shown here.
Figure 2. USB-1208FS functional block diagram
Software features
For information on the features of InstaCal and the other software included with your USB-1208FS, refer to the
Quick Start Guide that shipped with your device.

USB-1208FS User's Guide Introducing the USB-1208FS
10
Connecting a USB-1208FS to your computer is easy
Installing a data acquisition device has never been easier.
The USB-1208FS relies upon the Microsoft Human Interface Device (HID) class drivers. The HID class
drivers ship with every copy of Windows that is designed to work with USB ports. We use the Microsoft
HID because it is a standard, and its performance delivers full control and maximizes data transfer rates for
your USB-1208FS. No third-party device driver is required.
The USB-1208FS is plug-and-play. There are no jumpers to position, DIP switches to set, or interrupts to
configure.
You can connect the USB-1208FS before or after you install the software, and without powering down
your computer first. When you connect an HID to your system, your computer automatically detects it and
configures the necessary software. You can connect and power multiple HID peripherals to your system
using a USB hub.
You can run up to two USB-1208FS units on most computers.
You can connect your system to various devices using a standard four-wire cable. The USB connector
replaces the serial and parallel port connectors with one standardized plug and port combination.
You do not need a separate power supply module. The USB automatically delivers the electrical power
required by each peripheral connected to your system.
Data can flow two ways between a computer and peripheral over USB connections.

11
Chapter 2
Installing the USB-1208FS
What comes with your USB-1208FS shipment?
As you unpack your USB-1208FS, verify that the following components are included.
Hardware
USB-1208FS (shown with cable)
USB cable (2 meter length)
Additional documentation
In addition to this hardware user's guide, you should also receive the Quick Start Guide (available in PDF at
www.mccdaq.com/PDFmanuals/DAQ-Software-Quick-Start.pdf). This booklet supplies a brief description of
the software you received with your USB-1208FS and information regarding installation of that software.
Please read this booklet completely before installing any software or hardware.
Unpacking the USB-1208FS
As with any electronic device, you should take care while handling to avoid damage from static
electricity. Before removing the USB-1208FS from its packaging, ground yourself using a wrist strap or by
simply touching the computer chassis or other grounded object to eliminate any stored static charge.
If any components are missing or damaged, notify Measurement Computing Corporation immediately by
phone, fax, or e-mail:
Phone: [phone] and follow the instructions for reaching Tech Support.
Fax: [phone] to the attention of Tech Support
Email: [e-mail]

USB-1208FS User's Guide Installing the USB-1208FS
12
Installing the software
Refer to the Quick Start Guide for instructions on installing the software on the Measurement Computing Data
Acquisition Software CD. This booklet is available in PDF at www.mccdaq.com/PDFmanuals/DAQ-Software-
Quick-Start.pdf.
Installing the hardware
Be sure you are using the latest system software
Before you install your USB-1208FS, run Windows Update to update your operating system with the latest HID
and USB drivers.
To connect the USB-1208FS to your system, turn your computer on, and connect the USB cable to a USB port
on your computer or to an external USB hub that is connected to your computer. The USB cable provides power
and communication to the USB-1208FS.
When you connect the USB-1208FS for the first time, a series of Found New Hardware popup balloons
(Windows XP) or dialogs (other Windows versions) opens as the USB-1208FS is detected by your computer.
It is normal for multiple dialogs to open when you connect the USB-1208FS for the first time. The last popup
balloon or dialog states "Your new hardware is installed and ready to use," and the LED on the USB-1208FS
should flash and then remain lit. This indicates that communication is established between the USB-1208FS and
your computer.
You can install up to two USB-1208FS units on most computers. If you need to connect more than two USB-
1208FS units to your computer, contact Tech Support by phone, fax, or e-mail:
Phone: [phone] and follow the instructions for reaching Tech Support.
Fax: [phone] to the attention of Tech Support
Email: [e-mail]
Caution! Do not disconnect any device from the USB bus while the computer is communicating with the
USB-1208FS, or you may lose data and/or your ability to communicate with the USB-1208FS.
If the LED turns off
If the LED is illuminated but then turns off, the computer has lost communication with the USB-1208FS. To
restore communication, disconnect the USB cable from the computer, and then reconnect it. This should restore
communication, and the LED should turn back on.

13
Chapter 3
Functional Details
Theory of operation - analog input acquisition modes
The USB-1208FS can acquire analog input data in two different modes –software paced and continuous scan.
Maximum throughput is system-dependent
Maximum throughput may be lower in Windows operating systems that predate Windows XP.
Software paced mode
In software paced mode, you can acquire one analog sample at a time. You initiate the A/D conversion by
calling a software command. The analog value is converted to digital and returned to the computer. You can
repeat this procedure until you have the total number of samples that you want from one channel.
The maximum throughput sample rate in software paced mode is system-dependent.
Continuous scan mode
In continuous scan mode, you can acquire data from up to eight channels. The analog data is continuously
acquired and converted to digital values until you stop the scan. Data is transferred in blocks of 31 samples from
the USB-1208FS to the memory buffer on your computer.
The maximum continuous scan rate of 50 kS/s is an aggregate rate. The total acquisition rate for all channels
cannot exceed 50 kS/s. You can acquire data from one channel at 50 kS/s, two channels at 25 kS/s, and four
channels at 12.5 kS/s. You can start a continuous scan with either a software command or with an external
hardware trigger event.
External components
The USB-1208FS has the following external components, as shown in Figure 3.
USB connector
LED
Screw terminal banks (2)
Figure 3. USB-1208FS external components

USB-1208FS User's Guide Functional Details
14
USB connector
The USB connector is on the right side of the USB-1208FS. This connector provides +5 V power and
communication. The voltage supplied through the USB connector is system-dependent, and may be less than
5 V. No external power supply is required.
LED
The LED on the front of the housing indicates the communication status of the USB-1208FS. It uses up to 5 mA
of current and cannot be disabled. The table below defines the function of the USB-1208FS LED.
LED Illumination
LED Illumination
Indication
Steady green
The USB-1208FS is connected to a computer or external USB hub.
Blinks continuously
Data is being transferred.
Screw terminal wiring
The USB-1208FS has two rows of screw terminals—one row on the top edge of the housing, and one row on
the bottom edge. Each row has 20 connections. Pin numbers are identified in Figure 4.
Figure 4. USB-1208FS Screw terminal pin numbers
Screw terminal –pins 1-20
The screw terminals on the top edge of the USB-1208FS (pins 1 to 20) provide the following connections:
Eight analog input connections (CH0 IN to CH7 IN)
Two analog output connections (D/A OUT 0 to D/A OUT 1)
One external trigger source (TRIG_IN)
One SYNC terminal for external clocking and multi-unit synchronization (SYNC)
One calibration terminal (CAL)
Five analog ground connections (AGND)
One ground connection (GND)
One external event counter connection (CTR)

USB-1208FS User's Guide Functional Details
15
Screw terminal –pins 21-40
The screw terminals on the bottom edge of the (pins 21 to 40) provide the following connections:
16 digital I/O connections (PortA0 to Port A7, and Port B0 to Port B7)
One power connection (PC+5 V)
Three ground connections (GND)
Main connector and pin out
Connector type
Screw terminal
Wire gauge range
16 AWG to 30 AWG
8-channel single-ended mode pin out
4-channel differential mode pin out
Analog input terminals (CH0 IN - CH7 IN)
You can connect up to eight analog input connections to the screw terminal containing pins 1 to 20 (CH0 IN
through CH7 IN.) Refer to the pinout diagrams above for the location of these pins.
You can configure the analog input channels as eight single-ended channels or four differential channels. When
configured for differential mode, each analog input has 12-bit resolution. When configured for single-ended
mode, each analog input has 11-bit resolution, due to restrictions imposed by the A/D converter.
Single-ended configuration
When all of the analog input channels are configured for single-ended input mode, eight analog channels are
available. The input signal is referenced to signal ground (GND), and delivered through two wires:
The wire carrying the signal to be measured connects to CH# IN.
The second wire connects to AGND.
The input range for single-ended mode is ±10 V. No other ranges are supported in single-ended mode. Figure 5
illustrates a typical single-ended measurement connection.

USB-1208FS User's Guide Functional Details
16
Figure 5. Single-ended measurement connection
Battery voltage should read as expected when the hardware is configured for single-ended mode.
Single-ended measurements using differential channels
To perform a single-ended measurement using differential channels, connect the signal to "CHn IN HI" input,
and ground the associated "CHn IN LO" input.
Differential configuration
When all of the analog input channels are configured for differential input mode, four analog channels are
available. In differential mode, the input signal is measured with respect to the low input.
The input signal is delivered through three wires:
The wire carrying the signal to be measured connects to CH0 IN HI, CH1 IN HI, CH2 IN HI, or
CH3 IN HI.
The wire carrying the reference signal connects to CH0 IN LO, CH1 IN LO, CH2 IN LO, or
CH3 IN LO.
The third wire connects to GND.
A low-noise precision programmable gain amplifier (PGA) is available on differential channels to provide gains
of up to 20 and a dynamic range of up to 12-bits. Differential mode input voltage ranges are ±20 V, ±10 V,
±5 V, ±4 V, ±2.5 V, ±2.0 V, ±1.25 V, and ±1.0 V.
In differential mode, the following two requirements must be met for linear operation:
Any analog input must remain in the −10V to +20V range with respect to ground at all times.
The maximum differential voltage on any given analog input pair must remain within the selected voltage
range.
The input [common-mode voltage + signal] of the differential channel must be in the −10 V to +20 V range in
order to yield a useful result. For example, you input a 4 V pp sine wave to CHHI, and apply the same sine
wave 180° out of phase to CHLO. The common mode voltage is 0 V. The differential input voltage swings from
4 V-(-4 V) = 8 V to -4 V-4 V = -8V. Both inputs satisfy the -10 V to +20 V input range requirement, and the
differential voltage is suited for the ±10 V input range (see Figure 6).

USB-1208FS User's Guide Functional Details
17
Figure 6. Differential voltage example: common mode voltage of 0 V
If you increase the common mode voltage to 11 V, the differential remains at ±8 V. Although the [common-
mode voltage + signal] on each input now has a range of +7 V to +15 V, both inputs still satisfy the -10 V to
+20 V input requirement (see Figure 7).
Figure 7. Differential voltage example: common mode voltage of 11 V
If you decrease the common-mode voltage to -7 V, the differential stays at ±8 V. However, the solution now
violates the input range condition of -10 V to +20 V. The voltage on each analog input now swings from -3 V to
-11 V. Voltages between -10 V and -3 V are resolved, but those below -10 V are clipped (see Figure 8).
Figure 8. Differential voltage example: common mode voltage of -7 V
Since the analog inputs are restricted to a −10 V to +20 V signal swing with respect to ground, all ranges except
±20V can realize a linear output for any differential signal with zero common mode voltage and full scale signal
inputs. The ±20 V range is the exception. You cannot put −20 V on CHHI and 0 V on CHLO since this violates
the input range criteria.

USB-1208FS User's Guide Functional Details
18
The table below shows some possible inputs and the expected results.
Sample inputs and differential results
CHHI
CHLO
Result
-20 V
0 V
In Valid
-15 V
+5 V
In Valid
-10 V
0 V
-10 V
-10 V
+10 V
-20 V
0 V
+10 V
-10 V
0 V
+20 V
-20 V
+10 V
-10 V
+20 V
+10 V
0 V
+10 V
+15 V
-5 V
+20 V
+20 V
0
+20 V
For more information on analog signal connections
For more information on single-ended and differential inputs, refer to the Guide to Signal Connections (this
document is available on our web site at www.mccdaq.com/signals/signals.pdf)
Analog output terminals (D/A OUT 0 and D/A OUT 1)
You can connect up to two analog output connections to the screw terminal pins 13 and 14 (D/A OUT 0 and D/A
OUT 1). Refer to the pinout diagrams on page 15 for the location of these pins.
Each channel can be paced individually at rates up to 10,000 updates per second. Both channels can be paced
simultaneously using the same time base at 5000 updates per channel. The 0-4.096 V output range provides a
convenient 1 mV per LSB when setting the output voltage levels.
Digital I/O terminals (Port A0 to A7, and Port B0 to B7)
You can connect up to 16 digital I/O lines to the screw terminal containing pins 21 to 40 (Port A0 to Port A7,
and Port B0 to Port B7.) Refer to the pinout diagrams on page 15 for the location of these pins. You can
configure each digital port for either input or output.
When you configure the digital bits for input, you can use the digital I/O terminals to detect the state of any
TTL level input. Refer to the switch shown in Figure 9 and the schematic shown in Figure 10. If the switch is
set to the +5 V input, Port A0 reads TRUE (1). If you move the switch to GND, Port A0 reads FALSE.
Figure 9. Digital connection Port A0 detecting the state of a switch

USB-1208FS User's Guide Functional Details
19
Figure 10. Schematic showing switch detection by digital channel Port A0
For more information on digital signal connections
For more information on digital signal connections and digital I/O techniques, refer to the Guide to Signal
Connections (available on our web site at www.mccdaq.com/signals/signals.pdf).
Power terminals
The PC +5V connection (pin 30) is on the bottom screw terminal of the USB-1208FS. Refer to the pinout
diagrams on page 15 for the location of this pin. This terminal draws power from the USB connector. The +5 V
screw terminal is a 5 volt output that is supplied by the host computer.
Caution! The +5 V terminal is an output. Do not connect to an external power supply or you may damage
the USB-1208FS and possibly the computer.
The maximum total output current that can be drawn from all USB-1208FS connections (power, analog and
digital outputs) is 420 mA. This maximum applies to most personal computers and self-powered USB hubs.
Bus-powered hubs and notebook computers may limit the maximum available output current to 100 mA.
Just connecting the USB-1208FS to your computer draws 80 mA of current from the USB +5 V supply. Once
you start running applications with the USB-1208FS, each DIO bit can draw up to 2.5 mA, and each analog
output can draw 15 mA. The maximum amount of +5 V current available for experimental use, over and above
that required by the USB-1208FS, is the difference between the total current requirement of the USB (based on
the application), and the allowed current draw of the PC platform (500 mA for desktop PCs and self-powered
hubs, or 100 mA for bus-powered hubs and notebook computers).
With all outputs at their maximum output current, you can calculate the total current requirement of the USB-
1208FS USB +5 V as follows:
(USB-1208FS @ 80 mA) + (16 DIO @ 2.5 mA ea) + (2 AO @ 15 mA ea ) = 150 mA
For an application running on a PC or powered hub, the maximum available excess current is 500 mA−150 mA
= 350 mA. This number is the total maximum available current at the PC +5 V screw terminals. Measurement
Computing highly recommends that you figure in a safety factor of 20% below this maximum current loading
for your applications. A conservative, safe user maximum in this case would be in the 350-380 mA range.
Since laptop computers typically allow up to 100 mA, the USB-1208FS in a fully-loaded configuration may be
above that allowed by the computer. In this case, you must determine the per-pin loading in the application to
ensure that the maximum loading criteria is met. The per-pin loading is calculated by simply dividing the +5 V
by the load impedance of the pin in question.
Calibration terminal
The CAL connection (pin 16) is an output you should use only to calibrate the USB-1208FS. Refer to the pinout
diagrams on page 15 for the location of this pin. Calibration of the USB-1208FS is software-controlled via
InstaCal.

USB-1208FS User's Guide Functional Details
20
Ground terminals
The four analog ground (AGND) connections provide a common ground for all USB-1208FS input channels.
Four ground (GND) connections provide a common ground for the DIO, TRIG_IN, CTR, SYNC and PC +5V
connections. Refer to the pinout diagrams on page 15 for the location of the AGND and GND terminal pins.
External trigger terminal
The TRIG_IN connection (pin 18) can be configured for either rising or falling edge. Refer to the pinout
diagrams on page 15 for the location of the TRIG_IN terminal pin.
SYNC terminal
The SYNC connection (pin 19) is a bidirectional I/O signal. You can use it for two purposes:
Configure as an external clock input to externally source the A/D conversions. The SYNC terminal
supports TTL-level input signals of up to 50 kHz.
Configure as an output to synchronize with a second USB unit and acquire data from 16 channels.
Refer to the pinout diagrams on page 15 for the location of this pin. For more information on synchronizing
multiple units, refer to page 23.
Counter terminal
The CTR connection (pin 20) is input to the 32-bit external event. Refer to the pinout diagrams on page 15 for
the location of this pin. The internal counter increments when the TTL levels transition from low to high. The
counter can count frequencies of up to 1 MHz.
Accuracy
The overall accuracy of any instrument is limited by the error components within the system. Quite often,
resolution is incorrectly used to quantify the performance of a measurement product. While "12-bits" or "1 part
in 4096" does indicate what can be resolved, it provides little insight into the quality of an absolute
measurement. Accuracy specifications describe the actual results that can be realized with a measurement
device.
There are three types of errors which affect the accuracy of a measurement system:
offset
gain
nonlinearity.
The primary error sources in the USB-1208FS are offset and gain. Nonlinearity is small in the USB-1208FS,
and is not significant as an error source with respect to offset and gain.
Figure 11 shows an ideal, error-free, USB-1208FS transfer function. The typical calibrated accuracy of the
USB-1208FS is range-dependent, as explained in the "Specifications" chapter on page 25. We use a ±10 V
range here as an example of what you can expect when performing a measurement in this range.