Evaluation Board for the AD7798 16-Bit,
Low Power Sigma-Delta ADC (3 Channels)
EVAL-AD7798
Rev. 0
Evaluation boards are only intended for device evaluation and not for production purposes.
Evaluation boards are supplied “as is” and without warranties of any kind, express, implied, or
statutory including, but not limited to, any implied warranty of merchantability or fitness for a
particular purpose. No license is granted by implication or otherwise under any patents or other
intellectual property by application or use of evaluation boards. Information furnished by Analog
Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog
Devices for its use, nor for any infringements of patents or other rights of third parties that may result
from its use. Analog Devices reserves the right to change devices or specifications at any time
without notice. Trademarks and registered trademarks are the property of their respective owners.
Evaluation boards are not authorized to be used in life support devices or systems.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700 www.analog.com
Fax: 781.461.3113 ©2008 Analog Devices, Inc. All rights reserved.
FEATURES
Full-featured evaluation board for the AD7798
Standalone USB interface
Various linking options
PC software for control of AD7798
GENERAL DESCRIPTION
This data sheet describes the evaluation board for the AD7798
low power, 16-bit ∑-Δ ADC. The AD7798 is a complete analog
front end for low frequency measurement applications. It
contains three differential inputs and includes a low noise
instrumentation amplifier, a reference detect, and a low-side
power switch.
Features include an update rate that can be varied from 4.17 Hz
to 470 Hz, an on-board clock that eliminates the need for an
external clock, and a ∑-Δ conversion technique that allows
attainment of up to 16 bits of no missing codes performance.
The input signal is applied to an analog modulator, and the
modulator output is processed by an on-chip digital filter. The
analog input channel of the AD7798 accepts analog input
signals of ±VREF/gain, with a gain in the range of 1 to 128. When
the gain is 64 and the update rate is programmed to 16.7 Hz, the
rms noise is 85 nV. Simultaneous 50 Hz/60 Hz rejection is also
available at this data update rate.
Full data on the AD7798 is available in the AD7798 data sheet,
available from Analog Devices, Inc., and should be consulted in
conjunction with this document when using the evaluation board.
The evaluation board interfaces to the USB port of an IBM-
compatible PC. Software is available with the evaluation board
that allows the user to easily communicate with the AD7798.
The AD7798 evaluation board software should be installed
before connecting the AD7798 evaluation board to the PC.
Other components on the AD7798 evaluation board include the
ADP3303, which is a high precision, low power, 3.3 V output
voltage regulator used to power the USB/SPI interface.
FUNCTIONAL BLOCK DIAGRAM
AVDD
TEMPERATURE
SENSOR VOLTAGE
REGULATOR
SPI/USB
INTERFACE
AD7798
PC AND Lab VIEW ™
BASED
SOFTWARE
ANALOG
INPUTS
REFIN(±)
07432-001
Figure 1.
EVAL-AD7798
Rev. 0 | Page 2 of 2
TABLE OF CONTENTS
Features .............................................................................................. 1
General Description ......................................................................... 1
Functional Block Diagram .............................................................. 1
Revision History ............................................................................... 2
Evaluation Board Hardware ............................................................ 3
Power Supplies .............................................................................. 3
Links ............................................................................................... 3
Sockets ........................................................................................... 3
Setting Up the Evaluation Board ................................................ 3
Interfacing to the Evaluation Board ........................................... 4
Evaluation Board Software .............................................................. 5
Software Description.................................................................... 5
Installing the Software ..................................................................5
Using the Software ........................................................................5
Main Window ................................................................................6
Registers Window .........................................................................8
Other Registers Window ..............................................................9
Temp Demo Window ................................................................ 10
Evaluation Board Schematic and Artwork .................................. 11
Ordering Information .................................................................... 15
Bill of Materials ........................................................................... 15
Ordering Guide .......................................................................... 15
ESD Caution................................................................................ 15
REVISION HISTORY
4/08—Revision 0: Initial Version
EVAL-AD7798
Rev. 0 | Page 3 of 3
EVALUATION BOARD HARDWARE
POWER SUPPLIES
The evaluation board is powered via the 5 V supply from the
USB connector, J1. This 5 V supply can be used to power the
AD7798 directly or a 3.3 V regulated voltage from the on-board
ADP3303 high precision, low power, 3.3 V output voltage regulator
can be used to power the AD7798. Alternatively, the AD7798
can be powered using an external 3 V or 5 V power supply via J3.
LINKS
There are 10 link options that must be set for the required
operating setup before using the evaluation board. The
functions of these link options are outlined in Table 1.
SOCKETS
There are five sockets relevant to the operation of the AD7798
on this evaluation board. The functions of these sockets are
described in Table 3.
SETTING UP THE EVALUATION BOARD
Care should be taken before applying power and signals to the
evaluation board to ensure that all link positions are as per the
required operating mode. Table 2 shows the position in which
the links are set when the evaluation board is shipped.
Table 1. Evaluation Board Link Settings
Table 2. Initial Link and Switch Positions
Link No. Position Description
LK1, LK2 In AIN1(+) and AIN1(−) are shorted to the reference voltage.
LK3 to LK6 Out The demonstration circuit is disconnected from the AD7798.
LK7, LK8 In The reference voltage is set to 1.65 V (3.3 V/2).
LK9 B The 3.3 V supply is used as AVDD for the AD7798.
LK10 In The D2 LED is connected to the low-side power switch of the AD7798.
Table 3. Socket Functions
Link No. Default Description
LK1, LK2 In These links are used to connect the AIN1(+) and AIN1(−) inputs to a reference voltage that equals AVDD/2. With this
configuration, a noise analysis can be performed. With these links removed, an external voltage can be applied to
AIN1(+)/AIN1(−) using the SMB connectors.
LK3 to LK6 Out These links are used to connect the on-board temperature demonstration circuit to the ADC; all must be in place
when attempting to measure ambient temperature.
When LK3 and LK4 are inserted, the 1 kΩ thermistor is connected to AIN2(+)/AIN2(−). With LK5 and LK6 in place, a
5 kΩ precision resistor is used to generate the reference. This results in a ratiometric configuration.
LK7, LK8 In A resistor-divider network generates a voltage equal to AVDD/2, which can be used as the reference for the
AD7798. With LK7 and LK8 in place, AVDD/2 is connected to REFIN(+) and REFIN(−) is connected to GND. To use
another reference source, remove LK7 and LK8.
LK9 B
LK9 is used to select the power source for AVDD on the AD7798. LK9 in Position A selects an external power supply,
supplied via J3. LK9 in Position B selects the 3.3 V regulated output from the on-board ADP3303 voltage regulator.
LK9 in Position C selects the 5 V supply from the USB connector, J1.
LK10 In LK10 is used to test the on-chip low-side power switch. If LK10 is in place, enabling the low-side power switch
using PWR SW in the Registers window turns on the LED, D2. Clearing this bit turns off the LED.
Socket Description
AIN1(+) Subminiature BNC (SMB) connector. The analog input signal for the AIN1(+) input of the AD7798 is applied to this socket.
AIN1(−) Subminiature BNC (SMB) connector. The analog input signal for the AIN1(−) input of the AD7798 is applied to this socket.
REFIN(+) Subminiature BNC (SMB) connector. This socket is used in conjunction with REFIN(−) to apply an external reference
to the AD7798. The voltage for the REFIN(+) input of the AD7798 is applied to this socket.
REFIN(−) Subminiature BNC (SMB) connector. This socket is used in conjunction with REFIN(+) to apply an external reference
to the AD7798. The voltage for the REFIN(−) input of the AD7798 is applied to this socket.
J2 18-pin (2 × 9) straight header. This socket is used in conjunction with the prototype area to interface any signal to
the AD7798.
EVAL-AD7798
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INTERFACING TO THE EVALUATION BOARD
Interfacing to the evaluation board is via a standard USB connector,
J1. J1 is used to connect the evaluation board to the USB port of a
PC. A standard USB connector cable is included with the AD7798
evaluation board to allow the evaluation board to interface with
the USB port of the PC. Because the board is powered via the
USB connector, there is no need for an external power supply,
although one can be connected via J3 if preferred.
Communication between the AD7798 and the PC is via the
USB/SPI interface. The on-board USB controller (U2) controls
this communication.
To set up the USB/SPI interface, use the following procedure:
1. Install the AD7798 evaluation board software using the
supplied AD7798 evaluation board CD-ROM before
connecting the board to the PC.
2. After the AD7798 evaluation board software has been
installed, connect the board to the PC via J1 on the AD7798
evaluation board and via the USB port on the PC using the
supplied USB connector cable. The PC automatically finds
the new USB device and identifies it as the AD7798
evaluation board.
3. Follow the on-screen instructions that appear automatically.
If the Hardware Installation window shown in Figure 2
appears during the installation process, click Continue
Anyway to successfully complete the installation of the
AD7798 evaluation board.
07432-002
Figure 2. Hardware Installation Window
EVAL-AD7798
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EVALUATION BOARD SOFTWARE
SOFTWARE DESCRIPTION
The AD7798 evaluation board is shipped with a CD-ROM con-
taining software that can be installed onto a standard PC to control
the AD7798. The software uses the USB of the PC to communicate
with the AD7798 via the cable provided with the board.
The software allows the user to configure the AD7798 and to
read conversion data from the AD7798.
Data can be read from the AD7798 and displayed or stored for
later analysis. For further information, see the AD7798 data sheet.
INSTALLING THE SOFTWARE
Use the following procedure to install the software:
1. Start Windows® and insert the CD-ROM.
2. The installation software should launch automatically. If it
does not, use Windows Internet Explorer to locate the file
setup.exe on the CD-ROM. Double-clicking on this file
starts the installation procedure.
3. At the prompt, select a destination directory, which is
C:\Program Files\Analog Devices\AD7798 by default.
After the directory is selected, the installation procedure
copies the files into the relevant directories on the hard
drive. The installation program creates a program group
called Analog Devices with the subgroup AD7798 in the
Start menu of the taskbar.
4. Once the installation procedure is complete, double-click
the AD7798 icon to start the program.
USING THE SOFTWARE
Figure 3 shows the main window that is displayed when the
program starts. The Main Window section briefly describes the
menus on the main window. Following the Main Window section
are descriptions of the most commonly used evaluation software
windows.
The data that has been read can be exported to other packages,
such as MathCAD™ or Microsoft® Excel, for further analysis.
On power-up, the AD7798 evaluation board software configures
the device to have a gain of 64, the reference pins are connected
to AVDD/2, the AIN1(−)/AIN1(−) channel is selected, AIN1(−)
is connected to AVDD/2, and the update rate is set to 16.7 Hz.
EVAL-AD7798
Rev. 0 | Page 6 of 6
MAIN WINDOW
Menu Bar
File
Allows the user to read previously stored data for display or
analysis, write the current set of data to a file for later use, and
exit the program.
About
Provides information on the revision of software being used.
Buttons
Reset
Allows the user to reset the AD7798 and set the registers to the
power-up conditions as specified by the software (channel =
AIN1(−)/AIN1(−), gain = 64, update rate = 16.7 Hz).
Exit
Allows the user to exit the software. Serves the same purpose as
Quit in the File drop-down menu.
Sample
Allows the user to read a number of samples from the AD7798.
Noise analysis is then performed on the samples. These samples
can be stored for further analysis. The sample size is entered in
the Num Samples box.
Continuous
Allows the user to read a number of samples continuously. The
software gathers the number of samples specified by Num
Samples, performs noise analysis on the samples, and then
gathers the next group of samples.
Registers
Allows the user to access the configuration register, mode
register, and IO register.
Other Registers
Allows the user to access the ID register, status register, offset
register, and full-scale register.
Quick Analysis
Selects the Quick Analysis window. The Quick Analysis
window provides the user with access to the following subset of
AD7798 control bits: Channel, Update Rate, and Gain. For
access to all control bits, click Registers or Other Registers.
Temp Demo
Allows the user to access the temperature demonstration
software.
Num Samples
The number of samples to gather for analysis is entered in this box.
Get Samples
Serves the same purpose as the Sample button.
Wav efor m
The gathered conversions are displayed in graph form.
Histogram
The gathered samples are used to generate a histogram.
Codes
The gathered samples can be displayed as codes or in voltage
format. When Codes is clicked, the values are displayed as
codes, and the Codes button changes to Volts. To display the
information as volts, click Volts.
External Reference
The value of the external reference applied to the AD7798
should be entered in the External Reference box. The default
value is 1.65 V.
EVAL-AD7798
Rev. 0 | Page 7 of 7
07432-003
Figure 3. AD7798 Evaluation Software Main Window
EVAL-AD7798
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REGISTERS WINDOW
This window is accessed by clicking Registers, which allows
access to the configuration register, mode register, and IO
register. Figure 4 shows the Registers window. This window
allows the customer to change the update rate, gain, low-side
power switch, and so on. Consult the AD7798 data sheet for
further details on the bit functions.
07432-004
Figure 4. AD7798 Evaluation Software Registers Window
EVAL-AD7798
Rev. 0 | Page 9 of 9
OTHER REGISTERS WINDOW
This window is accessed by clicking Other Registers (see
Figure 5). It displays the contents of the offset calibration
register, ID register, full-scale calibration register, and status
register. To write to the offset and full-scale calibration
registers, the user must place the AD7798 in power-down or
idle mode using the Mode box in the Registers window (see
Figure 4).
07432-005
Figure 5. AD7798 Evaluation Software Other Registers Window
EVAL-AD7798
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TEMP DEMO WINDOW
This window is accessed by clicking Temp Demo. The AD7798
evaluation board has a temperature demonstration included on
board. To operate the temperature demonstration, LK3 to LK6
should be inserted and LK1, LK2, LK7, LK8, and LK10 should
be removed. With this configuration, a 1 kΩ thermistor is
connected to the AIN2(+)/AIN2(−) channel. In series with the
thermistor is a 5 kΩ precision resistor, which is connected to
the AD7798 reference pins. The thermistor and precision
resistor are connected to AVDD in a ratiometric configuration.
The temperature demonstration software saves the values in the
mode register, configuration register, and IO register and then
configures the AD7798 to operate with the AIN2(+)/AIN2(−)
channel. The gain is set to 1, and the reference voltage is
adjusted appropriately. The software continuously reads the
conversion from the AIN2(+)/AIN2(−) channel and converts
the result to temperature using a look-up table.
To exit the temperature demonstration, click Back. The software
sets the configuration register, mode register, and IO register to
their pretemperature demonstration values.
07432-006
Figure 6. AD7798 Evaluation Software Temperature Demo Window
EVAL-AD7798
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EVALUATION BOARD SCHEMATIC AND ARTWORK
07432-007
Figure 7. AD7798 Evaluation Board Schematic
EVAL-AD7798
Rev. 0 | Page 12 of 12
07432-008
Figure 8. AD7798 Evaluation Board—Component Layout Diagram
EVAL-AD7798
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07432-009
Figure 9. AD7798 Evaluation Board—Component Side Artwork
EVAL-AD7798
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07432-010
Figure 10. AD7798 Evaluation Board—Solder Side Artwork
EVAL-AD7798
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ORDERING INFORMATION
BILL OF MATERIALS
Table 4.
Qty Reference Designator Description Manufacturer/Part No.
Integrated Circuits
3 U1, U5, U6 AD7798BRUZ Analog Devices
1 U2 USB controller Cypress Semiconductor Corporation, CY7C68013A-56LFXC
1 U3 24LC64 Microchip Technology Inc., 24LC64-I/SN
1 U4 ADP3303ARZ-3.3 Analog Devices
1 Y1 24 MHz crystal AEL Crystals, X24M000000S244
2 D1, D2 Green LED Fairchild Semiconductor, QTLP630C-4
1 L1 Ferrite bead Meggitt Sigma, BMB2A0300AN1
1 D3 Diode Micro Commercial Components Corp., DL4001-TP
Capacitors
9 C1 to C9 Not inserted
2 C11, C12 1 μF ceramic Yageo Corporation, 223824619863
15 C13, C15, C18, C19, C21, C25 to C33, C43 0.1 μF ± 10% ceramic AVX Corporation, CM105X7R104K16AT
3 C14, C16, C17 10 μF tantalum AVX Corporation, TAJA106K010R
1 C20 2.2 μF tantalum EPCOS AG, B45196E2225K109
2 C22, C23 22 pF ceramic Yageo Corporation, 223886715229
1 C24 47 μF tantalum AVX Corporation, TAJC476K016R
Resistors
1 R1 1 kΩ thermistor EPCOS AG, B57620C102J62
3 R2, R24, R25 5 kΩ ± 0.1% Tyco International, Ltd., RN73C2A4K99BTG
9 R3 to R10, R17 0 Ω resistor Multicomp, MC 0.063W 0603 0R
4 R11 to R14 1 MΩ resistor Multicomp, MC 0.063W 0603 1% 1M
2 R15, R16 100 kΩ resistor Multicomp, MC 0.063W 0603 1% 100K
4 R18 to R21 10 kΩ resistor Multicomp, MC 0.063W 0603 1% 10K
2 R22, R23 1 kΩ resistor Multicomp, MC 0.063W 0603 1% 1K
2 R28, R29 2.2 kΩ resistor Multicomp, MC 0.063W 0603 1% 2K2
Links
10 LK1 to LK8, LK10 (2 × 1 way), LK9 (3 × 2 way) Pin headers Harwin Plc, M20-9983646
10 At LK1 to LK10 Shorting plugs Harwin Plc, M7566-05
Connectors
4 AIN1+, AIN1−, REFIN1+, REFIN1− SMB connector Not inserted
1 J1 USB mini-B connector Molex, 565790576
1 J2 18-pin (2 × 9) header Harwin Plc, M20-9983646
1 J3 Two-way terminal block Camden Electronics Ltd., CTB5000/2
ESD CAUTION
ORDERING GUIDE
Model Description
EVAL-AD7798EB Evaluation Board
EVAL-AD7798
Rev. 0 | Page 16 of 16
NOTES
©2008 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
EB07432-0-4/08(0)
