Evaluation Board for the AD7795 16-Bit,
Low Power Sigma-Delta ADC (6 Channels)
EVAL-AD7795
FEATURES
Full-featured evaluation board for the AD7795
Standalone USB interface
Various linking options
PC software for control of AD7795
processed by an on-chip digital filter. The analog input channel
of the AD7795 accepts analog input signals of ±V
REF
/gain, with
gain equal to 1 to 128. With a gain of 64 and the update rate
programmed to 16.7 Hz, the rms noise is 86 nV. Simultaneous
50 Hz/60 Hz rejection is also available at this data update rate.
Full details on the AD7795 are available in the
AD7795
data
sheet, available from Analog Devices, Inc., and should be
consulted in conjunction with this data sheet 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,
which allows the user to communicate easily with the AD7795.
Note that the AD7795 evaluation board software must be installed
before connecting the AD7795 evaluation board to the PC.
Other components on the AD7795 evaluation board include
the
ADP3303
high precision, low power, 3.3 V output, voltage
regulator, which is used to power the USB/SPI interface.
GENERAL DESCRIPTION
This data sheet describes the evaluation board for the AD7795,
a low power, 16-bit Σ-Δ ADC. The AD7795 is a complete
analog front end for low frequency measurement applications.
It contains six differential inputs and includes a low noise
instrumentation amplifier, an embedded reference, as well as
programmable current sources and a low-side power switch.
The update rate can be varied from 4.17 Hz to 500 Hz. It also
has an on-board clock, eliminating the need for an external
clock. It employs a Σ-Δ conversion technique to realize up to
16 bits of no missing codes performance. The input signal is
applied to an analog modulator. The modulator output is
FUNCTIONAL BLOCK DIAGRAM
AVDD
VOLTAGE
REGULATOR
TEMPERATURE
SENSOR
Figure 1.
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.
www.analog.com
Tel: 781.329.4700
Fax: 781.461.3113
©2008 Analog Devices, Inc. All rights reserved.
07421-001
ANALOG
INPUTS
AD7795
SPI/USB
INTERFACE
USB
CONNECTOR
PC AND
LabVIEW™
BASED
SOFTWARE
EVAL-AD7795
TABLE OF CONTENTS
Features .............................................................................................. 1
General Description ......................................................................... 1
Functional Block Diagram .............................................................. 1
Revision History ............................................................................... 2
Evaluation Board Hardware ............................................................ 3
Power Supplies .............................................................................. 3
Links ............................................................................................... 3
Setup Conditions .......................................................................... 3
Sockets ........................................................................................... 3
Interfacing to the Evaluation Board ........................................... 4
Evaluation Board Software .............................................................. 5
Software Description.................................................................... 5
Installing the Software ..................................................................5
Using the Software ........................................................................5
Main Window Options .................................................................6
Registers ..........................................................................................7
Other Registers ..............................................................................8
Temp Demo....................................................................................9
Evaluation Board Schematic and Artwork.................................. 10
Ordering Information .................................................................... 13
Bill of Materials ........................................................................... 13
Ordering Guide .......................................................................... 13
ESD Caution................................................................................ 13
REVISION HISTORY
4/08—Revision 0: Initial Version
Rev. 0 | Page 2 of 16
EVAL-AD7795
EVALUATION BOARD HARDWARE
POWER SUPPLIES
The AD7795 evaluation board is powered via the 5 V supply
from the USB connector, J1. This 5 V supply can be used to
power the AD7795 directly. A 3.3 V regulated voltage from the
on-board ADP3303, a high precision, low power, 3.3 V output,
voltage regulator, can also be used. Alternatively, the AD7795 can
be powered using an external 3 V or 5 V power supply via J3.
SETUP CONDITIONS
Care should be taken before applying power and signals to the
evaluation board to ensure that all link positions are set per the
required operating mode. Table 2 shows the position in which
all the links are initially set.
SOCKETS
There are five sockets relevant to the operation of the AD7795
on this evaluation board. The functions of these sockets are
outlined in Table 3.
LINKS
There are eight 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.
Table 1. Evaluation Board Link Settings
Link
LK1
Default
Position
In
LK2, LK3 LK4,
LK5, LK6
Out
LK7
Out
LK8
B
Description
This link is used to externally short AIN1(+) to AIN1(−). If V
BIAS
is enabled and directed to AIN1(−), a noise
analysis in this configuration can be performed. With this link removed, an external voltage can be applied to
AIN1(+)/AIN(−) using the SMB connectors.
These links are used to connect the on-board temperature demonstration circuit to the ADC, and must all be
in place when attempting to measure ambient temperature.
With the on-board temperature demonstration circuit selected in the software, LK6, when inserted, allows
current from the on-chip current source of the AD7795 to flow through the temperature demonstration
circuit. LK4 and LK5, when inserted, connect the 1 kΩ thermistor to AIN4(+)/AIN4(−), and with LK2 and LK3
inserted, the 5 kΩ precision resistor is used to generate the reference for the AD7795 so that a ratiometric
configuration of the temperature demonstration circuit is achieved.
LK7 is used to test the on-chip low-side power switch. If LK7 is in place, enabling the low-side power switch
via the
PWR SW
drop-down box in the
Registers
window (see Figure 4) turns on the LED, D2. Clearing this bit turns
off the LED.
LK8 is used to select the power source for AV
DD
on the AD7795. LK8 in Position A selects an external power
supply, supplied via J3. LK8 in Position B selects the 3.3 V regulated output from the on-board ADP3303
voltage regulator. LK8 in Position C selects the 5 V supply from the USB connector, J1.
Table 2. Initial Link and Switch Positions
Link No.
LK1
LK2, LK3
LK4, LK5
LK6
LK7
LK8
Position
In
Out
Out
Out
Out
B
Description
AIN1(+) and AIN1(−) are shorted.
Internal reference is used.
AIN4(+) and AIN4(−) are not connected to the temperature demonstration circuit.
IOUT2 is not connected to the temperature demonstration circuit.
LED D1 is not connected to the low-side power switch of the AD7795.
The 3.3 V supply is used as AV
DD
for the AD7795.
Table 3. Socket Functions
Socket
AIN1+
AIN1−
REFIN1+
REFIN1−
J2
Description
Subminiature BNC (SMB) connector. The analog input signal for the AIN1(+) input of the AD7795 is applied to this socket.
Subminiature BNC (SMB) connector. The analog input signal for the AIN1(−) input of the AD7795 is applied to this socket.
Subminiature BNC (SMB) connector. This socket is used in conjunction with REFIN1− to apply an external reference to the
AD7795. The voltage for the REFIN1(+) input of the AD7795 is applied to this socket.
Subminiature BNC (SMB) connector. This socket is used in conjunction with REFIN1− to apply an external reference to the
AD7795.The voltage for the REFIN(−) input of the AD7795 is applied to this socket.
30-pin (2 × 15) straight header. This socket is used in conjunction with the prototype area to interface any signal to the AD7795.
Rev. 0 | Page 3 of 16
EVAL-AD7795
INTERFACING TO THE EVALUATION BOARD
Interfacing to the evaluation board is via a standard USB connec-
tor, 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 AD7795 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 if preferred, via J3.
Communication between the AD7795 and the PC is via the
USB/SPI interface. The on-board USB controller (U2) controls
this communication.
1.
Install the AD7795 evaluation board software, using the
supplied AD7795 evaluation board CD-ROM, before
connecting the board to the PC.
When installation of the AD7795 evaluation board soft-
ware is complete, use the supplied USB connector cable to
connect the board to the PC via J1 on the AD7795 evaluation
board and the USB port on the PC. The PC automatically
detects the new USB device and identifies it as the
AD779x
Evaluation Board.
Follow the onscreen instructions that appear automatically.
During the installation process, if the
Hardware Installation
window appears (see Figure 2), click
Continue Anyway
to
successfully complete the installation of the AD7795 evalua-
tion board.
2.
3.
Figure 2. Hardware Installation
Rev. 0 | Page 4 of 16
07421-002
EVAL-AD7795
EVALUATION BOARD SOFTWARE
SOFTWARE DESCRIPTION
The AD7795 evaluation board is shipped with a CD-ROM
containing software that can be installed onto a standard PC
to control the AD7795. The software uses the USB of the PC to
communicate with the AD7795, via the cable that accompanies
the board.
The software allows you to configure the AD7795 and to read
conversion data from the AD7795.
Data can be read from the AD7795 and displayed or stored for
later analysis. For further information, see the AD7795 data
sheet available at www.analog.com.
Once 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
AD7795
in the
Start
menu of the taskbar.
Once the installation procedure is complete, double-click
the AD7795 icon to start the program.
4.
USING THE SOFTWARE
Figure 3 shows the main window that is displayed when the
program starts. The Main Window Options section briefly
describes the various menu and button options on the main
window. The Registers, Other Registers, and Temp Demo
sections describe 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 AD7795 evaluation board software configures
the device to have a gain of 64, the internal reference is selected,
the AIN1(−)/AIN1(−) channel is selected, the bias voltage is
enabled on AIN1(−), the update rate is set to 16.7 Hz, and chop
is enabled.
INSTALLING THE SOFTWARE
To install the software
1.
2.
Start Windows® and insert the CD-ROM.
The installation software should launch automatically. If it
does not, use Windows Explorer to locate the file
setup.exe
on the CD-ROM. Double-click this file to start the
installation procedure.
At the prompt, select a destination directory, which is
C:\Program Files\Analog Devices\AD7795
by default.
3.
Figure 3. AD7795 Evaluation Software Main Window
Rev. 0 | Page 5 of 16
07421-004
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