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Preliminary Technical Data
FEATURES
Full-Featured Evaluation Board for the AD7798 Stand alone USB Interface Various Linking Options PC Software for Control of AD7798
Evaluation Board for the AD7798, 16-Bit Low Power - ADC (3 Channels) EVAL-AD7798EB
Full data on the AD7798 is available in the AD7798 datasheet available from Analog Devices and should be consulted in conjunction with this Technical Note 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 easily communicate with the AD7798. N.B.!! 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, 3.3V output, voltage regulator, which is used to power the USB/SPI interface .
INTRODUCTION
This Technical Note describes the evaluation board for the AD7798 which is a Low Power, 16-Bit Sigma Delta 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, reference detect 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 sigmadelta 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 processed by an on-chip digital filter. The analog input channel of the AD7798 accepts analog input signals of +VREF/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 65 nV. Simultaneous 50 Hz/ 60 Hz rejection is available at this data update rate also.
OPERATING THE AD7798 EVALUATION BOARD
Power Supplies The board is powered via the 5V supply from the USB connector, J1. This 5V supply may be used to power the AD7798 directly or a 3.3V Regulated voltage from the on board ADP3303, high precision, low power, 3.3V output, voltage regulator may also be used. Alternatively, the AD7798 may be powered using an external 3V or 5V power supply via J3.
FUNCTIONAL BLOCK DIAGRAM
AVDD TEMPERATURE SENSOR VOLTAGE REGULATOR
ANALOG INPUTS
AD7798
SPI / USB INTERFACE
PC + LABVIEW BASED SOFTWARE
VOLTAGE REF
REFIN
Figure 1. Functional Block Diagram
Rev. Pr A
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. Specifications subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Trademarks and registered trademarks are the property of their respective companies.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. www.analog.com Tel: 781.329.4700 Fax: 781.326.8703 (c) 2004 Analog Devices, Inc. All rights reserved.
EVAL-AD7798EB
Preliminary Technical Data
There are ten link options which must be set for the required operating setup before using the evaluation board. The functions of these link options are outlined below. Table 1.Evaluation Board Link Settings
Link LK1, LK2 Default In Description
These links are used to connect the AIN1(+) and AIN1(-) inputs to a reference voltage which equals AVDD /2. With this configuration, a noise analysis can be performed. With these links removed, an external voltage may be applied to AIN1 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. When LK3 and LK4 are inserted, the 1K thermistor is connected to AIN2. With LK5 and LK6 in place, a 5K precision resistor is used to generate the reference. This results in a ratiometric configuration.
LK3, LK4, LK5, LK6
Out
LK7, LK8
In
A resistor divider network generates a voltage equal to AVDD /2 which can be used as the reference for the AD77998 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 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 3.3V regulated output from the onboard ADP3303 voltage regulator. LK9 in position C selects the 5V supply from the USB connector, J1. LK10 is used to test the on chip low side power switch. With LK10 in place, enabling the lowside power switch in software by setting a bit in the configuration register turns on the LED, D2 Clearing this bit turns off the LED.
LK9
B
LK10
In
SET-UP CONDITIONS
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 1 shows the position in which all the links are set when the evaluation board is sent out.
Table 2: Initial Link and Switch Positions Link No. LK1 - LK2 LK3 - LK6 LK7 - LK8 LK9 LK10 Position In Out In B In Function AIN1(+) and AIN1(-) are shorted to the reference voltage The demonstration circuit is disconnected from the AD7798 The reference voltage is set to 1.65V (3.3V/2) The 3.3V supply is used as AVDD for the AD7798. LED D2 is connected to the Low Side Power Switch of the AD7798
EVALUATION BOARD INTERFACING
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 PC's USB port. As the board is powered via the USB connector, there is no need for an external power supply, although one may be connected if preferred via J3. Communication between the AD7798 and the PC is over the USB/SPI interface. The onboard USB controller (U2) controls this communication. Remember, the AD7798 Evaluation Board Software should be installed (using the supplied AD7798 Evaluation Board CD ROM) BEFORE connecting the board to the PC.
Rev. Pr A | Page 2
Preliminary Technical Data
EVAL-AD7798EB
After the AD7798 Evaluation Board Software has been installed, connect the board to the PC via J1 on the AD7798 Evaluation Board and the USB port on the PC using the supplied USB connector cable. The PC will automatically find the new USB device and will identify it as: AD7798 Evaluation Board. Follow the onscreen instructions that appear automatically. During the installation process if the following window appears:
. Hit Continue Anyway in order to successfully complete the installation of the AD7798 Evaluation Board. SOCKETS There are five sockets relevant to the operation of the AD7798 on this evaluation board. The functions of these sockets are outlined in Table 3.
Table 3. Socket Functions Socket AIN1+ AIN1REFIN+ Function Sub-Miniature BNC (SMB) Connector. The analog input signal for the AIN1(+) input of the AD7798 is applied to this socket. Sub-Miniature BNC (SMB) Connector. The analog input signal for the AIN1(-) input of the AD7798 is applied to this socket. Sub-Miniature 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. Sub-Miniature 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. 34 Pin (2x16) pin Straight Header. This Socket is used in conjunction with the prototype area to interface
REFIN-
J2
any signal to the AD7798. It is specifically designed as a socket for a 34 pin header.
Rev. Pr A | Page 3

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