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PRELIMINARY
TECHNICAL
DATA
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AN-566 Preliminary Technical Data
TECHNICAL NOTE
One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106 • 781/329-4700 • World Wide Web Site: http://www.analog.com
INTRODUCTION
This application note describes a reference design based on the
24 Bit sigma-delta high dynamic range AD1555/AD1556 chip-
set. This chip-set allows direct acquisition of high dynamic
sensors like geophones or hydrophones. Acquisition of other
high dynamic range low frequency range ( up to few kHz )
sensors can also be done.
The intent of this note is to provide the detailed description of
this design. These guidelines can be used to ease the design
using the AD1555/AD1556 chip-set. The main goal of this
design is to give a baseline design that can be customized at
user convenience rather than a design which covers all acquisi-
tion needs.
This design is very dense ( less than 1 inch by 2 inches per
channel ), can be easily extended to do a multichannel acquisi-
tion system, and demonstrates the specific high accuracy
performances of the chip-set. Performance of 120dB dynamic
range in 408Hz bandwidth, equivalent input noise of 80nVrms
in 101 Hz bandwidth and distortion of -120 dB with a total
power dissipation lower than 90mW per channel typically can
be achieved.
The AD1555 is a complete sigma-delta modulator, combined
with a programmable gain amplifier intended for low fre-
quency, high dynamic range measurement applications. The
AD1555 outputs a ones-density bit stream proportional to the
analog input. When used in conjunction with the AD1556
digital filter/decimator, a high performance ADC is realized.
A full description of the AD1555/AD1556 is available in the
AD1555/AD1556 data sheet and should be consulted when
utilizing this application note. The data sheet provides detailed
information on the functionality of the AD1555/AD1556 chip-
set and will be referenced often in this application note.
A Geophone/Hydrophone acquisition reference design based on the
AD1555/AD1556 chipset
by Alain Guery
1.9" (48.3 mm)
0.9" (23 mm)
Figure 1. Implementation of the reference design.
GENERAL DISCUSSION
The implementation of this one channel acquisition AD1555/
AD1556 reference design occupies less than one 1 inch by 2
inches per channel on a one side board and is shown in figure
1. In multichannel applications, some components can be
shared to further reduce the estate per channel. This reference
design incliudes many functionalities like EMI/RFI filtering,
lightning protection, sensor and acquisition system build-in
testability, calibration, reference voltage and easy to use serial
digital interface. The schematic of the reference design is
shown in figure 2.
Input Filtering
Some filtering of the input signal coming from the sensor is
usually required before acquisition. Depending on the applica-
tion, the requirements of this filter can vary among EMI/RFI
filtering to prevent high frequency noise coming in and being
detected by the acquisition system, common mode filtering and
polarization when the sensor is floating, lightning protection
when sensors such as geophone are connected through long
cables and are exposed to the elements. It is difficult to cover
all the specific requirements of each application with the same
filter. The filter implemented in the reference design answers
some of these requirements and can be customized according
to each specific application.
