24-Bit ꢀ-ꢁ ADC
with Low Noise PGA
a
AD1555/AD1556
FEATURES
AD1555
Fourth Order ꢀ-ꢁ Modulator
Large Dynamic Range
high dynamic range measurement applications. The AD1555
outputs a ones-density bitstream proportional to the analog
input. When used in conjunction with the AD1556 digital filter/
decimator, a high performance ADC is realized.
116 dB Min, 120 dB Typical @ 1 ms
117 dB Typical @ 0.5 ms
Low Input Noise: 80 nV rms @ 4 ms with
Gain of 34,128
Low Distortion: –111 dB Max, –120 dB Typical
Low Intermodulation: 122 dB
Sampling Rate at 256 kSPS
Very High Jitter Tolerance
No External Antialias Filter Required
Programmable Gain Front End
Input Range: ꢂ2.25 V
The continuous-time analog modulator input architecture avoids
the need for an external antialias filter. The programmable gain
front end simplifies system design, extends the dynamic range,
and reduces the system board area. Low operating power and
standby modes makes the AD1555 ideal for remote battery-pow-
ered data acquisition systems.
The AD1555 is fabricated on Analog Devices’ BiCMOS process
that has high performance bipolar devices along with CMOS
transistors. The AD1555 and AD1556 are packaged, respectively,
in 28-lead PLCC and 44-lead MQFP packages and are specified
from –55°C to +85°C (AD1556 and AD1555 B Grade) and from
0°C to 85°C (AD1555 A Grade).
Robust Inputs
Gain Settings: 1, 2.5, 8.5, 34, 128
Common-Mode Rejection (DC to 1 kHz)
93 dB Min, 101 dB Typical @ Gain of 1
77 mW Typical Low Power Dissipation
Standby Modes
0
f
= 24.4Hz
IN
–20
–40
SNR = 116.7dB
THD = –120.6dB
AD1556
–60
FIR Digital Filter/Decimator
Serial or Parallel Selection of Configuration
Output Word Rates: 250 SPS to 16 kSPS
6.2 mW Typ Low Power Dissipation
70 ꢃW in Standby Mode
–80
–100
–120
–140
–160
Reference Design and Evaluation Board with
Software Available
APPLICATIONS
Seismic Data Acquisition Systems
Chromatography
–180
–200
0
50
100 150 200 250 300 350 400 450 500
FREQUENCY – Hz
Automatic Test Equipment
Figure 1. FFT Plot, Full-Scale AIN Input, Gain of 1
GENERAL DESCRIPTION
The AD1555 is a complete sigma-delta modulator, combined
with a programmable gain amplifier intended for low frequency,
FUNCTIONAL BLOCK DIAGRAM
REFIN
REFCAP2 REFCAP1 AGND3
PGA0...PGA4
H/S
ERROR
CB0...CB4
MFLG
MODE CONTROL
LOGIC
PGA
CONFIGURATION
REGISTER
INPUT SHIFT
REGISTER
DIN
CONTROL
REF DIVIDER
SCLK
CS
OVERVOLTAGE
DETECTION
STATUS
REGISTER
DAC
R/W
CSEL
DATA
OUTPUT
MUX
DIGITAL
FILTER
PGA
INPUT
MUX
DOUT
DRDY
RSEL
TDATA
LOOP
FILTER
MUX
AIN (+)
AIN (–)
MDATA
MCLK
DATA
REGISTER
CLOCK
GENERATION
TIN (+)
TIN (–)
CLOCK DIVIDER
AD1555
AD1556
AGND1
PGAOUT MODIN AGND2 +V
–V
V
DGND
CLKIN SYNC BW0...BW2 RESET PWRDN GND V
L
A
A
L
REV. B
Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, norforanyinfringementsofpatentsorotherrightsofthirdpartiesthat
may result from its use. No license is granted by implication or otherwise
under any patent or patent rights of Analog Devices.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700
Fax: 781/326-8703
www.analog.com
© Analog Devices, Inc., 2002
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