AD1555_15 Datasheet, PDF (16/24 Pages) Analog Devices – 24-Bit ADC with Low Noise PGA

AD1555/AD1556
CIRCUIT DESCRIPTION
The AD1555/AD1556 chipset is a complete sigma-delta 24-bit
A/D converter with very high dynamic range intended for the
measurement of low frequency signals up to a few kHz such as
those in seismic applications.
The AD1555 contains an analog multiplexer, a fully differential
programmable gain amplifier and a fourth order sigma-delta
modulator. The analog multiplexer allows selection of one fully
differential input from two different external inputs, an internal
ground reference or an internal full-scale voltage reference. The
fully differential programmable gain amplifier (PGA) has five gain
settings of 1, 2.5, 8.5, 34, and 128, which allow the part to handle
a total of five different input ranges: 1.6 V rms, 636 mV rms,
187 mV rms, 47 mV rms, and 12.4 mV rms that are programmed
via digital input pins (CB0 to CB4). The modulator that operates
nominally at a sampling frequency of 256 kHz, outputs a bit-
stream whose ones-density is proportional to its input voltage.
This bitstream can be filtered using the AD1556, which is a
digital finite impulse low pass filter (FIR). The AD1556 outputs
the data in a 24-bit word over a serial interface. The cutoff
frequency and output rate of this filter can be programmed via
an on-chip register or by hardware through digital input pins.
The dynamic performance and the equivalent input noise vary
with gain and output rate as shown in Table I. The use of the
different PGA gain settings allows enhancement of the total system
dynamic range up to 146 dB (gain of 34 or 128 and F_O = 250 Hz).
The AD1555 operates from a dual analog supply (± 5 V),
while the digital part of the AD1555 operates from a +5 V
supply. The AD1556 operates from a single 3.3 V or 5 V
supply. Each device exhibits low power dissipation and can
be configured for standby mode.
Figure 7 illustrates a typical operating circuit.
MULTIPLEXER AND PROGRAMMABLE GAIN
AMPLIFIER (PGA)
Analog Inputs
The AD1555 has two sets of fully differential inputs AIN and
TIN. The common-mode rejection capability of these inputs
generally surpasses the performance of conventional program-
mable gain amplifiers. The very high input impedance, typically
higher than 140 MΩ, allows direct connection of the sensor to
the AD1555 inputs, even through serial resistances. Figure 7
illustrates such a configuration. The passive filter between the
sensor and the AD1555 is shown here as an example. Other
filter structures could be used, depending on the specific require-
ments of the application. Also, the Johnson noise (√4 k TRB) of
the serial resistance should be taken into consideration. For
instance, a 1 kΩ serial resistance reduces by approximately 1.3 dB
the dynamic performance of a system using a gain setting of
128 at an output word rate F_O = 500 Hz. For applications
where the sensor inputs must be protected against severe
AC SINE
TEST DC TEST
SOURCE SOURCE
+5V
100nF
SENSOR:
GEOPHONE,
HYDROPHONE...
3
14
–5V
ADG609
100nF
DB DA
15
15
9
8
3 _TEM
6P
AD780
+V_IN
2
V_OUT GND O/P
4
8
+5V
100nF
22␮F
UNUSED AD1555 PINS MUST BE LEFT
UNCONNECTED;
UNUSED AD1556 INPUT PINS MUST BE
TIED TO DGND OR V_L.
CLOCK SOURCE
1.024MHz
SERIAL DATA
INTERFACE
ADSP-21xxx OR ␮P
2
28 25
23
22
PGAOUT MODIN REFIN REFCAP1 _AGND3
7
TIN (+)
8
TIN (–)
CB0...CB4
15
MFLG
TO OTHER AD1555s
17
MDATA
R1
R3
5 _{AIN (+)}
AD1555
18
MCLK
T₁
C₁
C3
T₂
C₂
R2
R4
6
AIN (–)
+V_A AGND1 AGND2 –V_A
19
V_L
100nF
DGND
16
5
+5V
15⍀
10␮F
32
CB0...CB4
MFLG
MDATA
MCLK
16
CS
R/W ¹⁷
18
RSEL
30
TDATA
13
SCLK
19
DIN
14
DOUT
15
DRDY
ERROR ²⁰
AD1556
31
SYNC
25
RESET
TO OTHER AD1556s
HARDWARE
CONTROL
3, 26
1
27
4, 20, 21
+5V
10␮F
100nF
100nF
–5V
10␮F
RESETD 37
H/S ¹⁰
V_L
DGND
11, 22, 44 _100nF 12, 23, 24, 34
V_DIG
Figure 7. Typical Operating Circuit
–16–
REV. B