AD1556ASRL datasheet(16/24 Pages) AD

16 / 24 page

REV. B

AD1555/AD1556

–16–

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 FO = 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 M

Ω, 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 k

Ω 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 FO = 500 Hz. For applications

where the sensor inputs must be protected against severe

AIN (+)

AIN (–)

TIN (–)

AD1555

ADG609

AD780

O/P

GND

VOUT

TEM

+VIN

+5V

100nF

AC SINE

TEST

SOURCE

100nF

PGAOUT MODIN REFIN REFCAP1 AGND3

CLOCK SOURCE

1.024MHz

TDATA

SCLK

RSEL

DIN

DOUT

DRDY

ERROR

SERIAL DATA

INTERFACE

ADSP-21xxx OR

AD1556

HARDWARE

CONTROL

100nF

10 F

+5V

–5V

100nF

10 F

TIN (+)

SENSOR:

GEOPHONE,

HYDROPHONE...

CB0...CB4

MFLG

MDATA

MCLK

DGND

22 F

100nF

TO OTHER AD1556s

VDIG

100nF

11, 22, 44

12, 23, 24, 34

–VA

+VA

AGND2

AGND1

3, 26

4, 20, 21

127

MCLK

MDATA

MFLG

CB0...CB4

TO OTHER AD1555s

UNUSED AD1555 PINS MUST BE LEFT

UNCONNECTED;

UNUSED AD1556 INPUT PINS MUST BE

TIED TO DGND OR VL.

10 F

+5V

–5V

SYNC

RESET

DGND

DC TEST

SOURCE

RESETD

Figure 7. Typical Operating Circuit

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