AD7731BRUZ-REEL Analog Devices Inc, AD7731BRUZ-REEL Datasheet - Page 33

• Manufacturer Part Number: AD7731BRUZ-REEL
• Description: 24-BIT SIGMA DELTA ADC I.C.
• Manufacturer: Analog Devices Inc

Specifications of AD7731BRUZ-REEL

• Number Of Bits: 24
• Sampling Rate (per Second): 6.4k
• Data Interface: DSP, Serial, SPI™
• Number Of Converters: 1
• Power Dissipation (max): 125mW
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 24-TSSOP (0.173", 4.40mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• For Use With: EVAL-AD7731EBZ - BOARD EVALUATION FOR AD7731
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

REV. A
Placing the part in standby mode reduces the total current to
10 A typical when the part is operated from an external master
clock, provided this master clock is stopped. If the external
clock continues to run in standby mode, the standby current
increases to 400 A typical. If a crystal or ceramic resonator is
used as the clock source, then the total current in standby mode
is 400 A typical. This is because the on-chip oscillator circuit
continues to run when the part is in its standby mode. This is
important in applications where the system clock is provided by
the AD7731’s clock, so that the AD7731 produces an uninter-
rupted master clock even when it is in its standby mode.
Digital Outputs
The AD7731 has two digital output pins, D0 and D1. When
the DEN bit of the Mode Register is set to 1, these digital
outputs assume the logic status of bits D0 and D1 of the
Mode Register. It gives the user access to two digital port
pins which can be programmed over the normal serial inter-
face of the AD7731. The two outputs obtain their supply
voltage from AV
even in cases where DV
POWER SUPPLIES
There is no specific power sequence required for the AD7731,
either the AV
the latch-up performance of the AD7731 is very good, it is
important that power is applied to the AD7731 before signals at
REF IN, AIN or the logic input pins in order to avoid latch-up
caused by excessive current. If this is not possible, then the
current which flows in any of these pins should be limited to less
than 30 mA per pin and less than 100 mA cumulative. If sepa-
rate supplies are used for the AD7731 and the system digital
circuitry, then the AD7731 should be powered up first. If it is
not possible to guarantee this, then current limiting resistors
should be placed in series with the logic inputs to again limit the
current to less than 30 mA per pin and less than 100 mA total.
Grounding and Layout
Since the analog inputs and reference input are differential,
most of the voltages in the analog modulator are common-mode
voltages. The excellent Common-Mode Rejection of the part
will remove common-mode noise on these inputs. The analog
and digital supplies to the AD7731 are independent and sepa-
rately pinned out to minimize coupling between the analog and
digital sections of the device. The digital filter will provide rejec-
tion of broadband noise on the power supplies, except at integer
multiples of the modulator sampling frequency. The digital filter
also removes noise from the analog and reference inputs pro-
vided those noise sources do not saturate the analog modulator.
As a result, the AD7731 is more immune to noise interference
that a conventional high resolution converter. However, because
the resolution of the AD7731 is so high and the noise levels
from the AD7731 so low, care must be taken with regard to
grounding and layout.
The printed circuit board that houses the AD7731 should be
designed such that the analog and digital sections are separated
and confined to certain areas of the board. This facilitates the
use of ground planes which can be separated easily. A minimum
etch technique is generally best for ground planes as it gives the
best shielding. Digital and analog ground planes should only be
joined in one place. If the AD7731 is the only device requiring
REV. 0
DD
or the DV
DD
, thus the outputs operate to 5 V levels
DD
DD
= +3 V.
supply can come up first. While
–33–
an AGND to DGND connection, the ground planes should be
connected at the AGND and DGND pins of the AD7731. If the
AD7731 is in a system where multiple devices require AGND to
DGND connections, the connection should still be made at one
point only, a star ground point, which should be established as
close as possible to the AD7731.
Avoid running digital lines under the device as these will couple
noise onto the die. The analog ground plane should be allowed
to run under the AD7731 to avoid noise coupling. The power
supply lines to the AD7731 should use as large a trace as pos-
sible to provide low impedance paths and reduce the effects of
glitches on the power supply line. Fast switching signals like
clocks should be shielded with digital ground to avoid radiating
noise to other sections of the board and clock signals should
never be run near the analog inputs. Avoid crossover of digital
and analog signals. Traces on opposite sides of the board should
run at right angles to each other. This will reduce the effects of
feedthrough through the board. A microstrip technique is by far
the best but is not always possible with a double-sided board. In
this technique, the component side of the board is dedicated to
ground planes while signals are placed on the solder side.
Good decoupling is important when using high resolution ADCs.
All analog supplies should be decoupled with 10 F tantalum in
parallel with 0.1 F capacitors to AGND. To achieve the best
from these decoupling components, they have to be placed as
close as possible to the device, ideally right up against the device.
All logic chips should be decoupled with 0.1 F disc ceramic
capacitors to DGND. In systems where a common supply volt-
age is used to drive both the AV
is recommended that the system’s AV
supply should have the recommended analog supply decoupling
capacitors between the AV
and the recommended digital supply decoupling capacitor
between the DV
Evaluating the AD7731 Performance
A recommended layout for the AD7731 is outlined in the evalu-
ation board for the AD7731. The evaluation board package
includes a fully assembled and tested evaluation board, docu-
mentation, software for controlling the board over the printer
port of a PC and software for analyzing the AD7731’s perfor-
mance on the PC. The evaluation board order number is
EVAL-AD7731EB.
Noise levels in the signals applied to the AD7731 may also
affect performance of the part. The AD7731 allows a technique
for evaluating the true performance of the part, independent of
the analog input signal. This scheme should be used after a
calibration has been performed on the part.
The first method is to select the AIN6/AIN6 input channel
arrangement. In this case, the differential inputs to the AD7731
are internally shorted together to provide a zero differential
voltage for the analog modulator. External to the device, the
AIN6 input should be connected to a voltage which is within the
allowable common-mode range of the part.
The software in the evaluation board package allows the user to
look at the noise performance in terms of bits and nV. Once the
user has established that the noise performance of the part is
satisfactory in this mode, then an external input voltage can be
applied to the device incorporating more of the signal chain.
DD
pin of the AD7731 and DGND.
DD
pin of the AD7731 and AGND
DD
and DV
DD
supply is used. This
DD
of the AD7731, it
AD7731