AD7719BR Analog Devices Inc, AD7719BR Datasheet - Page 35

• Manufacturer Part Number: AD7719BR
• Description: IC ADC 16BIT 24BIT DUAL 28-SOIC
• Manufacturer: Analog Devices Inc

Specifications of AD7719BR

• Number Of Bits: 16/24
• Sampling Rate (per Second): 105
• Data Interface: DSP, MICROWIRE™, QSPI™, Serial, SPI™
• Number Of Converters: 2
• Power Dissipation (max): 4.5mW
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-SOIC (0.300", 7.50mm Width)
• For Use With: EVAL-AD7719EB - BOARD EVAL FOR AD7719
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Internally in the AD7719, the coefficients are normalized before
being used to scale the words coming out of the digital filter. The
offset calibration coefficient is subtracted from the result prior
to the multiplication by the gain coefficient.
From an operational point of view, a calibration should be treated
like another ADC conversion. A zero-scale calibration (if required)
should always be carried out before a full-scale calibration. System
software should monitor the relevant ADC RDY0/1 bit in the
Status register to determine end of calibration via a polling
sequence or interrupt driven routine.
Grounding and Layout
Since the analog inputs and reference input on the main ADC
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 AD7719 are independent
and separately pinned out to minimize coupling between the
analog and digital sections of the device. The AD7719 can be
operated with 5 V analog and 3 V digital supplies, or vice versa.
The digital filter will provide rejection 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 provided these noise sources do
not saturate the analog modulator. As a result, the AD7719 is
more immune to noise interference than a conventional high
resolution converter. However, because the resolution of the
AD7719 is so high, and the noise levels from the AD7719 are so
low, care must be taken with regard to grounding and layout.
The printed circuit board that houses the AD7719 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 that can be easily separated. A minimum
etch technique is generally best for ground planes as it gives the
best shielding.
Although the AD7719 has separate pins for analog and digital
ground, the AGND and DGND pins are tied together within
the device via the substrate. The user must not tie these pins
external to separate ground planes unless the ground planes are
connected together near the AD7719.
In systems where the AGND and DGND are connected some-
where else in the system, i.e., the system power supply, they should
not be connected again at the AD7719 as a ground loop will
result. In these situations, it is recommended that the AD7719’s
AGND and DGND pins be tied to the AGND plane. In any
layout, it is important that the user keep in mind the flow of
currents in the system, ensuring that the return paths for all
currents are as close as possible to the paths the currents took to
reach their destinations. Avoid forcing digital currents to flow
through the AGND sections of the layout.
The PWRGND pin is tied internally to AGND on the AD7719.
The PWRGND pad internally has a resistance of less then 50 mΩ
to the PWRGND pin, while the resistance back to the AGND
pad is >3 Ω. This means that 19.5 mA of the maximum speci-
fied current (20 mA) will flow to PWRGND with the remaining
0.5 mA flowing to AGND. PWRGND and AGND should be
tied together at the AD7719 and it is important to minimize the
resistance on the ground return lines.
REV. A
–35–
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 AD7719 to prevent noise coupling. The power
supply lines to the AD7719 should use as wide a trace as possible
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 µF tantalum in
parallel with 0.1 µF 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 µF ceramic capacitors to
DGND. In systems where a common supply voltage is used to
drive both the AV
mended that the system’s AV
should have the recommended analog supply decoupling capaci-
tors between the AV
recommended digital supply decoupling capacitor between the
DV
APPLICATIONS
The AD7719 provides a low cost, high resolution analog-to-
digital function. Because the analog-to-digital function is provided
by a Σ-∆ architecture, it makes the part more immune to noisy
environments, making it ideal for use in sensor measurement and
industrial and process control applications. Given the architecture
used in the AD7719, where the signal chain is chopped and
the device is factory-calibrated at final test, field calibration
can be avoided due to the extremely low offset and gain drifts
exhibited by this converter. It also provides a programmable
gain amplifier, a digital filter, and system calibration options. Thus,
it provides far more system-level functionality than off-the-shelf
integrating ADCs without the disadvantage of having to supply a
high quality integrating capacitor. In addition, using the AD7719
in a system allows the system designer to achieve a much higher
level of resolution because noise performance of the AD7719 is
significantly better than that of integrating ADCs.
The on-chip PGA allows the AD7719 to handle an analog input
voltage range as low as 10 mV full-scale with V
differential inputs of the part allow this analog input range to
have an absolute value anywhere between AGND + 100 mV and
AV
directly to the input of the AD7719. The programmable gain
front end on the AD7719 allows the part to handle unipolar
analog input ranges from 0 mV to 20 mV to 0 V to 2.5 V and
bipolar inputs of ±20 mV to ±2.5 V. Because the part operates
from a single supply, these bipolar ranges are with respect to a
biased-up differential input. Another key advantage of the AD7719 is
that it contains two Σ-∆ converters operating in parallel; thus the
user does not need to interrupt the main channel when a second-
ary measurement on a different variable needs to be performed.
DD
DD
– 100 mV. It allows the user to connect the transducer
pin of the AD7719 and DGND.
DD
DD
and DV
pin of the AD7719 and AGND, and the
DD
DD
of the AD7719, it is recom-
supply be used. This supply
REF
AD7719
= 1.25 V. The