AD7194 Analog Devices, AD7194 Datasheet - Page 52

• Manufacturer Part Number: AD7194
• Description: 8-Channel, 4.8 kHz, Ultralow Noise, 24-Bit Sigma-Delta ADC with PGA
• Manufacturer: Analog Devices

Specifications of AD7194

• Resolution (bits): 24bit
• # Chan: 8
• Sample Rate: n/a
• Interface: Ser,SPI
• Analog Input Type: Diff-Uni
• Ain Range: (2Vref/PGA Gain) p-p
• Adc Architecture: Sigma-Delta
• Pkg Type: CSP

AD7194
GROUNDING AND LAYOUT
Because the analog inputs and reference inputs are differential,
most of the voltages in the analog modulator are common-
mode voltages. The high common-mode rejection of the part
removes common-mode noise on these inputs. The analog and
digital supplies to the AD7194 are independent and separately
pinned out to minimize coupling between the analog and
digital sections of the device. The digital filter provides rejection
of broadband noise on the power supplies, except at integer
multiples of the modulator sampling frequency.
Connect an R-C filter to each analog input pin to provide rejection
at the modulator sampling frequency. A 100 Ω resistor in series
with each analog input, a 0.1 μF capacitor between the analog
input pins, and a 0.01 μF capacitor from each analog input to
AGND are advised.
The digital filter also removes noise from the analog and
reference inputs provided that these noise sources do not
saturate the analog modulator. As a result, the AD7194 is
more immune to noise interference than a conventional high
resolution converter. However, because the resolution of the
AD7194 is so high and the noise levels from the converter so
low, care must be taken with regard to grounding and layout.
The printed circuit board (PCB) that houses the ADC must be
designed so 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 because it
gives the best shielding.
Although the AD7194 has separate pins for analog and digital
ground, the AGND and DGND pins are tied together internally
via the substrate. Therefore, the user must not tie these two pins
to separate ground planes unless the ground planes are connected
together near the AD7194.
In systems in which the AGND and DGND are connected
somewhere else in the system (that is, the power supply of the
system), they should not be connected again at the AD7194
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because a ground loop results. In these situations, it is recom-
mended that the ground pins of the AD7194 be tied to the
AGND plane.
In any layout, the user must keep in mind the flow of currents
in the system, ensuring that the 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.
Avoid running digital lines under the device because this
couples noise onto the die, and allows the analog ground plane
to run under the AD7194 to prevent noise coupling. The power
supply lines to the AD7194 must use as wide a trace as possible
to provide low impedance paths and reduce the effects of
glitches on the power supply line. Shield fast switching signals,
like clocks, with digital ground to prevent radiating noise to
other sections of the board, and never run clock signals near the
analog inputs. Avoid crossover of digital and analog signals.
Run traces on opposite sides of the board at right angles to each
other. This reduces 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,
whereas signals are placed on the solder side.
Good decoupling is important when using high resolution
ADCs. Decouple all analog supplies with 10 μF tantalum
capacitors in parallel with 0.1 μF capacitors to AGND. To
achieve the best results from these decoupling components,
place them as close as possible to the device, ideally right up
against the device. Decouple all logic chips with 0.1 μF ceramic
capacitors to DGND. In systems in which a common supply
voltage is used to drive both the AV
it is recommended that the system AV
this supply, place the recommended analog supply decoupling
capacitors between the AV
and the recommended digital supply decoupling capacitor
between the DV
DD
pin of the AD7194 and DGND.
DD
pin of the AD7194 and AGND
DD
DD
and DV
supply be used. For
DD
of the AD7194,