AD7657 Analog Devices, AD7657 Datasheet - Page 29

• Manufacturer Part Number: AD7657
• Description: 250 kSPS, 6-Channel, Simultaneous Sampling, Bipolar, 14-Bit A/D Converter
• Manufacturer: Analog Devices

Specifications of AD7657

• Resolution (bits): 14bit
• # Chan: 6
• Sample Rate: 250kSPS
• Interface: Par,Ser,SPI
• Analog Input Type: SE-Bip
• Ain Range: Bip (Vref) x 2,Bip (Vref) x 4,Bip 10V,Bip 5.0V
• Adc Architecture: SAR
• Pkg Type: QFP

APPLICATION HINTS
LAYOUT
The printed circuit board that houses the AD7656/AD7657/
AD7658 should be designed so that the analog and digital
sections are separated and confined to certain areas of the board.
At least one ground plane should be used. It could be common
or split between the digital and analog sections. In the case of
the split plane, the digital and analog ground planes should be
joined in only one place, preferably underneath the AD7656/
AD7657/AD7658, or at least as close as possible to each part.
If the AD7656/AD7657/AD7658 are in a system where multiple
devices require analog-to-digital ground connections, the con-
nection should still be made at only one point, a star ground
point, which should be established as close as possible to the
AD7656/AD7657/AD7658. Good connections should be made
to the ground plane. Avoid sharing one connection for multiple
ground pins. Individual vias or multiple vias to the ground
plane should be used for each ground pin.
Avoid running digital lines under the devices because doing so
couples noise onto the die. The analog ground plane should be
allowed to run under the AD7656/AD7657/AD7658 to avoid
noise coupling. Fast-switching signals like CONVST or clocks
should be shielded with digital ground to avoid radiating noise
to other sections of the board, and they should never run near
analog signal paths. Crossover of digital and analog signals
should be avoided. Traces on different but close layers of the
board should run at right angles to each other to reduce the
effect of feedthrough through the board.
The power supply lines to the AV
pins on the AD7656/AD7657/AD7658 should use as large a
trace as possible to provide low impedance paths and reduce the
effect of glitches on the power supply lines. Good connections
should be made between the AD7656/AD7657/AD7658 supply
pins and the power tracks on the board; this should involve the
use of a single via or multiple vias for each supply pin.
Good decoupling is also important to lower the supply
impedance presented to the AD7656/AD7657/AD7658 and to
reduce the magnitude of the supply spikes. Decoupling ceramic
capacitors, typically 100 nF, should be placed on all of the power
supply pins, V
capacitors should be placed close to, ideally right up against,
these pins and their corresponding ground pins. Additionally,
low ESR 10 μF capacitors should be placed on each of the
supply pins. Avoid sharing these capacitors between pins. Use
big vias to connect the capacitors to the power and ground
planes. Use wide, short traces between the via and the capacitor
pad, or place the via adjacent to the capacitor pad to minimize
DD
, V
SS
, AV
CC
, DV
CC
CC
, and V
, DV
CC
DRIVE
, V
DRIVE
. These decoupling
, V
DD
, and V
Rev. C | Page 29 of 32
SS
parasitic inductances. Recommended decoupling capacitors are
100 nF, low ESR, ceramic capacitors (Farnell 335-1816) and
10 μF, low ESR, tantalum capacitors (Farnell 197-130) for the
AV
be placed where the AV
An alternative reduced decoupling arrangement is outlined in
the Typical Connection Diagram section. This decoupling
arrangement groups the AV
the decoupling capacitors to be shared between the supply pairs.
Group the six AV
Pin 35, Pin 40 and Pin 41, and Pin 46 and Pin 47. Connect the
supply pins in each pair together; their location on the AD7656/
AD7657/AD7658 pin configuration easily facilitates this. For
the AD7656, decouple each pair with a 100 μF capacitor; for the
AD7657, decouple each pair with a 33 μF capacitor; for the
AD7658, decouple each pair with a 22 μF capacitor. For this
minimum decoupling configuration, all other supply and
reference pins should be decoupled with a 10 μF decoupling
capacitor.
POWER SUPPLY CONFIGURATION
As outlined in the Absolute Maximum Ratings section, the
analog inputs should not be applied to the AD7656/AD7657/
AD7658 until after the AD7656/AD7657/AD7658 power
supplies have been applied to the device. However, if a
condition exists where the system analog signal conditioning
circuitry supplies are different to the V
the AD7656/AD7657/AD7658 or if the analog inputs may be
applied prior to the AD7656/AD7657/AD7658 supplies being
established then an analog input series resister and Schottky
diodes in series with the V
mended, see Figure 36. This configuration should also be
used if AV
prior to V
CC
decoupling. A large tantalum decoupling capacitor should
DD
ANALOG
CC
INPUTS
and V
is applied to the AD7656/AD7657/AD7658
Figure 36. Power Supply Configuration
CC
SS
core supply pins into three pairs, Pin 34 and
.
240Ω
CC
AD7656/AD7657/AD7658
supply enters the board.
DD
CC
V1
V2
V3
V4
V5
V6
and V
supply pins into pairs and allows
AD7656/
AD7657/
AD7658
SS
supplies are recom-
V
V
V
V
DD
DD
DD
SS
SS
and V
SS
supplies of