AD7664 Analog Devices, AD7664 Datasheet - Page 19

• Manufacturer Part Number: AD7664
• Manufacturer: Analog Devices

Specifications of AD7664

• Resolution (bits): 16bit
• # Chan: 1
• Sample Rate: 570kSPS
• Interface: Par,Ser,SPI
• Analog Input Type: Diff-Uni
• Ain Range: (Vref) p-p,Uni (Vref)
• Adc Architecture: SAR
• Pkg Type: CSP,QFP

APPLICATION HINTS
Bipolar and Wider Input Ranges
In some applications, it is desired to use a bipolar or wider ana-
log input range like, for instance, ± 10 V, ± 5 V, or 0 V to 5 V.
Although the AD7664 has only one unipolar range, by simple
modifications of the input driver circuitry, bipolar and wider
input ranges can be used without any performance degradation.
Figure 22 shows a connection diagram that allows this.
Component values required and resulting full-scale ranges are
shown in Table III.
For applications where accurate gain and offset are desired, they
can be calibrated by acquiring a ground and a voltage reference
using an analog multiplexer, U2, as shown for bipolar input
ranges in Figure 22.
Input Range
± 10 V
± 5 V
0 V to –5 V
Layout
The AD7664 has very good immunity to noise on the power
supplies, as can be seen in Figure 9. However, care should still
be taken with regard to grounding layout.
The printed circuit board that houses the AD7664 should 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. Digital and
analog ground planes should be joined in only one place,
preferably underneath the AD7664, or, at least, as close as
possible to the AD7664. If the AD7664 is in a system where
multiple devices require analog-to-digital ground connections,
the connection should still be made at one point only, a star
ground point that should be established as close as possible to
the AD7664.
REV. E
Figure 22. Using the AD7664 in 16-Bit Bipolar and/or
Wider Input Ranges
ANALOG
INPUT
Table III. Component Values and Input Ranges
2.5V REF
U2
R1 (k )
1
1
1
R2
R3
C
REF
U1
R2 (k )
8
4
2
R1
R4
1 F
100nF
10nF
5
R3 (
10
10
None
IN
INGND
REF
REFGND
k
AD7664
)
R4 (
8
6.67
0
k
)
–19–
It is recommended to avoid running digital lines under the
device, because these couple noise onto the die. The analog
ground plane should be allowed to run under the AD7664 to avoid
noise coupling. Fast switching signals like CNVST or clocks
should be shielded with digital ground to avoid radiating
noise to other sections of the board and 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. This reduces the
effect of feedthrough through the board.
The power supplies’ lines to the AD7664 should use as large a
trace as possible to provide low impedance paths and reduce the
effect of glitches on the power supplies’ lines. Good decoupling
is also important to lower the supplies’ impedance presented to
the AD7664 and to reduce the magnitude of the supply spikes.
Decoupling ceramic capacitors, typically 100 nF, should be
placed on each power supply’s pins AVDD, DVDD, and OVDD
close to, and ideally right up against, these pins and their corre-
sponding ground pins. Additionally, low ESR 10 µF capacitors
should be located in the vicinity of the ADC to further reduce low
frequency ripple.
The DVDD supply of the AD7664 can be either a separate supply
or come from the analog supply AVDD or the digital interface
supply OVDD. When the system digital supply is noisy, or fast
switching digital signals are present, it is recommended that, if no
separate supply is available, to connect the DVDD digital supply
to the analog supply, AVDD, through an RC filter, as shown
in Figure 5, and to connect the system supply to the interface
digital supply, OVDD, and the remaining digital circuitry. When
DVDD is powered from the system supply, it is useful to insert
a bead to further reduce high frequency spikes.
The AD7664 has five different ground pins: INGND, REFGND,
AGND, DGND, and OGND. INGND is used to sense the
analog input signal. REFGND senses the reference, voltage and
should be a low impedance return to the reference because it
carries pulsed currents. AGND is the ground to which most
internal ADC analog signals are referenced. This ground must
be connected with the least resistance to the analog ground
plane. DGND must be tied to the analog or digital ground
plane, depending on the configuration. OGND is connected to
the digital system ground.
Evaluating the AD7664 Performance
A recommended layout for the AD7664 is outlined in the
EVAL-AD7664 evaluation board for the AD7664. The evaluation
board package includes a fully assembled and tested evaluation
board, documentation, and software for controlling the board
from a PC via the EVAL-CONTROL-BRD2.
AD7664