EVAL-CONTROLBOARD AD [Analog Devices], EVAL-CONTROLBOARD Datasheet - Page 19

• Manufacturer Part Number: EVAL-CONTROLBOARD
• Description: 16-Bit, 100 kSPS CMOS ADC
• Manufacturer: AD [Analog Devices]

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 AD7660 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 23 shows a connection diagram which allows that. Com-
ponent values required and resulting full-scale ranges are shown in
Table II.
Input Range
± 10 V
± 5 V
0 V to –5 V
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 in Figure 23. Also, C
can be used as a one-pole antialiasing filter.
Layout
The AD7660 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 AD7660 should be
designed so 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 AD7660, or, at least, as close as possible to the
AD7660. If the AD7660 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,
which should be established as close as possible to the AD7660.
ANALOG
INPUT
Table II. Component Values and Input Ranges
2.5V REF
U2
R1
1 kΩ
2 kΩ
8 kΩ
R2
R3
C
REF
R2
8 kΩ
8 kΩ
8 kΩ
U1
R1
C
R4
F
100nF
100nF
R3
10 kΩ
10 kΩ
None
IN
INGND
REF
REFGND
AD7660
R4
84 kΩ
6.67 kΩ
0 Ω
F
It is recommended to 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 AD7660 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 will reduce the effect of feedthrough
through the board.
The power supply lines to the AD7660 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 decoupling is
also important to lower the supplies impedance presented to
the AD7660 and reduce the magnitude of the supply spikes.
Decoupling ceramic capacitors, typically 100 nF, should be
placed on each power supplies 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 AD7660 can be either a separate
supply or come from the analog supply, AVDD, or from the
digital interface supply, OVDD. When the system digital supply
is noisy, or fast switching digital signals are present, it is recom-
mended if no separate supply available, to connect the DVDD
digital supply to the analog supply AVDD through an RC filter
as shown in Figure 6, and connect the system supply to the inter-
face 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 AD7660 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 con-
nected with the least resistance to the analog ground plane.
DGND must be tied to the analog or digital ground plane depend-
ing on the configuration. OGND is connected to the digital
system ground.
Evaluating the AD7660 Performance
A recommended layout for the AD7660 is outlined in the evalu-
ation board for the AD7660. The evaluation board package
includes a fully assembled and tested evaluation board, docu-
mentation, and software for controlling the board from a PC
via the Eval-Control Board.
AD7660