AD7650ACPZRL Analog Devices Inc, AD7650ACPZRL Datasheet - Page 18

• Manufacturer Part Number: AD7650ACPZRL
• Description: 1MSPS, 16-Bit ADC Int Ref, 2.5LSB
• Manufacturer: Analog Devices Inc

Specifications of AD7650ACPZRL

• Number Of Bits: 16
• Sampling Rate (per Second): 570k
• Data Interface: Serial, Parallel
• Number Of Converters: 1
• Power Dissipation (max): 77mW
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 48-VFQFN, CSP Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• For Use With: EVAL-AD7650CBZ - BOARD EVALUATION FOR AD7650
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

AD7650
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 AD7650 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 20 shows a connection diagram which allows that. Compo-
nents 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 for bipolar input
ranges in Figure 20.
Layout
The AD7650 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 AD7650 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 AD7650 or, at least, as close as possible to the
AD7650. If the AD7650 is in a system where multiple devices
require analog-to-digital ground connections, the connection
ANALOG
INPUT
Table II. Component Values and Input Ranges
2.5V REF
U2
R1
250 Ω
500 Ω
1 kΩ
R2
R3
C
R2
2 kΩ
2 kΩ
2 kΩ
REF
U1
R1
R4
1mF
100nF
10nF
5V
R3
10 kΩ
10 kΩ
None
IN+
IN–
REF
REFGND
AD7650
R4
8 kΩ
6.67 kΩ
0 Ω
should still be made at one point only, a star ground point,
which should be established as close as possible to the AD7650.
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 AD7650 to avoid noise
coupling. Fast switching signals like CNVST or clocks should be
shielded with digital ground to avoid radiating noise to other sec-
tions 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 supplies lines to the AD7650 should use as large
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 AD7650 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 correspond-
ing 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 AD7650 can either be 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, connect the DVDD digital
supply to the analog supply, AVDD, through an RC filter as
shown in Figure 5, 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 AD7650 has five different ground pins: IN–, REFGND,
AGND, DGND, and OGND. IN– 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 AD7650 Performance
A recommended layout for the AD7650 is outlined in the
evaluation board for the AD7650. 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 Board.