AD7764 Analog Devices, AD7764 Datasheet - Page 27

• Manufacturer Part Number: AD7764
• Description: 24-Bit, 312 kSPS, 109 dB Sigma Delta ADC with On-Chip Buffers and Serial Interface
• Manufacturer: Analog Devices

Specifications of AD7764

• Resolution (bits): 24bit
• # Chan: 1
• Sample Rate: 40MSPS
• Interface: Ser,SPI
• Analog Input Type: Diff-Uni
• Ain Range: 6.5 V p-p
• Adc Architecture: Sigma-Delta
• Pkg Type: SOP

DECOUPLING AND LAYOUT INFORMATION
SUPPLY DECOUPLING
The decoupling of the supplies applied to the AD7764 is
important in achieving maximum performance. Each supply
pin must be decoupled to the correct ground pin with a 100 nF,
0603 case size capacitor.
Pay particular attention to decoupling Pin 7 (AV
the nearest ground pin (Pin 8). The digital ground pin AGND2
(Pin 20) is routed directly to ground. Also, connect REFGND
(Pin 26) directly to ground.
The DV
decoupled to the ground plane at a point away from the device.
It is advised to decouple the supplies that are connected to the
following supply pins through 0603 size, 100 nF capacitors to a
star ground point linked to Pin 23 (AGND1):
A layout decoupling scheme for these supplies, which connect
to the right side of the AD7764, is shown in Figure 51. Note the
star-point ground created at Pin 23.
REFERENCE VOLTAGE FILTERING
A low noise reference source, such as the
(4.096 V), is suitable for use with the AD7764. The reference
voltage supplied to the AD7764 should be decoupled and
filtered as shown in Figure 52.
The recommended scheme for the reference voltage supply is a
200 Ω series resistor connected to a 100 μF tantalum capacitor,
followed by a 10 nF decoupling capacitor very close to the
V
•
•
•
•
REF
+ pin.
V
AV
AV
AV
REF
DD
DD
DD
DD
+ (Pin 27)
4 (Pin 25)
1 (Pin 24)
2 (Pin 21)
(Pin 17) and AV
STAR-POINT
PIN 23
Figure 51. Supply Decoupling
GND
PIN 15
GND
(PIN 25)
AV
DD
DD
3 (Pin 28) supplies should be
4
VIA TO GND
FROM PIN 20
GND
AV
V
AV
AV
REF
DD
DD
DD
ADR444
+ (PIN 27)
3 (PIN 28)
1 (PIN 24)
2 (PIN 21)
DD
2) directly to
or
ADR434
Rev. A | Page 27 of 32
DIFFERENTIAL AMPLIFIER COMPONENTS
The components recommended for use around the on-chip
differential amplifier are detailed in Table 7. Matching the
components on both sides of the differential amplifier is
important to minimize distortion of the signal applied to the
amplifier. A tolerance of 0.1% or better is required for these
components. Symmetrical routing of the tracks on both sides
of the differential amplifier also assists in achieving stated
performance. Figure 53 shows a typical layout for the com-
ponents around the differential amplifier. Note that the traces
for both differential paths are made as symmetrical as possible
and that the feedback resistors and capacitors are placed on the
underside of the PCB to enable the simplest routing.
LAYOUT CONSIDERATIONS
Whereas using the correct components is essential to achieve
optimum performance, the correct layout is equally as
important. The AD7764 product page on www.analog.com
contains the Gerber files for the AD7764 evaluation board. The
Gerber files should be used as a reference when designing any
system using the AD7764.
The use of ground planes should be carefully considered. To
ensure that the return currents through the decoupling
capacitors are flowing to the correct ground pin, the ground
side of the capacitors should be as close to the ground pin
associated with that supply, as recommended in the Supply
Decoupling section.
7.5V
Figure 53. Typical Layout Structure for Surrounding Components
10µF
+
100nF
V
V
IN
IN
A+
A–
Figure 52. Reference Connection
2
V
IN
ADR444
GND
4
R
IN
V
OUT
R
6
IN
100µF
200Ω
R
+
FB
C
FB
100nF
AD7764
V
PIN 27
REF
+