AD7765 Analog Devices, AD7765 Datasheet - Page 26

• Manufacturer Part Number: AD7765
• Description: 24-Bit, 156 kSPS, 112 dB Sigma-Delta ADC with On-Chip Buffers and Serial Interface
• Manufacturer: Analog Devices

Specifications of AD7765

• 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

AD7765
DECOUPLING AND LAYOUT INFORMATION
SUPPLY DECOUPLING
The decoupling of the supplies applied to the AD7765 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 AD7765, is shown in Figure 45. 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 AD7765. The reference
voltage supplied to the AD7765 should be decoupled and
filtered, as shown in Figure 46.
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
V
AV
AV
AV
REF
DD
DD
DD
DD
REF
+ (Pin 27)
4 (Pin 25)
1 (Pin 24)
2 (Pin 21)
+ pin.
(Pin 17) and AV
STAR-POINT
Figure 45. AD7765 Supply Decoupling
PIN 23
GND
PIN 15
GND
(PIN 25)
AV
DD
DD
4
3 (Pin 28) supplies should be
VIA TO GND
FROM PIN 20
GND
AV
V
AV
AV
REF
DD
DD
DD
+ (PIN 27)
3 (PIN 28)
1 (PIN 24)
2 (PIN 21)
ADR444
DD
2) directly to
or
ADR434
Rev. A | Page 26 of 32
7.5V
DIFFERENTIAL AMPLIFIER COMPONENTS
The correct components 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 47 shows a
typical layout for the components 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
While using the correct components is essential to achieving
optimum performance, the correct layout is just as important.
The AD7765 product page on
Gerber files for the AD7765 evaluation board. These files
should be downloaded and used as a reference when designing
any system using the AD7765.
The use of ground planes should also 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.
Figure 47. Typical Layout Structure for Surrounding Components
10µF
+
100nF
V
V
IN
IN
A–
A+
Figure 46. Reference Connection
2
V
IN
ADR444
GND
4
R
V
IN
www.analog.com
OUT
R
6
IN
100µF
200Ω
+
R
FB
C
FB
100nF
contains the
V
PIN 27
REF
+