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EVAL-AD7764EBZ View Datasheet(PDF) - Analog Devices

Part Name
Description
MFG CO.
'EVAL-AD7764EBZ' PDF : 32 Pages View PDF
AD7764
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 (AVDD2) directly to
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 DVDD (Pin 17) and AVDD3 (Pin 28) supplies should be
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):
VREF+ (Pin 27)
AVDD4 (Pin 25)
AVDD1 (Pin 24)
AVDD2 (Pin 21)
A layout decoupling scheme for the these supplies, which
connect to the right hand side of the AD7764, is shown in
Figure 46. Note the star-point ground created at Pin 23.
AVDD4
(PIN 25)
GND
PIN 23
STAR-POINT
GND
AVDD3 (PIN 28)
VREF+ (PIN 27)
AVDD1 (PIN 24)
GND
AVDD2 (PIN 21)
PIN 15
VIA TO GND
FROM PIN 20
Figure 46. Supply Decoupling
REFERENCE VOLTAGE FILTERING
A low noise reference source, such as the ADR444 or ADR34
(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 47.
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 VREF+ pin.
7.5V
10µF+ 100nF
ADR444
2 +VIN
VOUT 6 200
GND
4
+
100µF 100nF
VREF+
PIN 27
Figure 47. Reference Connection
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 48 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 the feedback resistors and capacitors are placed on
the underside of the PCB to enable the simplest routing.
VINA–
RIN
RFB
CFB
VINA+
RIN
Figure 48. Typical Layout Structure for Surrounding Components
LAYOUT CONSIDERATIONS
While 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.
Rev. 0 | Page 26 of 32
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