AD775 Analog Devices, AD775 Datasheet - Page 9

• Manufacturer Part Number: AD775
• Description: 8-Bit 20 MSPS/ 60 mW Sampling A/D Converter
• Manufacturer: Analog Devices

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POWER SUPPLY CONNECTIONS AND DECOUPLING
The analog and digital supplies of the AD775 have been sepa-
rate to prevent the typically large transients associated with the
on-chip digital circuitry from coupling into the analog supplies
(AV
conditions, AV
external to the part, preferably a common source somewhere on
the PC board.
Each supply should be decoupled by a 0.1 F capacitor located
as close to the device pin as possible. Surface-mount capacitors,
by virtue of their low parasitic inductance, are preferable to
through-hole types. A larger capacitor (10 F electrolytic)
should be located somewhere on the board to help decouple
large, low frequency supply noise. For specific layout informa-
tion, refer to the AD775 Evaluation Board section of the data
sheet.
ANALOG
C2
DD
TP1
+5VA
INPUT
, AV
1
3
5
A
3
J1
A
V
AD680
IN
TP2
SS
GND
R4
49.9
1
U1
C1
V
A
). However, in order to avoid possible latch-up
OUT
A
V
4
2
6
DD
3
2
499
500
500
AD817
EE
R3
R1
R2
C5
TP11
TP8
TP7
2
and DV
A
4
10k
R8
A
V
V
A
7
CC
U3
EE
C6
A
6
10k
V
R7
2
3
CC
DD
R14
500
10k
R9
AD822
+5VA
R13
+5VA
C19
20
must share a common supply
8
22 F
R15
499
A
1/2 U2
5
6
C8
AD822
4.99k
10k
C4
390pF
R6
C16
R12
V
1
C9
CC
4
V
V
+5VA
A
CC
A
EE
C20
1/2 U2
Q1
2N3904
C3
390pF
CR1
1N4148
R11
Figure 17. AD775 Evaluation Board Schematic
A
75
7
R10
20
V
V
J3
78M05
J2
IN
EE
GND
U4
C13
J7
V
TP4
V
OUT
A
RT
J4
C7
10pF
VRB
R5
20
TP3
A
TP5
C14
V
C12
IN
+5VA
C21
C11
TP9
C15
D
J6
J5
D
D
CLOCK
(
TP12
TP6
J8
13
14
15
16
17
18
19
20
21
22
23
24
A
D
)
–9–
49.9
V
V
V
AV
V
V
DV
R16
DV
AV
AV
AV
AV
+5VA
TP10
RTS
RT
IN
RBS
RB
+5V
AD775
SS
SS
DD
DD
DD
DD
SS
+5V
APPLICATIONS
AD775 EVALUATION BOARD
Figures 17 through 22 show the schematic and printed circuit
board (PCB) layout for the AD775 evaluation board. Referring
to Figure 17, the input signal is buffered by U3, an AD817 op
amp configured as a unity-gain follower. The signal is then ac-
coupled and dc-biased by adjusting potentiometer R14. Video
and imaging applications would typically use a dc-restoration
circuit instead of the manual potentiometer adjustment. Q1, an
emitter-follower, buffers the input signal and provides ample
current to drive a simple low-pass filter. The filtering is included
to limit wideband noise and highlight the fact that the AD775
can be driven from a nonzero source impedance.
The reference circuit is similar to the one shown in Figure 11
with the exception that R1 and R2 allow precise adjustment of
U5
DV
DV
5
CLK
OE
DD
D7
D6
D5
D4
D3
D2
D1
D0
SS
1/6
U7
12
11
10
9
8
7
6
5
4
3
2
1
6
J9
NOTES
C18
D
9
1/6
D
U7
= 47 F ELECTROLYTIC CAPACITOR
= 0.1 F CERAMIC CAPACITOR
J10
D
UNLESS OTHERWISE NOTED
UNLESS OTHERWISE NOTED
8
10
1
2
4
5
7
9
3
6
8
74ALS541
A1
A2
A3
A4
A5
A6
A7
A8
GND
G1
U6
ENABLE
V
G2
Y1
Y3
Y4
Y5
Y6
Y7
CC
Y2
Y8
TP13
19
18
17
16
15
14
13
12
11
20
+5V
C22
D
AD775
D
D
P2-40 PIN IDC
1
40
D