AD775 Datasheet, PDF (10/12 Pages) Analog Devices – 8-Bit 20 MSPS, 60 mW Sampling A/D Converter

AD775
V_RT and V_RB. Note that the V_RT and V_RB traces (see Figures 19
and 20) are run in parallel and in the same proximity. Any noise
coupling is likely to be common mode to both signals and would
result in an offset error but not a gain error. The entire reference
circuit is powered by a single +5 V supply. The minimum volt-
age for V_RB is determined by the impedance of the AD822 out-
put stage and the amount of current flowing through the
internal resistor ladder of the AD775.
The sampling clock is buffered by U7, a 74HC04 inverter. It is
recommended that the output loading of the inverter is mini-
mized in order to maintain fast transition times on the clock. An
additional inverter is used to provide a buffered clock signal
whose rising edges indicate that data is valid. A 74ALS541
buffers the eight digital data outputs of the AD775 to improve
the load driving capability.
The multilayer PCB board layout shows some of the important
design guidelines recommended for the AD775. The most im-
portant aspect is the power and ground distribution. While the
AD775 has separate analog and digital power and ground pins,
the AD775 should be treated as an entirely analog component.
The ground plane is joined close to the ADC in order to main-
tain a low potential difference across the analog and digital
ground pins. Because the power and grounds are derived from a
common point, a slit in the ground plane is used to minimize
any interaction between the analog and digital return currents.
The power for the AD775, AV_DD and DV_DD, are derived from
the same supply. Separate traces are run to AV_DD and DV_DD
and joined together at the source. While not used on the evalua-
tion board, a ferrite bead or inductor can effectively isolate noise
generated by digital circuitry such as the output buffers. In cases
where only a single supply is available, the inductor should not
be placed between AV_DD and DV_DD. Instead, both supplies of
the AD775 should be connected together and isolated from
entirely digital components.
Table I. Components List
Reference Designator
Description
R1, R2, R14
R3, R15
R4, R13, R16
R5, R10
R6–R9
R11
R12
CR1
C1, C2, C5, C6, C9, C12–C15, C18
C20, C22, C23
C3, C4
C7
C8
C11, C16, C19, C21
Q1
U1
U2
U3
U4
U5
U6
U7
J1, J8
Potentiometer
Resistor, 1%, 499 Ω
Resistor, 1%, 49.9 Ω
Resistor, 1%, 20 Ω
Resistor, 1%, 10 kΩ
Resistor, 1%, 75 Ω
Resistor, 1%, 4.99 kΩ
Diode, 1N4148
Ceramic Cap, Z5U, 0.1 µF
Capacitor, Mica, 390 pF
Capacitor, Mica, 10 pF
Capacitor, Tantalum, 22 µF, 16 V
Capacitor, Alum. Electrolytic, 47 µF, 16 V
Transistor, 2N3904
AD680JT
AD822AN
AD817AN
78M05
AD775
74ALS541N
74HC04N
BNC Jack
Quantity
3
2
3
2
4
1
1
1
13
2
1
1
4
1
1
1
1
1
1
1
1
2
–10–
REV. 0