AD775

REV. 0

–10–

Table I. Components List

Reference Designator

Description

Quantity

R1, R2, R14

Potentiometer

3

R3, R15

Resistor, 1%, 499

Ω

2

R4, R13, R16

Resistor, 1%, 49.9

Ω

3

R5, R10

Resistor, 1%, 20

Ω

2

R6–R9

Resistor, 1%, 10 k

Ω

4

R11

Resistor, 1%, 75

Ω

1

R12

Resistor, 1%, 4.99 k

Ω

1

CR1

Diode, 1N4148

1

C1, C2, C5, C6, C9, C12–C15, C18

C20, C22, C23

Ceramic Cap, Z5U, 0.1

µF13

C3, C4

Capacitor, Mica, 390 pF

2

C7

Capacitor, Mica, 10 pF

1

C8

Capacitor, Tantalum, 22

µF, 16 V

1

C11, C16, C19, C21

Capacitor, Alum. Electrolytic, 47

µF, 16 V

4

Q1

Transistor, 2N3904

1

U1

AD680JT

1

U2

AD822AN

1

U3

AD817AN

1

U4

78M05

1

U5

AD775

1

U6

74ALS541N

1

U7

74HC04N

1

J1, J8

BNC Jack

2

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-

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.

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

the AD775 should be connected together and isolated from

entirely digital components.