ADS774

11

®

In particular, the unused input pin should not be connected

to any capacitive load, including high impedance switches.

Even a few pF on the unused pin can degrade acquisition

time.

Coupling between analog input and digital lines should be

minimized by careful layout. For instance, if the lines must

cross, they should do so at right angles. Parallel analog and

digital lines should be separated from each other by a pattern

connected to common.

If external full scale and offset potentiometers are used, the

potentiometers and associated resistors should be as close as

possible to the ADS774.

POWER SUPPLY DECOUPLING

On the ADS774, +5V (to Pin 1) is the only power supply

required for correct operation. Pin 7 is not connected inter-

nally, so there is no problem in existing ADC774 sockets

as a logic input to select modes of control over the sampling

function as described above. When used in an existing

ADC774 socket, the –15V on pin 11 selects the ADC774

Emulation Mode. Since pin 11 is used as a logic input, it is

immune to typical supply variations.

INSTALLATION

LAYOUT PRECAUTIONS

Analog (pin 9) and digital (pin 15) commons are not con-

nected together internally in the ADS774, but should be

connected together as close to the unit as possible and to an

analog common ground plane beneath the converter on the

component side of the board. In addition, a wide conductor

pattern should run directly from pin 9 to the analog supply

common, and a separate wide conductor pattern from pin 15

to the digital supply common.

If the single-point system common cannot be established

directly at the converter, pin 9 and pin 15 should still be

connected together at the converter. A single wide conductor

pattern then connects these two pins to the system common.

In either case, the common return of the analog input signal

should be referenced to pin 9 of the ADC. This prevents any

voltage drops that might occur in the power supply common

returns from appearing in series with the input signal.

The speed of the ADS774 requires special caution regarding

whichever input pin is unused. For 10V input ranges, pin 14

(20V Range) must be unconnected, and for 20V input

ranges, pin 13 (10V Range) must be unconnected. In both

cases, the unconnected input should be shielded with ground

plane to reduce noise pickup.

S/H CONTROL MODE

ADC774 EMULATION MODE

(Pin 11 Connected to +5V)

(Pin 11 Connected to 0V to –15V)

SYMBOL

PARAMETER

MIN

TYP

MAX

MIN

TYP

MAX

UNITS

Throughput Time:

12-bit Conversions

8

8.5

8

8.5

µs

8-bit Conversions

6

6.3

6

6.3

µs

Conversion Time:

12-bit Conversions

6.4

6.4

µs

8-bit Conversions

4.4

4.4

µs

Acquisition Time

1.4

1.4

µs

Aperture Delay

20

1600

ns

Aperture Uncertainty

0.3

10

ns

TABLE VI. Conversion Timing, T

FIGURE 9. Signal Acquisition and Conversion Timing.

Signal

Acquisition

Conversion

Signal

Acquisition

t

AP

Signal

Acquisition

Conversion

Signal

Acquisition

t

C

t

AQ

t

C

t

AQ

t

AP

R/C

S/H Control Mode

Pin 11 connected to +5V.

ADC774 Emulation Mode*

Pin 11 connected to V

*In the ADC774 Emulation Mode, a convert command triggers a delay that

allows the ADS774 enough time to acquire the input signal before converting.