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11/20/01

SPT7864

DIFFERENTIAL CLOCK INPUT

The SPT7864 clock can be driven differentially or single-

The peak-to-peak value should be 0.8 V. In order to pre-

serve accuracy at high input frequency, it is important that

the clock have low jitter. The differential clock input is made

to allow a low-jitter clock design. To ensure low jitter, the

differential input should be a pure sine wave with low white

noise floor.

SINGLE-ENDED CLOCK INPUT

biased to 1.5 V, and should externally be decoupled to

ground by a capacitor. A CMOS logic level clock (5 V or

3 V) is applied at the CLK node. (To get an inverted clock

input, CLK should be decoupled and the clock signal ap-

be close to 50%. Consecutive pipeline stages in the ADC

are clocked in antiphase. With a 50% duty cycle, every

stage has the same time for settling. If the duty cycle devi-

ates from 50%, every second stage has a shorter time for

settling; thus it operates less accurately, causing degrada-

tion of SNR.

In order to preserve accuracy at high input frequency, it is

important that the clock have low jitter and steep edges.

Rise/fall times should be kept shorter than 2 ns whenever

possible. Overshoot should be minimized. Low jitter is es-

pecially important when converting high-frequency input

signals. Jitter causes the noise floor to rise proportionally

to input signal frequency. Jitter may be caused by crosstalk

on the PCB. It is therefore recommended that the clock

trace on the PCB be made as short as possible.

DIGITAL OUTPUTS

The digital output data appears in offset binary code at

CMOS logic levels. Full-scale negative input results in out-

put code 000...0. Full-scale positive input results in output

code 111...1. Output data are available 6 clock cycles after

the data are sampled. The analog input is sampled one

Output data should be sampled as shown in the timing dia-

gram (figure 5). The OR pin is an out-of-range pin; if the

outputs go either over or under range, OR is set high.

PCB LAYOUT AND DECOUPLING

A well designed PCB is necessary to get good spectral

purity from any high-performance ADC. A multilayer PCB

with a solid ground plane is recommended for optimum

performance. If the system has a split analog and digital

ground plane, it is recommended that all ground pins on

the ADC be connected to the analog ground plane. It is our

experience that this gives the best performance. The

power supply pins should be bypassed using 100 nF

surface mounted capacitors as close to the package pins

as possible. Analog and digital supply pins should be

separately filtered.

Figure 3 – Driving Differential Inputs with a

Differential Configuration

Figure 4 – Driving Differential Inputs with a

Single-Ended Configuration

N–1

N

N+1

N+2

N+3

Data

N–1

Data

N

Data

N+1

Data

N+2

Data

Clock

Clock

Figure 5 – Timing Diagram