ADC12DC080

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SNAS405B – SEPTEMBER 2007 – REVISED NOVEMBER 2008

Generally, analog and digital lines should cross each other at 90° to avoid crosstalk. To maximize accuracy in

high speed, high resolution systems, however, avoid crossing analog and digital lines altogether. It is important to

keep clock lines as short as possible and isolated from ALL other lines, including other digital lines. Even the

generally accepted 90° crossing should be avoided with the clock line as even a little coupling can cause

problems at high frequencies. This is because other lines can introduce jitter into the clock line, which can lead to

degradation of SNR. Also, the high speed clock can introduce noise into the analog chain.

Best performance at high frequencies and at high resolution is obtained with a straight signal path. That is, the

signal path through all components should form a straight line wherever possible.

Be especially careful with the layout of inductors and transformers. Mutual inductance can change the

characteristics of the circuit in which they are used. Inductors and transformers should not be placed side by

side, even with just a small part of their bodies beside each other. For instance, place transformers for the analog

input and the clock input at 90° to one another to avoid magnetic coupling.

The analog input should be isolated from noisy signal traces to avoid coupling of spurious signals into the input.

Any external component (e.g., a filter capacitor) connected between the converter's input pins and ground or to

the reference input pin and ground should be connected to a very clean point in the ground plane.

All analog circuitry (input amplifiers, filters, reference components, etc.) should be placed in the analog area of

the board. All digital circuitry and dynamic I/O lines should be placed in the digital area of the board. The

ADC12DC080 should be between these two areas. Furthermore, all components in the reference circuitry and

the input signal chain that are connected to ground should be connected together with short traces and enter the

ground plane at a single, quiet point. All ground connections should have a low inductance path to ground.

DYNAMIC PERFORMANCE

To achieve the best dynamic performance, the clock source driving the CLK input must have a sharp transition

region and be free of jitter. Isolate the ADC clock from any digital circuitry with buffers, as with the clock tree

shown in Figure 22. The gates used in the clock tree must be capable of operating at frequencies much higher

than those used if added jitter is to be prevented.

As mentioned in Clock Input, it is good practice to keep the ADC clock line as short as possible and to keep it

well away from any other signals. Other signals can introduce jitter into the clock signal, which can lead to

reduced SNR performance, and the clock can introduce noise into other lines. Even lines with 90° crossings

have capacitive coupling, so try to avoid even these 90° crossings of the clock line.

Figure 22. Isolating the ADC Clock from other Circuitry with a Clock Tree

Copyright © 2007–2008, Texas Instruments Incorporated

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