SIGNAL PATH

Tips, tricks, and techniques from the analog signal-path experts

No. 111

increasing in complexity as the latest high-performance proces-

sors and DSPs enable new signal-processing techniques. As system

requirements for speed and resolution increase, more capable Analog-to-Digital

Converters (ADCs) emerge, and these in turn require higher-performance

Analog Front Ends (AFEs). In many systems the AFE can be considered a

key limiting factor in the overall system performance. Applications such as

medical ultrasound, RADAR, Radio Frequency Identification (RFID), and

video imaging similarly demand high-performance AFEs. AFE designers

today are faced with the challenge of selecting the best amplifier to drive the

ADC, including how to maximize the dynamic range of the signal path and

how to choose the best filter for a given application. This article will address

the design of high-speed data acquisition systems, including some of the

limiting factors in the overall system performance created by the AFE and

the clock driving the ADC.

A generic AFE signal path including a source (Vs), Low-Noise Amplifier

(LNA), ADC driver, channel filter, sampling clock, and ADC stages are

shown in Figure 1.

A key measure of any data-acquisition-system performance is the Effective

Number Of Bits (ENOB) of resolution it delivers. The ENOB is maximized

by minimizing the noise and distortion added by each stage of the AFE to the

processed signal. A measure of the noise added by a particular stage is the noise

factor, F, which is the total input referred noise of the stage divided by the input

noise due to the previous stage. The often-quoted Noise Figure, NF, is 10 log F.

Selecting Amplifiers, ADCs, and Clocks

for High-Performance Signal Paths

— By Mike Ewer, Principal Applications Engineer

LNA

ADC

Driver

ADC

Channel

Filter

CLK

Clock

Driver

CLK

Figure 1. AFE Signal Path

Feature Article ............... 1-9

GHz Amplifiers..................10

GSPS A/D Converters......11

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