LT1818 datasheet(21/28 Pages) LINER

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LT1806/LT1807

18067fc

TYPICAL APPLICATIONS

Driving A/D Converter

The LT1806/LT1807 have 60ns settling time to 0.01% on

a 2V step signal, and 20Ω output impedance at 100MHz,

that makes them ideal for driving high speed A/D convert-

ers. With the rail-to-rail input and output, and low supply

voltage operation, the LT1806/LT1807 are also desirable

for single supply applications. As shown in the application

on the front page of this data sheet, the LT1807 drives a

10Msps, 12-bit, LTC1420 ADC in a gain of 20. Driving the

LTC1420 differentially will optimize the signal-to-noise

ratio, SNR, and the total harmonic distortion, THD, of the

A/D converter. The lowpass ﬁlter, R5, R6 and C3 reduce

noise or distortion products that might come from the input

signal. High quality capacitors and resistors, NPO chip

capacitor and metal ﬁlm surface mount resistors, should

be used since these components can add to distortion.

The voltage glitch of the converter, due to its sampling

nature is buffered by the LT1807, and the ability of the

ampliﬁer to settle it quickly will affect the spurious free

dynamic range of the system. Figure 2 depicts the LT1806

driving LTC1420 at noninverting gain of 2 conﬁguration.

The FFT responses show a better than 92dB of spurious

free dynamic range, SFDR.

Figure 2. Noninverting A/D Driver

–

LT1806

LTC1420

PGA GAIN = 1

REF = 2.048V

12 BITS

10Msps

–5V

•

VIN

1.5VP-P

470pF

–AIN

18067 F02

+AIN

49.9Ω

Figure 3. 4096 Point FFT Response

FREQUENCY (MHz)

–120

–100

–80

–60

–40

–20

123

18067 F03

VS = 5V

AV = 2

fSAMPLE = 10Msps

fIN = 1.4086MHz

SFDR = 92.5dB

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