LT1809CS6 datasheet(18/24 Pages) LINER

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LT1809/LT1810

TYPICAL APPLICATIO S

Driving A/D Converters

The LT1809/LT1810 have a 27ns settling time to 0.1% of

a 2V step signal and 20

Ω output impedance at 100MHz

making it ideal for driving high speed A/D converters. With

the rail-to-rail input and output and low supply voltage

operation, the LT1809 is also desirable for single supply

applications. As shown in Figure 2, the LT1809 drives a

10Msps, 12-bit ADC, the LTC1420. The lowpass filter, R3

and C1, reduces the noise and distortion products that

might come from the input signal. High quality capacitors

and resistors, an NPO chip capacitor and metal-film sur-

face mount resistors, should be used since these compo-

nents can add to distortion. The voltage glitch of the

converter, due to its sampling nature, is buffered by the

LT1809 and the ability of the amplifier to settle it quickly

will affect the spurious-free dynamic range of the system.

Figure 2 to Figure 7 depict the LT1809 driving the LTC1420

at different configurations and voltage supplies. The FFT

responses show better than 90dB of SFDR for a

±5V

supply, and 80dB on a 5V single supply for the 1.394MHz

signal.

–

LT1809

LTC1420

PGA GAIN = 1

REF = 2.048V

12 BITS

10Msps

–5V

•

VIN

1VP-P

470pF

–AIN

1809 F02

+AIN

49.9

Ω

Figure 2. Noninverting A/D Driver

Figure 3. 4096 Point FFT Response

FREQUENCY (MHz)

–120

–100

–80

–60

–40

–20

123

1809 F03

VS = ±5V

AV = +2

fSAMPLE = 10Msps

fIN = 1.394MHz

SFDR = 90dB

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