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LTC1406

FUNCTIONAL BLOCK DIAGRA

APPLICATIONS INFORMATION

Conversion Details

The LTC1406 uses an internal sample-and-hold circuit and

a pipeline quantizing architecture to convert an analog

signal to an 8-bit parallel output. With CLK high the input

switches are closed and the analog input will be acquired

on the input sampling capacitors CS (see Figure 1).

On the falling edge of CLK the input switches open, captur-

ing the input signal. The sampling capacitors are then

shorted together and the charge is transferred to the hold

TRACK-AND-

HOLD AMP

DIGITAL

DATA

8-BIT

PIPELINE

ADC

2.5k

AVDD

VBIAS

VREF

OVDD

DVDD

OGND

AGND

SHDN

1.95k

2.2V

OUTPUT

DRIVERS

OF/UF

1406 BD

CLOCK

CIRCUITRY

CLK

AIN

–

DGND

–

CLK

1406 F01

CLK

TO NEXT STAGE

CLK

AIN

–

CLK

Figure 1. Input Sample-and-Hold Amplifier

capacitors CH resulting in a differential DC voltage on the

output of the track-and-hold amplifier that is proportional

to the input signal. This differential voltage is fed into a

comparator that determines the most significant bit and

subtracts the result. The residue is then amplified by two

and passed to the next stage via a similar sample-and-hold

circuit. This continues down the eight pipeline stages. The

comparator outputs are then combined in a digital error

correction circuit. The 8-bit word is available at the output,

five clock cycles after the sampling edge.

Dynamic Performance

The LTC1406 has excellent wideband sampling capability.

The sample-and-hold amplifier has a small-signal input

bandwidth of 250MHz allowing the ADC to undersample

input signals with frequencies well beyond the converter’s

Nyquist frequency. FFT (Fast Fourier Transform) test tech-

niques are used to test the ADC’s frequency response,

distortion and noise at the rated throughput. By applying

a low distortion sine wave and analyzing the digital output

using an FFT algorithm, the ADC’s spectral content can be

examined for frequencies outside the fundamental. Figure

2 shows a typical LTC1406 FFT plot.

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