LTC1404CS8 datasheet(10/24 Pages) LINER

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LTC1404

APPLICATIONS INFORMATION

Dynamic Performance

The LTC1404 has excellent high speed sampling capabil-

ity. FFT (Fast Fourier Transform) test techniques 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 algo-

rithm, the ADC’s spectral content can be examined for

frequencies outside the fundamental. Figure 2a shows a

typical LTC1404 FFT plot.

Conversion Details

The LTC1404 uses a successive approximation algorithm

and an internal sample-and-hold circuit to convert an

analog signal to a 12-bit serial output based on a precision

internal reference. The control logic provides easy inter-

face to microprocessors and DSPs through 3-wire con-

nections.

A rising edge on the CONV input starts a conversion. At the

start of a conversion the successive approximation regis-

ter (SAR) is reset. Once a conversion cycle has begun, it

cannot be restarted.

During conversion, the internal 12-bit capacitive DAC

output is sequenced by the SAR from the most significant

bit (MSB) to the least significant bit (LSB). Referring to

Figure 1, the AIN input connects to the sample-and-hold

capacitor during the acquired phase and the comparator

offset is nulled by the feedback switch. In this acquire

phase, it typically takes 160ns for the sample-and-hold

capacitor to acquire the analog signal. During the convert

phase, the comparator feedback switch opens, putting the

comparator into the compare mode. The input switches

connect CSAMPLE to ground, injecting the analog input

charge onto the summing junction. This input charge is

successively compared with the binary-weighted charges

supplied by the capacitive DAC. Bit decisions are made by

the high speed comparator. At the end of a conversion, the

DAC output balances the AIN input charge. The SAR

contents (a 12-bit data word) which represent the input

voltage, are presented through the serial pin DOUT.

1404 F01

SAMPLE

DOUT

CDAC

VDAC

DAC

AIN

CSAMPLE

COMP

SAMPLE

HOLD

Figure 1. AIN Input

FREQUENCY (kHz)

60 90 120 150 180 210 240 270 300

1404 F02a

–10

–20

–30

–40

–50

– 60

–70

–80

–90

–100

–110

–120

fSAMPLE = 600kHz

fIN = 99.169kHz

SINAD = 72dB

THD = – 88dB

Figure 2a. LTC1404 Nonaveraged, 4096 Point FFT

Plot with 100kHz Input Frequency in Bipolar Mode

Figure 2b. LTC1404 Nonaveraged, 4096 Point FFT

Plot with 300kHz Input Frequency in Bipolar Mode

FREQUENCY (kHz)

60 90 120 150 180 210 240 270 300

1404 F02b

–10

–20

–30

–40

–50

– 60

–70

–80

–90

–100

–110

–120

fSAMPLE = 600kHz

fIN = 298.681kHz

SINAD = 71dB

THD = – 84dB

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