LTC1418AIN datasheet(9/28 Pages) LINER

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LTC1418

APPLICATIONS INFORMATION

Referring to Figure 1, the AIN+ and AIN– inputs are con-

nected to the sample-and-hold capacitors (CSAMPLE) dur-

ing the acquire phase and the comparator offset is nulled by

the zeroing switches. In this acquire phase, a minimum

delay of 1

µs will provide enough time for the sample-and-

hold capacitors to acquire the analog signal. During the

convert phase the comparator zeroing switches open,

putting the comparator into compare mode. The input

switches the CSAMPLE capacitors to ground, transferring

the differential analog input charge onto the summing

junction. This input charge is successively compared with

the binary weighted charges supplied by the differential

capacitive DAC. Bit decisions are made by the high speed

comparator. At the end of a conversion, the differential

DAC output balances the AIN

+ and AIN– input charges. The

SAR contents (a 14-bit data word) which represent the

difference of AIN+ and AIN– are loaded into the 14-bit

output latches.

DYNAMIC PERFORMANCE

The LTC1418 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 2 shows a

typical LTC1418 FFT plot.

Signal-to-Noise Ratio

The signal-to-noise plus distortion ratio [S/(N + D)] is the

ratio between the RMS amplitude of the fundamental input

frequency to the RMS amplitude of all other frequency

components at the A/D output. The output is band limited

to frequencies from above DC and below half the sampling

frequency. Figure 2a shows a typical spectral content with

a 200kHz sampling rate and a 10kHz input. The dynamic

performance is excellent for input frequencies up to and

beyond the Nyquist limit of 100kHz.

FREQUENCY (kHz)

305070

–20

–40

–60

–80

–100

–120

1418 F02a

100

fSAMPLE = 200kHz

fIN = 9.9609375kHz

SFDR = 99.32

SINAD = 82.4

Figure 2a. LTC1418 Nonaveraged, 4096 Point FFT,

Input Frequency = 10kHz

FREQUENCY (kHz)

305070

–20

–40

–60

–80

–100

–120

1418 F02b

100

fSAMPLE = 200kHz

fIN = 97.509765kHz

SFDR = 94.29

SINAD = 81.4

Figure 2b. LTC1418 Nonaveraged, 4096 Point FFT,

Input Frequency = 97.5kHz

Effective Number of Bits

The effective number of bits (ENOBs) is a measurement of

the resolution of an ADC and is directly related to the

S/(N + D) by the equation:

N = [S/(N + D) – 1.76]/6.02

where N is the effective number of bits of resolution and

S/(N + D) is expressed in dB. At the maximum sampling

rate of 200kHz the LTC1418 maintains near ideal ENOBs

up to the Nyquist input frequency of 100kHz (refer to

Figure 3).

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