LTC1867IGN datasheet(11/16 Pages) LINER

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LTC1863/LTC1867

18637f

DC Performance

One way of measuring the transition noise associated with

a high resolution ADC is to use a technique where a DC

signal is applied to the input of the ADC and the resulting

output codes are collected over a large number of conver-

sions. For example, in Figure 2 the distribution of output

codes is shown for a DC input that had been digitized 4096

times. The distribution is Gaussian and the RMS code

transition noise is about 0.74LSB.

APPLICATIO S I FOR ATIO

1867 F01a

CH0

GND

LTC1863/

LTC1867

REFCOMP

2000pF

µF

Ω

ANALOG

INPUT

1000pF

1867 F01b

CH0

CH1

LTC1863/

LTC1867

REFCOMP

1000pF

µF

Ω

DIFFERENTIAL

ANALOG

INPUTS

Figure 1a. Optional RC Input Filtering for Single-Ended Input

Figure 1b. Optional RC Input Filtering for Differential Inputs

Figure 2. LTC1867 Histogram for 4096 Conversions

Dynamic Performance

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.

Signal-to-Noise Ratio

The Signal-to-Noise and Distortion Ratio (SINAD) 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 3 shows a typical SINAD of 87.9dB with

a 200kHz sampling rate and a 1kHz input. When an

external 5V is applied to REFCOMP (tie VREF to GND), a

signal-to-noise ratio of 90dB can be achieved.

CODE

–4

18637 GO3

–2

–3

–1

2500

2000

1500

1000

500

276

2152

579

122

935

Figure 3. LTC1867 Nonaveraged 4096 Point FFT Plot

FREQUENCY (kHz)

–20

–40

–60

–80

–100

–120

–140

18637 G04

100

SNR = 88.8dB

SINAD = 87.9dB

THD = 95dB

fSAMPLE = 200ksps

INTERNAL REFERENCE

Total Harmonic Distortion

Total Harmonic Distortion (THD) is the ratio of the RMS

sum of all harmonics of the input signal to the fundamental

itself. The out-of-band harmonics alias into the frequency

band between DC and half the sampling frequency. THD is

expressed as:

THD

VVV

log

...

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