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LTC1863CGN

12-/16-Bit, 8-Channel 200ksps ADCs

Linear Technology 

LTC1863/LTC1867

APPLICATIONS INFORMATION

ANALOG

50Ω

INPUT

2000pF

CH0

LTC1863/

LTC1867

GND

10µF

REFCOMP

1867 F01a

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

algorithm, the ADC’s spectral content can be examined

for frequencies outside the fundamental.

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

Signal-to-Noise Ratio

50Ω

DIFFERENTIAL

ANALOG

INPUTS

50Ω

1000pF

1000pF

1000pF

CH0

LTC1863/

LTC1867

CH1

10µF

REFCOMP

1867 F01b

Figure 1b. Optional RC Input Filtering for Differential Inputs

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 conversions. 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.

2500

2000

2152

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.

0

SNR = 88.8dB

–20

SINAD = 87.9dB

THD = 95dB

fSAMPLE = 200ksps

–40

INTERNAL REFERENCE

–60

–80

–100

–120

–140

0

25

50

75

FREQUENCY (kHz)

100

18637 G04

Figure 3. LTC1867 Nonaveraged 4096 Point FFT Plot

1500

1000

935

579

500

276

1 26

0

–4 –3 –2 –1 0

122

50

12 34

CODE

18637 GO3

Figure 2. LTC1867 Histogram for 4096 Conversions

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 = 20log V22 + V32 + V42...+ VN2

V1

18637fc

12

For more information www.linear.com/LTC1863

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