Datasheet

LTC1863/LTC1867
11
18637fa
APPLICATIONS INFORMATION
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.
1867 F01a
CH0
GND
LTC1863/
LTC1867
REFCOMP
2000pF
10μF
50Ω
ANALOG
INPUT
1000pF
1867 F01b
CH0
CH1
LTC1863/
LTC1867
REFCOMP
1000pF
1000pF
10μF
50Ω
50Ω
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
algorithm, 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 V
REF
to GND), a
signal-to-noise ratio of 90dB can be achieved.
CODE
–4
COUNTS
4
18637 GO3
–2–3
0–1
321
2500
2000
1500
1000
500
0
1
26
276
2152
579
122
5
0
935
Figure 3. LTC1867 Nonaveraged 4096 Point FFT Plot
FREQUENCY (kHz)
0
0
–20
–40
–60
–80
–100
–120
–140
75
18637 G04
25 50 100
AMPLITUDE (dB)
SNR = 88.8dB
SINAD = 87.9dB
THD = 95dB
f
SAMPLE
= 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 V
V
N
=
++ +
20
2
2
3
2
4
22
1
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