LTC1863L Linear Technology, LTC1863L Datasheet - Page 11

• Manufacturer Part Number: LTC1863L
• Description: (LTC1863L / LTC1867L) 12-/16-Bit 8-Channel 175ksps ADCs
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
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
used since these components can add distortion. NPO and
silver mica type dielectric capacitors have excellent linear-
ity. Carbon surface mount resistors can also generate
distortion from self heating and from damage that may
occur during soldering. Metal film surface mount resis-
tors are much less susceptible to both problems.
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 1.6LSB.
Figure 2. LTC1867L Histogram for 4096 Conversions
1200
1000
800
600
400
200
0
20
7
58
21
170
22
U
465
23
1044
24
895
U
25
CODE
830
26
333
27
V
INTERNAL REF
261
28
DD
W
= 2.7V
23 9
29
1863L7L G12
30
31
1
U
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
components at the A/D output. The output is band limited
to frequencies from above DC and below half the sampling
frequency. Figure 3a shows a typical SINAD of 81.4dB with
a 175kHz sampling rate and a 1kHz input. Higher SINAD can
be obtained with a 3V supply. For example, when an exter-
nal 3V is applied to REFCOMP (tie V
of 83.5dB can be achieved as shown in Figure 3b.
Figure 3a. LTC1867L Nonaveraged 4096 Point
FFT Plot with 2.7V Supply
Figure 3b. LTC1867L Nonaveraged 4096 Point
FFT Plot with 3V Supply
–120
–140
–100
–100
–120
–140
–20
–40
–60
–80
–20
–60
–40
–80
0
0
0
0
LTC1863L/LTC1867L
21.875
21.875
FREQUENCY (kHz)
FREQUENCY (kHz)
43.75
43.75
f
f
SNR = 84.7dB
SINAD = 83.5dB
THD = 90dB
REFCOMP = EXT 3V
SAMPLE
IN
f
f
SNR = 82.9dB
SINAD = 81.4dB
THD = 86.8dB
SAMPLE
IN
= 1kHz
= 1kHz
65.625
65.625
= 175ksps
REF
= 175ksps
1863L7L F03b
1863L7L G03
to GND), a SINAD
87.5
87.5
11
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