LTC1856 Linear Technology, LTC1856 Datasheet - Page 21

• Manufacturer Part Number: LTC1856
• Description: 100ksps ADC Converters
• Manufacturer: Linear Technology

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APPLICATIO S I FOR ATIO
SIGNAL-TO-NOISE AND DISTORTION 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 12 shows a typical SINAD of 87dB with
a 100kHz sampling rate and a 1kHz input.
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:
where V
quency and V
second through Nth harmonics.
THD = 20log
1
is the RMS amplitude of the fundamental fre-
2
through V
U
V
2
2
+
U
V
3
N
2
are the amplitudes of the
V
+
1
V
4
2
W
Figure 12. LTC1856 Nonaveraged 4096 Point FFT Plot
...
+
V
N
–100
–110
–120
–130
2
–40
–50
–60
–80
–90
–10
–20
–30
–70
0
0
U
5
10
15
FREQUENCY (kHz)
20
25
BOARD LAYOUT, POWER SUPPLIES
AND DECOUPLING
Wire wrap boards are not recommended for high resolu-
tion or high speed A/D converters. To obtain the best
performance from the LTC1854/LTC1855/LTC1856, a
printed circuit board is required. Layout for the printed
circuit board should ensure the digital and analog signal
lines are separated as much as possible. In particular, care
should be taken not to run any digital track alongside an
analog signal track or underneath the ADC. The analog
input should be screened by AGND.
In applications where the MUX is connected to the ADC, it
is possible to get noise coupling into the ADC from the
trace connecting the MUXOUT to the ADC. Therefore,
reducing the length of the traces connecting the MUXOUT
pins (Pins 10, 11) to the ADC pins (Pins 12, 13) can
minimize the problem. The unused MUX inputs should be
grounded to prevent noise coupling into the inputs.
Figure 13 shows the power supply grounding that will help
obtain the best performance from the 12-bit/14-bit/16-bit
ADCs. Pay particular attention to the design of the analog
and digital ground planes. The DGND pin of the LTC1854/
30
f
f
SINAD = 87dB
THD = –101dB
SAMPLE
IN
= 1kHz
35
LTC1854/LTC1855/LTC1856
40 45
= 100kHz
185456 F12
50
21
185456fa