LTC1604 Linear Technology, LTC1604 Datasheet - Page 17

• Manufacturer Part Number: LTC1604
• Description: High Speed/ 16-Bit/ 333ksps Sampling A/D Converter with Shutdown
• Manufacturer: Linear Technology

APPLICATIONS
Signal-to-Noise Ratio
The signal-to-noise plus distortion ratio [S/(N + D)] 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 18a shows a typical spectral content
with a 333kHz sampling rate and a 5kHz input. The
dynamic performance is excellent for input frequencies up
to and beyond the Nyquist limit of 167kHz.
Effective Number of Bits
The effective number of bits (ENOBs) is a measurement of
the resolution of an ADC and is directly related to the
S/(N + D) by the equation:
where N is the effective number of bits of resolution and
S/(N + D) is expressed in dB. At the maximum sampling
rate of 333kHz the LTC1604 maintains above 14 bits up to
the Nyquist input frequency of 167kHz (refer to Figure 19).
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 V1 is the RMS amplitude of the fundamental fre-
quency and V2 through Vn are the amplitudes of the
second through nth harmonics. THD vs Input Frequency is
shown in Figure 20. The LTC1604 has good distortion
performance up to the Nyquist frequency and beyond.
N = [S/(N + D) – 1.76]/6.02
THD
20
Log
V
U
2
2
INFORMATION
V
U
3
2
V
1
V
4
2
W
Vn
2
U
Figure 19. Effective Bits and Signal/(Noise + Distortion)
vs Input Frequency
Figure 18b. Even with Inputs at 100kHz, the LTC1604’s
Dynamic Linearity Remains Robust
–140
–100
–120
–20
–60
–80
Figure 20. Distortion vs Input Frequency
–40
–100
–110
–10
–20
–30
–40
–50
–60
–70
–80
–90
0
16
15
14
13
12
11
10
0
9
8
0
1k
1k
f
f
SINAD = 89dB
THD = –96dB
SAMPLE
IN
20
= 97.152kHz
40
= 333kHz
INPUT FREQUENCY (Hz)
FREQUENCY (kHz)
10k
60
10k
FREQUENCY (Hz)
80 100 120 140 160
100k
100k
1604 F18b
1604 F19
2ND
1604 G04
THD
3RD
LTC1604
1M
1M
98
92
86
80
74
68
62
56
50
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