Datasheet

LTC1604
17
1604fa
APPLICATIONS INFORMATION
Figure 18b. Even with Inputs at 100kHz, the LTC1604’s
Dynamic Linearity Remains Robust
Figure 19. Effective Bits and Signal/(Noise + Distortion)
vs Input Frequency
Figure 20. Distortion vs Input Frequency
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:
N = [S/(N + D) – 1.76]/6.02
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:
THD=20Log
V2
2
+ V3
2
+ V4
2
+...Vn
2
V1
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.
FREQUENCY (kHz)
AMPLITUDE (dB)
–60
–40
–20
1604 F18b
–80
–100
–120
–140
0
06020 40 80 100 120 140 160
f
SAMPLE
= 333kHz
f
IN
= 97.152kHz
SINAD = 89dB
THD = –96dB
FREQUENCY (Hz)
1k
EFFECTIVE BITS
SINAD (dB)
16
15
14
13
12
11
10
9
8
98
92
86
80
74
68
62
56
50
10k 100k 1M
1604 F19
INPUT FREQUENCY (Hz)
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
AMPLITUDE (dB BELOW THE FUNDAMENTAL)
1604 G04
1k
10k
100k 1M
THD
3RD
2ND