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LTC1603CG#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20
16
LTC1603
1603f
APPLICATIONS INFORMATION
WUU
U
1603 F16
A
IN
+
V
SS
OV
DD
DGNDAV
DD
LTC1603
DIGITAL
SYSTEM
ANALOG
INPUT
CIRCUITRY
AGND
5 TO 8
2
34 29
DV
DD
OGND
2810
1
REFCOMP
4
47µF
V
REF
3
2.2µF
A
IN
–
10µF
36
10µF
AV
DD
35
10µF
10µF
+
–
9
10µF
DC PERFORMANCE
The noise of an ADC can be evaluated in two ways: signal-
to-noise raio (SNR) in frequency domain and histogram in
time domain. The LTC1603 excels in both. Figure 18a
demonstrates that the LTC1603 has an SNR of over 90dB
in frequency domain. The noise in the time domain histo-
gram is the transition noise associated with a high resolu-
tion ADC which can be measured with a fixed DC signal
applied to the input of the ADC. The resulting output codes
are collected over a large number of conversions. The
shape of the distribution of codes will give an indication of
the magnitude of the transition noise. In Figure 17 the
distribution of output codes is shown for a DC input that
has been digitized 4096 times. The distribution is Gaussian
and the RMS code transition noise is about 0.66LSB. This
corresponds to a noise level of 90.9dB relative to full scale.
Adding to that the theoretical 98dB of quantization error
for 16-bit ADC, the resultant corresponds to an SNR level
of 90.1dB which correlates very well to the frequency
domain measurements in DYNAMIC PERFORMANCE
section.
DYNAMIC PERFORMANCE
The LTC1603 has excellent high speed sampling capabil-
ity. Fast fourier transform (FFT) test techniques are used
to test the ADC’s frequency response, distortions 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. Figures 18a and 18b
show typical LTC1603 FFT plots.
CODE
–5
–4
–3 –2 –1 0 1 2 3 4 5
COUNT
2500
2000
1500
1000
500
0
1603 F17
Figure 17. Histogram for 4096 Conversions
Figure 18a. This FFT of the LTC1603’s Conversion of a
Full-Scale 10kHz Sine Wave Shows Outstanding Response
with a Very Low Noise Floor When Sampling at 250ksps
Figure 16. Power Supply Grounding Practice
FREQUENCY (kHz)
0
AMPLITUDE (dB)
–60
–40
–20
60
1603 F18a
–80
–100
20 40 80 100 120
–120
–140
0
f
SAMPLE
= 250kHz
f
IN
= 9.959kHz
SINAD = 90.2dB
THD = –103.2dB
1603f
17
LTC1603
APPLICATIONS INFORMATION
WUU
U
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 250kHz sampling rate and a 5kHz input. The
dynamic performance is excellent for input frequencies up
to and beyond the Nyquist limit of 125kHz.
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 250kHz the LTC1603 maintains above 14 bits up to
the Nyquist input frequency of 125kHz (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 Log
VVV Vn
V
=
+++
20
234
1
222 2
...
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 LTC1603 has good distortion
performance up to the Nyquist frequency and beyond.
Figure 18b. Even with Inputs at 100kHz, the LTC1603’s
Dymanic Linearity Remains Robust
Figure 20. Distortion vs Input Frequency
Figure 19. Effective Bits and Signal/(Noise + Distortion)
vs Input Frequency
FREQUENCY (kHz)
AMPLITUDE (dB)
–60
–40
–20
1603 F18b
–80
–100
–120
–140
0
06020 40 80 100 120
f
SAMPLE
= 250kHz
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
1603 F19
INPUT FREQUENCY (Hz)
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
AMPLITUDE (dB BELOW THE FUNDAMENTAL)
1603 F20
1k
10k
100k 1M
THD
3RD
2ND
18
LTC1603
1603f
Intermodulation Distortion
If the ADC input signal consists of more than one spectral
component, the ADC transfer function nonlinearity can
produce intermodulation distortion (IMD) in addition to
THD. IMD is the change in one sinusoidal input caused by
the presence of another sinusoidal input at a different
frequency.
If two pure sine waves of frequencies fa and fb are applied
to the ADC input, nonlinearities in the ADC transfer func-
tion can create distortion products at the sum and differ-
ence frequencies of mfa ±nfb, where m and n = 0, 1, 2, 3,
APPLICATIONS INFORMATION
WUU
U
etc. For example, the 2nd order IMD terms include
(fa – fb). If the two input sine waves are equal in magni-
tude, the value (in decibels) of the 2nd order IMD products
can be expressed by the following formula:
IMD fa fb Log
Amplitude
±
()
=
±
20
at (fa fb)
Amplitude at fa
Peak Harmonic or Spurious Noise
The peak harmonic or spurious noise is the largest spec-
tral component excluding the input signal and DC. This
value is expressed in decibels relative to the RMS value of
a full-scale input signal.
Full-Power and Full-Linear Bandwidth
The full-power bandwidth is that input frequency at which
the amplitude of the reconstructed fundamental is
reduced by 3dB for a full-scale input signal.
The full-linear bandwidth is the input frequency at which
the S/(N + D) has dropped to 84dB (13.66 effective bits).
The LTC1603 has been designed to optimize input band-
width, allowing the ADC to undersample input signals with
frequencies above the converter’s Nyquist Frequency. The
noise floor stays very low at high frequencies; S/(N + D)
becomes dominated by distortion at frequencies far
beyond Nyquist.
Figure 21. Intermodulation Distortion Plot
FREQUENCY (kHz)
020
AMPLITUDE (dB)
80 100
0
–20
–40
–60
–80
–100
–120
–140
1603 F21
40 60 120
f
SAMPLE
= 250kHz
f
IN1
= 29.3kHz
f
IN2
= 32.4kHz
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20

LTC1603CG#PBF

Mfr. #:
Buy LTC1603CG#PBF
Manufacturer:
Analog Devices Inc.
Description:
Analog to Digital Converters - ADC 16-B, 250ksps Smpl A/D Conv w/
Lifecycle:
New from this manufacturer.
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