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LTC1418AIG#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P27P28-P30
LTC1418
10
1418fa
For more information www.linear.com/LTC1418
Referring to Figure 1, the A
IN
+
and A
IN
–
inputs are connected
to the sample-and-hold capacitors (C
SAMPLE
) during the
acquire phase and the comparator offset is nulled by the
zeroing switches. In this acquire phase, a minimum delay
of 1µs will provide enough time for the sample-and-hold
capacitors to acquire the analog signal. During the convert
phase, the comparator zeroing switches open, putting the
comparator into compare mode. The input switches the
C
SAMPLE
capacitors to ground, transferring the differential
analog input charge onto the summing junction. This input
charge is successively compared with the binary weighted
charges supplied by the differential capacitive DAC. Bit
decisions are made by the high speed comparator. At the
end of a conversion, the differential DAC output balances
the A
IN
+
and A
IN
–
input charges. The SAR contents (a
14-bit data word) which represent the difference of A
IN
+
and A
IN
–
are loaded into the 14-bit output latches.
DYNAMIC PERFORMANCE
The LTC1418 has excellent high speed sampling capability.
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
algorithm, the ADC’s spectral content can be examined
for frequencies outside the fundamental. Figure 2a shows
a typical LTC1418 FFT plot.
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 2b shows a typical spectral content with
a 200kHz sampling rate and a 10kHz input. The dynamic
performance is excellent for input frequencies up to and
beyond the Nyquist limit of 100kHz.
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 200kHz, the LTC1418 maintains near
ideal ENOBs up to the Nyquist input frequency of 100kHz
(refer to Figure 3).
APPLICATIONS INFORMATION
Figure 2a. LTC1418 Nonaveraged, 4096 Point FFT,
Input Frequency = 10kHz
FREQUENCY (kHz)
0 10 30 50 70 90
AMPLITUDE (dB)
0
–20
–40
–60
–80
–100
–120
20 40 60 80
1418 F02a
100
f
SAMPLE
= 200kHz
f
IN
= 9.9609375kHz
SFDR = 99.32
SINAD = 82.4
Figure 2b. LTC1418 Nonaveraged, 4096 Point FFT,
Input Frequency = 97.5kHz
FREQUENCY (kHz)
0 10 30 50 70 90
AMPLITUDE (dB)
0
–20
–40
–60
–80
–100
–120
20 40 60 80
1418 F02b
100
f
SAMPLE
= 200kHz
f
IN
= 97.509765kHz
SFDR = 94.29
SINAD = 81.4
LTC1418
11
1418fa
For more information www.linear.com/LTC1418
APPLICATIONS INFORMATION
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
+...V
N
2
V1
where V1 is the RMS amplitude of the fundamental
frequency and V2 through V
N
are the amplitudes of the
second through nth harmonics. THD vs Input Frequency
is shown in Figure 4. The LTC1418 has good distortion
performance up to the Nyquist frequency and beyond.
Figure 3. Effective Bits and Signal/(Noise + Distortion)
vs Input Frequency
INPUT FREQUENCY (Hz)
1k
EFECTIVE BITS
14
13
12
11
10
9
8
7
6
5
4
3
2
10k 100k 1M
1418 F03
Figure 4. Distortion vs Input Frequency
INPUT FREQUENCY (Hz)
AMPLITUDE (dB BELOW THE FUNDAMENTAL)
0
–20
–40
–60
–80
–100
–120
1418 F04
1k 100k 1M10k
THD
2ND
3RD
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 function
can create distortion products at the sum and difference
frequencies of mfa ±nfb, where m and n = 0, 1, 2, 3, etc.
For example, the 2nd order IMD terms include (fa + fb).
If the two input sine waves are equal in magnitude, the
value (in decibels) of the 2nd order IMDproducts can be
expressed by the following formula:
IMD fa+ fb
( )
=20Log
Amplitude at fa + fb
( )
Amplitude at fa
Peak Harmonic or Spurious Noise
The peak harmonic or spurious noise is the largest spectral
component excluding the input signal and DC. This value
is expressed in decibels relative to the RMS value of a
full-scale input signal.
FREQUENCY (kHz)
0
–120
AMPLITUDE (dB)
–100
–80
–60
–40
20 50
70
100
1418 F05
–20
0
10 30 40
60
80 90
f
SAMPLE
= 200kHz
f
IN1
= 97.65625kHz
f
IN2
= 104.248046kHz
Figure 5. Intermodulation Distortion Plot
LTC1418
12
1418fa
For more information www.linear.com/LTC1418
APPLICATIONS INFORMATION
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 77dB (12.5 effec-
tive bits). The LTC1418 has been designed to optimize
input bandwidth, 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.
DRIVING THE ANALOG INPUT
The differential analog inputs of the LTC1418 are easy
to drive. The inputs may be driven differentially or as a
single-ended input (i.e., the A
IN
–
input is grounded). The
A
IN
+
and A
IN
–
inputs are sampled at the same instant. Any
unwanted signal that is common mode to both inputs will
be reduced by the common mode rejection of the sample-
and-hold circuit. The inputs draw only one small current
spike while charging the sample-and-hold capacitors at the
end of conversion. During conversion, the analog inputs
draw only a small leakage current. If the source imped-
ance of the driving circuit is low then the LTC1418 inputs
can be driven directly. As source impedance increases so
will acquisition time (see Figure 6). For minimum acquisi-
tion time, with high source impedance, a buffer amplifier
must be used. The only requirement is that the amplifier
driving the analog input(s) must settle after the small
current spike before the next conversion starts—1µs for
full throughput rate.
Choosing an Input Amplifier
Choosing an input amplifier is easy if a few requirements
are taken into consideration. First, choose an amplifier that
has a low output impedance (<100Ω) at the closed-loop
bandwidth frequency.
For example, if an amplifier is used
in a gain of 1 and has a closed-loop bandwidth of 10MHz,
then the output impedance at 10MHz must be less than
100Ω. The second requirement is that the closed-loop
bandwidth must be greater than 5MHz to ensure adequate
small-signal settling for full throughput rate. If slower op
amps are used, more settling time can be provided by
increasing the time between conversions.
The best choice for an op amp to drive the LTC1418 will
depend on the application. Generally, applications fall into
two categories: AC applications where dynamic specifica-
tions are most critical and time domain applications where
DC accuracy and settling time are most critical. The fol-
lowing list is a summary of the op amps that are suitable
for driving the LTC1418. More detailed information is
available at www.linear.com.
LT
®
1354: 12MHz, 400V/µs Op Amp. 1.25mA maximum
supply current. Good AC and DC specifications. Suitable
for dual supply application.
LT1357: 25MHz, 600V/µs Op Amp. 2.5mA maximum
supply current. Good AC and DC specifications. Suitable
for dual supply application.
LT1366/LT1367: Dual/Quad Precision Rail-to-Rail Input
and Output Op Amps. 375µA supply current per amplifier.
1.8V to ±15V supplies. Low input offset voltage: 150µV.
Good for low power and single supply applications with
sampling rates of
20ksps and under.
LT1498/LT1499: 10MHz, 6V/µs, Dual/Quad Rail-to-Rail
Input and Output Op Amps. 1.7mA supply current per
amplifier. 2.2V to ±15V supplies. Good AC performance,
input noise voltage = 12nV/√Hz (typ).
Figure 6. t
ACQ
vs Source Resistance
SOURCE RESISTANCE (Ω)
1
ACQUISITION TIME (μs)
10
1 100 1k 10k
1418 F06
0.1
10
100
100k
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LTC1418AIG#PBF

Mfr. #:
Buy LTC1418AIG#PBF
Manufacturer:
Analog Devices Inc.
Description:
Analog to Digital Converters - ADC L Pwr, 14-B, 200ksps ADC w/ Serial & Par
Lifecycle:
New from this manufacturer.
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