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LTC1276ACN#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24
10
LTC1273
LTC1275/LTC1276
127356fa
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CONVERSION DETAILS
The LTC1273/LTC1275/LTC1276 use a successive ap-
proximation algorithm and an internal sample-and-hold
circuit to convert an analog signal to a 12-bit parallel or
2-byte output. The ADCs are complete with a precision
reference and an internal clock. The control logic provides
easy interface to microprocessors and DSPs. (Please refer
to the Digital Interface section for the data format.)
Conversion start is controlled by the CS, RD and HBEN
inputs. At the start of conversion the successive approxi-
mation register (SAR) is reset and the three-state data
outputs are enabled. Once a conversion cycle has begun
it cannot be restarted.
During conversion, the internal 12-bit capacitive DAC
output is sequenced by the SAR from the most significant
bit (MSB) to the least significant bit (LSB). Referring to
Figure 1, the A
IN
input connects to the sample-and-hold
capacitor during the acquire phase, and the comparator
offset is nulled by the feedback switch. In this acquire
phase, a minimum delay of 600ns will provide enough
time for the sample-and-hold capacitor to acquire the
analog signal. During the convert phase, the comparator
feedback switch opens, putting the comparator into the
compare mode. The input switch switches C
SAMPLE
to
ground, injecting the analog input charge onto the sum-
ming junction. This input charge is successively com-
pared with the binary-weighted charges supplied by the
Figure 2. LTC1275 Nonaveraged, 1024 Point FFT Plot
FREQUENCY (kHz)
0
–120
AMPLITUDE (dB)
–100
–80
–60
–40
40 80
120
160
LTC1273/75/76 • F02
–20
0
20 60 100
140
f
SAMPLE
= 300kHz
f
IN
= 29.37kHz
Figure 1. A
IN
Input
V
DAC
LTC1273/75/76 • F01
+
–
C
DAC
DAC
SAMPLE
HOLD
C
SAMPLE
S
A
R
12-BIT
LATCH
COMPARATOR
SAMPLE
SI
A
IN
capacitive DAC. Bit decisions are made by the high speed
comparator. At the end of a conversion, the DAC output
balances the A
IN
input charge. The SAR contents (a 12-bit
data word) which represent the A
IN
are loaded into the
12-bit output latches.
DYNAMIC PERFORMANCE
The LTC1273/LTC1275/LTC1276 have an exceptionally
high speed sampling capability. FFT (Fast Fourier Trans-
form) test techniques are used to characterize 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 2 shows a typical LTC1275
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 2 shows a typical spectral content with
a 300kHz sampling rate and a 29kHz input. The dynamic
performance is excellent for input frequencies up to the
Nyquist limit of 150kHz.
11
LTC1273
LTC1275/LTC1276
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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 300kHz the LTC1273/LTC1275/LTC1276 maintain
very good ENOBs up to the Nyquist input frequency of
150kHz. Refer to Figure 3.
INPUT FREQUENCY (Hz)
10k
0
EFFECTIVE BITS
3
5
7
10
100k 2M
LTC1273/75/76 • F03
1
4
6
9
12
11
8
2
1M
62
56
74
68
50
S/(N + D) (dB)
f
SAMPLE
= 300kHz
Figure 3. Effective Bits and Signal to (Noise + Distortion)
vs Input Frequency
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
√V
2
2
+ V
3
2
+ V
4
2
... + V
N
2
V
1
where V
1
is the RMS amplitude of the fundamental fre-
quency and V
2
through V
N
are the amplitudes of the
second through Nth harmonics. THD versus input fre-
quency is shown in Figure 4. The LTC1273/LTC1275/
LTC1276 have good distortion performance up to Nyquist
and beyond.
Figure 4. Distortion vs Input Frequency
INPUT FREQUENCY (Hz)
–80
AMPLITUDE (dB BELOW THE FUNDAMENTAL)
–60
–40
–20
0
1k 100k 1M 10M
LTC1273/75/76 • F04
–100
10k
–90
–70
–50
–30
–10
f
SAMPLE
= 300kHz
THD
2nd HARMONIC
3rd HARMONIC
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 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) and
(fa – fb) while the 3rd order IMD terms include (2fa + fb),
(2fa – fb), (fa + 2fb), and (fa – 2fb). If the two input sine
waves are equal in magnitude, the value (in decibels) of the
2nd order IMD products can be expressed by the following
formula:
IMD (fa ± fb) = 20log
Amplitude at (fa ± fb)
Amplitude at fa
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LTC1273
LTC1275/LTC1276
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Figure 5 shows the IMD performance at a 30kHz input.
FREQUENCY (kHz)
0
–120
AMPLITUDE (dB)
–100
–80
–60
–40
40 80
120
160
LTC1273/75/76 • F05
–20
0
20 60 100
140
f
SAMPLE
= 300kHz
f
IN1
= 29.37kHz
f
IN2
= 32.446kHz
Figure 5. Intermodulation Distortion Plot
the analog input must settle after the small current spike
before the next conversion starts. Any op amp that settles
in 600ns to small current transients will allow maximum
speed operation. If slower op amps are used, more settling
time can be provided by increasing the time between
conversions. Suitable devices capable of driving the ADCs’
A
IN
input include the LT1190/LT1191, LT1007, LT1220,
LT1223 and LT1224 op amps.
The analog input tolerates source resistance very well.
Here again, the only requirement is that the analog input
must settle before the next conversion starts. For larger
source resistance, full DC accuracy can be obtained if
more time is allowed between conversions. For more
information, see the Acquisition Time vs Source Resis-
tance curve in the Typical Performance Characteristics
section. For optimum frequency domain performance
[e.g., S/(N + D)], keep the source resistance below 100Ω.
Internal Reference
The LTC1273/LTC1275/LTC1276 have an on-chip, tem-
perature compensated, curvature corrected, bandgap ref-
erence which is factory trimmed to 2.42V. It is internally
connected to the DAC and is available at pin 2 to provide
up to 1mA current to an external load.
For minimum code transition noise the reference output
should be decoupled with a capacitor to filter wideband
noise from the reference (10µF tantalum in parallel with a
0.1µF ceramic).
I
n the LTC1275, the V
REF
pin can be driven above its
normal value with a DAC or other means to provide input
span adjustment or to improve the reference temperature
drift. Figure 6 shows an LT1006 op amp driving the
V
REF(OUT)
≥ 2.45V
3Ω
INPUT RANGE
±1.033V
REF(OUT)
–
+
LT1006
LTC1275
A
IN
AGND
V
REF
10µF
LTC1273/75/76 • F06
Figure 6. Driving the V
REF
with the LT1006 Op Amp
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 re-
duced 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 68dB (11 effective bits). The
LTC1273/LTC1275/LTC1276 have been designed to opti-
mize input bandwidth, allowing ADCs to undersample
input signals with frequencies above the converters’ Nyquist
Frequency. The noise floor stays very low at high frequen-
cies; S/(N + D) becomes dominated by distortion at
frequencies far beyond Nyquist.
Driving the Analog Input
The analog inputs of the LTC1273/LTC1275/LTC1276 are
easy to drive. They draw only one small current spike while
charging the sample-and-hold capacitor at the end of
conversion. During conversion the analog input draws no
current. The only requirement is that the amplifier driving
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24

LTC1276ACN#PBF

Mfr. #:
Buy LTC1276ACN#PBF
Manufacturer:
Analog Devices Inc.
Description:
Analog to Digital Converters - ADC 12-B, 300ksps Smpl A/D Convs w/ Ref
Lifecycle:
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