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LTC1278-4CSW#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16
10
LTC1278
APPLICATIONS INFORMATION
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frequency is shown in Figure 4. The LTC1278 has good
distortion performance up to the Nyquist frequency and
beyond.
Figure 4. Distortion vs Input Frequency
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
Figure 5 shows the IMD performance at a 100kHz input.
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
LTC1278 has been designed to optimize input bandwidth,
allowing ADC to undersample input signals with frequen-
cies above the converter’s Nyquist Frequency. The noise
floor stays very low at high frequencies; S/(N + D) be-
comes dominated by distortion at frequencies far beyond
Nyquist.
Driving the Analog Input
The analog input of the LTC1278 is easy to drive. It draws
only one small current spike while charging the sample-
and-hold capacitor at the end of conversion. During con-
version the analog input draws no current. The only
requirement is that the amplifier driving the analog input
must settle after the small current spike before the next
Figure 5. Intermodulation Distortion Plot
INPUT FREQUENCY (Hz)
10k
–100
AMPLITUDE (dB BELOW THE FUNDAMENTAL)
–80
–60
–40
–20
100k 2M1M
LT1278 G6
–90
–70
–50
–30
–10
0
f
SAMPLE
= 500kHz
3RD HARMONIC
THD
2ND HARMONIC
11
LTC1278
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S
A
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INPUT VOLTAGE (V)
0V
OUTPUT CODE
–1
LSB
LTC1278 • F8b
011...111
011...110
000...001
000...000
100...000
100...001
111...110
1
LSB
BIPOLAR
ZERO
111...111
FS/2 – 1LSB–FS/2
FS = 5V
1LSB = FS/4096
Figure 7. Supplying a 2.5V Reference Voltage to the LTC1278
with the LT1019A-2.5
Figure 8a. LTC1278 Unipolar Transfer Characteristics
conversion starts. Any op amp that settles in 200ns to
small current transients will allow maximum speed opera-
tion. If slower op amps are used, more settling time can be
provided by increasing the time between conversions.
Suitable devices capable of driving the ADC’s A
IN
input
include the LT1360, LT1220, LT1223 and LT1224 op
amps.
Internal Reference
The LTC1278 has an on-chip, temperature compensated,
curvature corrected, bandgap reference, 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).
The V
REF
pin can be driven with a DAC or other means to
provide input span adjustment in bipolar mode. The V
REF
pin must be driven to at least 2.45V to prevent conflict with
the internal reference. The reference should be driven to
no more than 4.8V to keep the input span within the ±5V
supplies.
Figure 6 shows an LT1006 op amp driving the reference
pin. (In the unipolar mode, the input span is already 0V to
5V with the internal reference so driving the reference is
not recommended, since the input span will exceed the
supply and codes will be lost at the full scale.) Figure 7
shows a typical reference, the LT1019A-2.5 connected to
the LTC1278. This will provide an improved drift (equal to
the maximum 5ppm/°C of the LT1019A-2.5) and a ±2.582V
full scale.
Figure 6. Driving the V
REF
with the LT1006 Op Amp
INPUT VOLTAGE (V)
0V
OUTPUT CODE
FS – 1LSB
LTC1278 F8a
111...111
111...110
111...101
111...100
000...000
000...001
000...010
000...011
1
LSB
UNIPOLAR
ZERO
1LSB =
FS
4096
5V
4096
=
UNIPOLAR/BIPOLAR OPERATION AND ADJUSTMENT
Figure 8a shows the ideal input/output characteristics for
the LTC1278. The code transitions occur midway between
successive integer LSB values (i.e., 0.5LSB, 1.5LSB,
2.5LSB, ... FS – 1.5LSB). The output code is naturally
binary with 1LSB = FS/4096 = 5V/4096 = 1.22mV. Figure
8b shows the input/output transfer characteristics for the
bipolar mode in two’s complement format.
Figure 8b. LTC1278 Bipolar Transfer Characteristics
V
REF(OUT)
≥ 2.45V
3Ω
INPUT RANGE
±1.033V
REF(OUT)
5V
–5V
–
+
LT1006
LTC1278
A
IN
AGND
V
REF
10µF
LTC1278 F6
3Ω
INPUT RANGE
±2.58V
(= ±1.033 × V
REF
)
LTC1278
A
IN
AGND
V
REF
10µF
LTC1278 F7
LT1019A-2.5
V
IN
GND
V
OUT
5V
5V
–5V
12
LTC1278
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Figure 9a. Full-Scale Adjust Circuit
LTC1278
A
IN
AGND
LTC1278 F9a
R4
100Ω
FULL-SCALE
ADJUST
R3
10k
R2
10k
R1
50Ω
V1
–
+
A1
ADDITIONAL PINS OMITTED FOR CLARITY
±20LSB TRIM RANGE
Unipolar Offset and Full-scale Error Adjustments
In applications where absolute accuracy is important, then
offset and full-scale errors can be adjusted to zero. Offset
error must be adjusted before full-scale error. Figure 9a
shows the extra components required for full-scale error
adjustment. If both offset and full-scale adjustments are
needed, the circuit in Figure 9b can be used. For zero offset
error apply 0.61mV (i.e., 1/2LSB) at the input and adjust
the offset trim until the LTC1278 output code flickers
between 0000 0000 0000 and 0000 0000 0001. For zero
full-scale error apply an analog input of 4.99817V (i.e., FS
– 1 1/2LSB or last code transition) at the input and adjust
R5 until the LTC1278 output code flickers between 1111
1111 1110 and 1111 1111 1111.
Bipolar Offset and Full-scale Error Adjustments
Bipolar offset and full-scale errors are adjusted in a similar
fashion to the unipolar case. Again, bipolar offset must be
adjusted before full-scale error. Bipolar offset error ad-
justment is achieved by trimming the offset of the op amp
driving the analog input of the LTC1278 while the input
voltage is 1/2LSB below ground. This is done by applying
an input voltage of –0.61mV (–0.5LSB) to the input in
Figure 9c and adjusting the R8 until the ADC output code
flickers between 0000 0000 0000 and 1111 1111 1111.
For full-scale adjustment, an input voltage of 2.49817V
(FS – 1.5LSBs) is applied to the input and R5 is adjusted
until the output code flickers between 0111 1111 1110
and 0111 1111 1111.
BOARD LAYOUT AND BYPASSING
Wire wrap boards are not recommended for high resolu-
tion or high speed A/D converters. To obtain the best
performance from the LTC1278, a printed circuit board is
required. Layout for the printed circuit board should
ensure that digital and analog signal lines are separated as
much as possible. In particular, care should be taken not
to run any digital track alongside an analog signal track or
underneath the ADC. The analog input should be screened
by AGND.
High quality tantalum and ceramic bypass capacitors
should be used at the AV
DD
and V
REF
pins as shown in
Figure 10. For the bipolar mode, a 0.1µF ceramic provides
adequate bypassing for the V
SS
pin. The capacitors must
be located as close to the pins as possible. The traces
connecting the pins and the bypass capacitors must be
kept short and should be made as wide as possible.
Input signal leads to A
IN
and signal return leads from
AGND (Pin 3) should be kept as short as possible to
minimize input noise coupling. In applications where this
is not possible, a shielded cable between source and ADC
is recommended.
Figure 9c. LTC1278 Bipolar Offset and Full-Scale Adjust Circuit
Figure 9b. LTC1278 Unipolar Offset and Full-Scale Adjust Circuit
A
IN
LTC1278 F9b
R2
10k
R4
100k
R1
10k
10k
5V
R9
20Ω
ANALOG
INPUT
0V TO 5V
R3
100k
5V
R8
10k
OFFSET
ADJUST
R6
400Ω
R5
4.3k
FULL-SCALE
ADJUST
R7
100k
–
+
LTC1278
A
IN
LTC1278 F9c
R2
10k
R4
100k
R1
10k
ANALOG
INPUT
R3
100k
5V
R8
20k
OFFSET
ADJUST
R6
200Ω
R5
4.3k
FULL-SCALE
ADJUST
R7
100k
–
+
LTC1278
–5V
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16

LTC1278-4CSW#TRPBF

Mfr. #:
Buy LTC1278-4CSW#TRPBF
Manufacturer:
Analog Devices Inc.
Description:
Analog to Digital Converters - ADC 12-Bit, 400ksps Sampling A/D Converter with Shutdown
Lifecycle:
New from this manufacturer.
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