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LTC1402CGN#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24
10
LTC1402
APPLICATIONS INFORMATION
WUU
U
Figure 1. Acquisition Time vs Source Resistance
in Bipolar and Unipolar Modes
LinearView is a trademark of Linear Technology Corporation.
second requirement is that the closed-loop bandwidth
must be greater than 40MHz to ensure adequate small-
signal settling for full throughput rate. If slower op amps
are used, more time for settling can be provided by
increasing the time between conversions. The best choice
for an op amp to drive the LTC1402 will depend on the
application. Generally, applications fall into two catego-
ries: AC applications where dynamic specifications are
most critical, and time domain applications where DC
accuracy and settling time are most critical. The following
list is a summary of the op amps that are suitable for
driving the LTC1402. More detailed information is avail-
able in the Linear Technology Databooks and on the
LinearView
TM
CD-ROM.
LT
®
1206: 60MHz Current Feedback Amplifier with Shut-
down Pin (Amplifier Draws 200µA While in Shutdown).
±5V to ±15V supplies. Distortion is –80dB to 1MHz
(2V
P-P
into 30Ω). Good for AC applications. Dual avail-
able with shutdown as LT1207. Output swings to within
2V
BE
of the supply rails.
LT1223: 100MHz Video Current Feedback Amplifier. 6mA
supply current. ±5V to ±15V supplies. Low distortion at
frequencies above 400kHz. Low noise. Good for AC appli-
cations.
LT1227: 140MHz Video Current Feedback Amplifier. 10mA
supply current; has shutdown pin (draws 120µA while in
shutdown). ±5V to ±15V supplies. Lowest distortion
(–92dB) at frequencies above 400kHz. Low noise. Best for
AC applications.
LT1229/LT1230: Dual and Quad 100MHz Current Feed-
back Amplifiers. ±2V to ±15V supplies. Low noise. Good
AC specifications, 6mA supply current each amplifier.
LT1360: 50MHz Voltage Feedback Amplifier. 3.8mA sup-
ply current. ±5V to ±15V supplies. Good AC and DC
specifications. 70ns settling to 0.5LSB.
LT1363: 70MHz, 1000V/µs Op Amps. 6.3mA supply cur-
rent. Good AC and DC specifications. 60ns settling to
0.5LSB.
LT1364/LT1365: Dual and Quad 70MHz, 1000V/µs Op
Amps. 6.3mA supply current per amplifier. 60ns settling
to 0.5LSB.
SOURCE RESISTANCE (Ω)
300
ACQUISITION TIME (ns)
900
1100
1000
1300
1500
200
100
800
500
700
600
400
1200
1400
10 1k 10k
5V
100k
1402 F01
0
100
±5V
DRIVING THE ANALOG INPUT
The differential analog inputs of the LTC1402 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 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 charg-
ing the sample-and-hold capacitors at the end of conver-
sion. During conversion, the analog inputs draw only a
small leakage current. If the source impedance of the
driving circuit is low, then the LTC1402 inputs can be
driven directly. As source impedance increases, so will
acquisition time (see Figure 1). For minimum acquisition
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 (settling time must
be 50ns for full throughput rate).
CHOOSING AN INPUT AMPLIFIER
Choosing an input amplifier is easy if a few requirements
are taken into consideration. First, to limit the magnitude
of the voltage spike seen by the amplifier from charging
the sampling capacitor, 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 unity-gain bandwidth of 50MHz, then the
output impedance at 50MHz must be less than 100Ω. The
11
LTC1402
APPLICATIONS INFORMATION
WUU
U
less susceptible to both problems. When high amplitude
unwanted signals are close in frequency to the desired signal
frequency, a multiple pole filter is required. Figure 3 shows
a simple implementation using an LTC1560-1, a fifth␣ order
elliptic continuous-time 1MHz filter.
BIPOLAR AND UNIPOLAR INPUT RANGES
The ±2V bipolar input range of the LTC1402 is optimized
for low noise and low distortion. Most op amps also
perform best over this same range, allowing direct cou-
pling to the analog inputs and eliminating the need for
special translation circuitry. The inputs of the LTC1402
may also be driven fully differential in bipolar mode with
a single supply. Each input should not swing more than
2V
P-P
individually to get the best performance from single
supply amplifiers.
The 0V to 4V range is ideal for single ended input use with
single supply applications.
LTC1402
A
IN
+
A
IN
–
V
REF
AGND1
AGND2
GAIN
1402 F02
3
4
2
6
5
7
10µF
68pF
51Ω
ANALOG
INPUT
Figure 2. RC Input Filter
LTC1560-1
1
2
3
4
8
7
6
5
0.1µF
5V
0.1µF
–5V
V
IN
LTC1402
A
IN
+
A
IN
–
V
REF
AGND1
AGND2
GAIN
1402 F03
3
4
2
6
5
7
10µF
Figure 3. 1MHz Fifth Order Elliptic Lowpass Filter
LT1630: Dual 30MHz Rail-to-Rail Voltage FB Amplifier.
2.7V to ±15V supplies. Very high A
VOL
, 500µV offset and
520ns settling to 0.5LSB for a 4V swing. THD and noise
are –93dB to 40kHz and below 1LSB to 320kHz (A
V
= 1,
2V
P-P
into 1kΩ, V
S
= 5V), making the part excellent for AC
applications (to 1/3 Nyquist) where rail-to-rail perfor-
mance is desired. Quad version is available as LT1631.
LT1632: Dual 45MHz Rail-to-Rail Voltage FB Amplifier.
2.7V to ±15V supplies. Very high A
VOL
, 1.5mV offset and
400ns settling to 0.5LSB for a 4V swing. It is suitable for
applications with a single 5V supply. THD and noise are
–93dB to 40kHz and below 1LSB to 800kHz (A
V
= 1,
2V
P-P
into 1kΩ, V
S
= 5V), making the part excellent for AC
applications where rail-to-rail performance is desired.
Quad version is available as LT1633.
LT1813: Dual 100MHz 750V/µs 3mA Voltage Feedback
Amplifier. 5V to ±5V supplies. Distortion is –86dB to
100kHz and –77dB to 1MHz with ±5V supplies (2V
P-P
into
500Ω). Excellent part for fast AC applications with
±5V␣ supplies.
INPUT FILTERING AND SOURCE IMPEDANCE
The noise and the distortion of the input amplifier and
other circuitry must be considered since they will add to
the LTC1402 noise and distortion. The small-signal band-
width of the sample-and-hold circuit is 80MHz. Any noise
or distortion products that are present at the analog inputs
will be summed over this entire bandwidth. Noisy input
circuitry should be filtered prior to the analog inputs to
minimize noise. A simple 1-pole RC filter is sufficient for
many applications. For example, Figure 2 shows a 68pF
capacitor from A
IN
+
to ground and a 51Ω source resistor
to limit the input bandwidth to 47MHz. The 68pF capacitor
also acts as a charge reservoir for the input sample-and-
hold and isolates the ADC input from sampling glitch-
sensitive circuitry.
High quality capacitors and resistors should be used since
these components can add distortion. NPO and silver mica
type dielectric capacitors have excellent linearity.
Carbon surface mount resistors can generate distortion
from self heating and from damage that may occur during
soldering. Metal film surface mount resistors are much
12
LTC1402
APPLICATIONS INFORMATION
WUU
U
ADC can then be adjusted to match the peak input signal,
maximizing the signal-to-noise ratio. The filtering of the
internal LTC1402 reference amplifier will limit the band-
width and settling time of this circuit. A settling time of
5ms should be allowed after a reference adjustment.
DIFFERENTIAL INPUTS
The LTC1402 has a unique differential sample-and-hold
circuit that allows inputs from –2.5V to 5V. The ADC will
always convert the difference of A
IN
+
– A
IN
–
independent
of the common mode voltage. The common mode rejec-
tion holds up at extremely high frequencies, see Figure 7.
The only requirement is that both inputs not exceed
–2.5V or 5V. Integral nonlinearity errors (INL) and differ-
ential nonlinearity errors (DNL) are independent of the
common mode voltage. However, the bipolar zero error
(BZE) will vary. The change in BZE is typically less than
0.1% of the common mode voltage. Figure 5b shows the
use of bipolar mode with single 5V supply.
Figure 5a. Using the LT1019-2.5 as an External Reference
LTC1402
LT1019-2.5
A
IN
+
ANALOG INPUT
5V
5V
A
IN
–
V
REF
V
OUT
V
IN
AGND2
GAIN
1402 F04a
3
4
5
6
7
10µF
LTC1402
LT1019-2.5
A
IN
+
V
IN
2.5V ±2.048V
5V
A
IN
–
V
REF
BIP
V
IN
AGND2
14
V
SS
GAIN
1402 F04a
3
4
5
6
7
10µF
10µF
2.5V
5V
8
Figure 5b. Bipolar Mode with Single Supply
Figure 6. Driving V
REF
with a 12 Bit, V
OUT
DAC
LTC1402
A
IN
+
ANALOG INPUT
5V
A
IN
–
V
REF
AGND2
GAIN
1402 F06
3
4
5
6
7
10µF
LTC1451
INTERNAL REFERENCE
The LTC1402 has an on-chip, temperature compensated,
curvature corrected, bandgap reference that is factory
trimmed to 2.048V. It is connected internally to a reference
amplifier, see Figure 4. The reference amplifier amplifies
the voltage at the V
REF
pin by 2 to create the required
internal reference voltage of 4.096V. This provides buffer-
ing for the high speed capacitive DAC. The reference
amplifier output V
REF
, (Pin 5) must be bypassed with a
capacitor to ground. The reference amplifier is stable with
capacitors of 1µF or greater. For the best noise perfor-
mance, a 10µF ceramic or a 10µF tantalum in parallel with
a 0.1µF ceramic is recommended.
The V
REF
pin can be driven with an external reference as
shown in Figure 5a. The GAIN pin (Pin 7) is tied to the
positive supply to disable the internal reference buffer.
A DAC may also be used to drive V
REF
as shown in
Figure 6. This is useful in applications where the peak
input signal amplitude may vary. The input span of the
Figure 4. LTC1402 Reference Circuit
64k
64k
REFERENCE
AMP
10µF
GAIN
V
REF
BANGAP
REFERENCE
5
7
AGND2
6
4.096V
LTC1402
+
–
2.048V
1402 F04
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LTC1402CGN#TRPBF

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Buy LTC1402CGN#TRPBF
Manufacturer:
Analog Devices Inc.
Description:
Analog to Digital Converters - ADC Serial 12-B, 2.2Msps Smpl ADC w/ SD
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