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LTC1409CSW#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20
10
LTC1409
APPLICATIONS INFORMATION
WUU
U
FREQUENCY (Hz)
0
AMPLITUDE (dB)
0
–20
–40
–60
–80
–100
–120
100k 200k 300k 400k
LTC1409 • F05
50k 150k 250k 350k
fb – fa
2fb – fa
2fa – fb
2fa
2fb
3fb
fa + 2fb
3fa
2fa
+ fb
fa + fb
f
SAMPLE
= 800kHz
f
IN1
= 88.19580078kHz
f
IN2
= 111.9995117kHz
Figure 5. Intermodulation Distortion Plot
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 68dB (11 effective bits). The
LTC1409 has been designed to optimize input bandwidth,
allowing the ADC to undersample input signals with fre-
quencies 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 LTC1409 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 impedance of the driving circuit is low then the
LTC1409 inputs can be driven directly. As source imped-
ance increases so will acquisition time (see Figure 6). For
minimum acquisition time, with high source impedance,
a buffer amplifier should be used. The only requirement
is that the amplifier driving the analog input(s) must
settle after the small current spike before the next conver-
sion starts (settling time must be 150ns for full through-
put 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 band-
width 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 should be less than
100Ω. The second requirement is that the closed-loop
SOURCE RESISTANCE (kΩ)
0.01
ACQUISITION TIME (µs)
1
LTC1409 • F06
0.1
0.01
0.1
110
100
10
Figure 6. Acquisition Time vs Source Resistance
11
LTC1409
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S
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AT
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bandwidth must be greater than 20MHz 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 LTC1409 will
depend on the application. Generally applications fall into
two categories: AC applications where dynamic specifi-
cations 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 LTC1409, more detailed informa-
tion is available in the Linear Technology databooks and
the LinearView
TM
CD-ROM.
LT
®
1220: 30MHz unity-gain bandwidth voltage feed-
back amplifier. ±5V to ±15V supplies. Excellent DC
specifications, 90ns settling to 0.5LSB.
LT1223: 100MHz video current feedback amplifier. 6mA
supply current. ±5V to ±15V supplies. Low distortion up
to and above 400kHz. Low noise. Good for AC
applications.
LT1227: 140MHz video current feedback amplifier. 10mA
supply current ±5V to ±15V supplies. Lowest distortion
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 specs. 6mA supply current for each amplifier.
LT1360: 37MHz voltage feedback amplifier. 3.8mA sup-
ply current. Good AC/DC specs. ±5V to ±15V supplies.
70ns settling to 0.5LSB.
LT1363: 50MHz, 450V/µs op amps. 6.3mA supply cur-
rent. Good AC/DC specs. 60ns settling to 0.5LSB.
LT1364/LT1365: Dual and quad 50MHz, 450V/µs op
amps. 6.3mA supply current per amplifier. 60ns settling
to 0.5LSB.
Input Filtering
The noise and the distortion of the input amplifier and
other circuitry must be considered since they will add to
the LTC1409 noise and distortion. The small-signal band-
width of the sample-and-hold circuit is 20MHz. 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 7 shows a 1000pF
capacitor from +A
IN
to ground and a 100Ω source resistor
to limit the input bandwidth to 1.6MHz. The 1000pF
capacitor also acts as a charge reservoir for the input
sample-and-hold and isolates the ADC input from sam-
pling 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 also generate distortion from self heating and from
damage that may occur during soldering. Metal film
surface mount resistors are much less susceptible to both
problems.
When high amplitude unwanted signals are close in fre-
quency to the desired signal frequency, a multiple pole filter
LTC1409
+A
IN
–A
IN
V
REF
REFCOMP
AGND
LTC1409 • F07
1
2
3
4
5
LTC1560-1
1
2
3
4
8
7
6
5
10µF
0.1µF
5V
0.1µF
–5V
V
IN
Figure 7b. 500kHz 5th Order Elliptic Lowpass Filter
LinearView is a trademark of Linear Technology Corporation.
LTC1409
+A
IN
–A
IN
V
REF
REFCOMP
AGND
LTC1409 • F07b
1
2
3
4
5
10µF
1000pF
50Ω
ANALOG INPUT
Figure 7a. RC Input Filter
12
LTC1409
U
S
A
O
PP
L
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is required. Figure 7b shows a simple implementation using
a LTC1560 5th order elliptic continuous time filter.
Input Range
The ±2.5V input range of the LTC1409 is optimized for low
noise and low distortion. Most op amps also perform best
over this same range, allowing direct coupling to the
analog inputs and eliminating the need for special transla-
tion circuitry.
Some applications may require other input ranges. The
LTC1409 differential inputs and reference circuitry can ac-
commodate other input ranges often with little or no addi-
tional circuitry. The following sections describe the reference
and input circuitry and how they affect the input range.
Internal Reference
The LTC1409 has an on-chip, temperature compensated,
curvature corrected, bandgap reference that is factory
trimmed to 2.500V. It is connected internally to a reference
amplifier and is available at V
REF
(Pin 3) see Figure 8a. A
4k resistor is in series with the output so that it can be
easily overdriven by an external reference or other cir-
cuitry. The reference amplifier gains the voltage at the V
REF
pin by 1.625 to create the required internal reference
voltage. This provides buffering between the V
REF
pin and
the high speed capacitive DAC. The reference amplifier
compensation pin, REFCOMP (Pin 4), must be bypassed
with a capacitor to ground. The reference amplifier is
stable with capacitors of 1µF or greater. For the best noise
performance, a 10µF ceramic or 10µF tantalum in parallel
with 0.1µF ceramic is recommended (see Figure 8b).
LTC1409
+A
IN
ANALOG INPUT
5V
–A
IN
V
REF
REFCOMP
AGND
LTC1409 • F08b
1
2
3
4
5
10µF
V
IN
V
OUT
LT1019A-2.5
Figure 8b. Using the LT1019-2.5 as an External Reference
The V
REF
pin can be driven with a DAC or other means
shown in Figure 9. This is useful in applications where the
peak input signal amplitude may vary. The input span of
the ADC can then be adjusted to match the peak input
signal, maximizing the signal-to-noise ratio. The filtering
of the internal LTC1409 reference amplifier will limit the
bandwidth and settling time of this circuit. A settling time
of 5ms should be allowed for, after a reference adjust-
ment.
R2
40k
R3
64k
REFERENCE
AMP
10µF
REFCOMP
AGND
V
REF
R1
4k
BANGAP
REFERENCE
3
4
5
2.5V
4.0625V
LTC1409
LTC1409 • F08a
Figure 8a. LTC1409 Reference Circuit
LTC1409
+A
IN
ANALOG INPUT
–A
IN
V
REF
REFCOMP
AGND
LTC1409 • F09
1
2
3
4
5
10µF
LTC1450
12-BIT 
RAIL-TO-RAIL DAC
1.25V TO 3V
Figure 9.Driving V
REF
with a DAC
Differential Inputs
The LTC1409 has a unique differential sample-and-hold
circuit that allows rail-to-rail inputs. The ADC will always
convert the difference of +A
IN
– (–A
IN
) independent of the
common mode voltage. The common mode rejection
holds up to extremely high frequencies, see Figure 10a.
The only requirement is that both inputs can not exceed
the AV
DD
or AV
SS
power supply voltages. Integral
nonlinearity errors (INL) and differential nonlinearity er-
rors (DNL) are independent of the common mode voltage,
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20

LTC1409CSW#TRPBF

Mfr. #:
Buy LTC1409CSW#TRPBF
Manufacturer:
Analog Devices Inc.
Description:
Analog to Digital Converters - ADC 12-bit, 800ksps SAR ADC with +/-2.5V Input
Lifecycle:
New from this manufacturer.
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