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LTC1416CG#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20
10
LTC1416
APPLICATIONS INFORMATION
WUU
U
Driving the Analog Input
The differential analog inputs of the LTC1416 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 un-
wanted 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 LTC1416
inputs can be driven directly. As source impedance
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
conversion starts (settling time must be 400ns for full
throughput rate).
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 bandwidth
must be greater than 10MHz 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 LTC1416 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
following list is a summary of the op amps that are suitable
for driving the LTC1416. More detailed information is
available in the Linear Technology Databooks and the
LinearView
TM
CD-ROM.
LT
®
1220: 30MHz unity-gain bandwidth voltage feedback
amplifier. ±5V to ±15V supplies, excellent DC specifica-
tions.
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, ±5V to ±15V supplies, lowest distortion at
frequencies above 400kHz, low noise, best for AC applica-
tions.
LT1229/LT1230: Dual and quad 100MHz current feedback
amplifiers. ±2V to ±15V supplies, low noise, good AC
specs, 6mA supply current each amplifier.
LT1360: 50MHz voltage feedback amplifier. 3.8mA supply
current, good AC and DC specs, ±5V to ±15V supplies.
LT1363: 70MHz, 1000V/µs op amps. 6.3mA supply cur-
rent, good AC and DC specs.
LT1364/LT1365: Dual and quad 70MHz, 100V/µs op amps.
6.3mA supply current per amplifier.
LinearView is a trademark of Linear Technology Corporation.
SOURCE RESISTANCE (Ω)
ACQUISITION TIME (µs)
10 1k 10k 100k
1416 F06
100
10
1
0.1
0.01
Figure 6. Acquisition Time vs Source Resistance
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
11
LTC1416
APPLICATIONS INFORMATION
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Input Filtering
The noise and the distortion of the input amplifier and
other circuitry must be considered since they will add to
the LTC1416 noise and distortion. The small-signal band-
width of the sample-and-hold circuit is 15MHz. 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 200Ω source resistor
to limit the input bandwidth to 800kHz. 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.
accommodate other input ranges often with little or no
additional circuitry. The following sections describe the
reference and input circuitry and how they affect the input
range.
Internal Reference
The LTC1416 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 (see Figure 8b). The reference amplifier gains the
voltage at the V
REF
pin by 1.625 to create the required
internal reference voltage. This provides buffering be-
tween the V
REF
pin and the high speed capacitive DAC. The
V
REF
REFCOMP
AGND
2.5V
4.0625V
3
4
5
22µF
1416 F08a
R1
4k
R2
80k
BANDGAP
REFERENCE
LTC1416
REF
AMP
R3
128k
Figure 8a. LTC1416 Reference Circuit
1
2
3
22µF
ANALOG
INPUT
1416 F08b
A
IN
+
A
IN
–
V
REF
LTC1416
REFCOMP
AGND
4
5
5V
LT1019A-2.5
V
OUT
V
IN
Figure 8b. Using the LT1019-2.5 as an External Reference
1
2
3
1000pF
200Ω
22µF
ANALOG
INPUT
1416 F07
A
IN
+
A
IN
–
V
REF
LTC1416
REFCOMP
AGND
4
5
Figure 7. RC Input Filter
Input Range
The ±2.5V input range of the LTC1416 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
LTC1416 differential inputs and reference circuitry can
12
LTC1416
APPLICATIONS INFORMATION
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reference amplifier compensation pin, REFCOMP (Pin 4),
must be bypassed with a capacitor to ground. The refer-
ence amplifier is stable with capacitors of 1µF or greater.
For the best noise performance, a 22µF ceramic or 22µF
tantalum in parallel with a 0.1µF ceramic is recommended.
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 LTC1416 reference amplifier will limit the
bandwidth and settling time of this circuit. A settling time
of 5ms should be allowed for after a reference adjustment.
INPUT FREQUENCY (Hz)
1k
COMMON MODE REJECTION (dB)
80
70
60
50
40
30
20
10
0
10k 100k
1416 G09
1M 2M
Figure 10a. CMRR vs Input Frequency
1
2
22µF
ANALOG INPUT
1416 F10b
A
IN
+
A
IN
–
LTC1416
REFCOMP
AGND
4
5
3
0V TO 5V
±2.5V
V
REF
converts a 0V to 5V analog input signal with no additional
translation circuitry.
Full-Scale and Offset Adjustment
Figure 11a shows the ideal input/output characteristics for
the LTC1416. The code transitions occur midway between
successive integer LSB values (i.e., –FS + 0.5LSB, –FS +
1.5LSB, –FS + 2.5LSB, . . . FS – 1.5LSB, FS – 0.5LSB). The
output is two’s complement binary with 1LSB = FS –
(–FS)/16384 = 5V/16384 = 305.2µV.
In applications where absolute accuracy is important,
offset and full-scale errors can be adjusted to zero. Offset
error must be adjusted before full-scale error. Figure 11b
shows the extra components required for full-scale error
adjustment. Zero offset is achieved by adjusting the offset
applied to the A
IN
–
input. For zero offset error, apply
–152µV (i.e., –0.5LSB) at A
IN
+
and adjust the offset at the
A
IN
–
input until the output code flickers between 0000
Figure 10b. Selectable 0V to 5V or ±2.5V Input Range
1
2
3
22µF
ANALOG
INPUT
1.25V TO 3V
1416 F09
A
IN
+
A
IN
–
V
REF
LTC1416
LTC1450
REFCOMP
AGND
4
5
Figure 9. Driving V
REF
with a DAC
Differential Inputs
The LTC1416 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 cannot exceed the
AV
DD
or AV
SS
power supply voltages. Integral nonlinearity
errors (INL) and differential 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.
Dynamic performance is also affected by the common
mode voltage. THD will degrade as the inputs approach
either power supply rail, from 90dB with a common mode
of 0V to 79dB with a common mode of 2.5V or –2.5V.
Differential inputs allow greater flexibility for accepting
different input ranges. Figure 10b shows a circuit that
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20

LTC1416CG#PBF

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Buy LTC1416CG#PBF
Manufacturer:
Analog Devices Inc.
Description:
Analog to Digital Converters - ADC 400ksps 14-Bit Parallel ADC
Lifecycle:
New from this manufacturer.
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