LTC1406 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1406' PDF : 16 Pages View PDF
LTC1406
APPLICATIONS INFORMATION
70
60
50
40
30
20
10
0
100k
1M
10M
INPUT FREQUENCY (Hz)
100M
1406 G08
Figure 7. Common Mode Rejection
vs Input Frequency
single input providing a ±1V bipolar input range centered
around AIN–. Likewise, AIN+ can be tied to a fixed voltage
and AIN– used as the single input. In any configuration the
maximum output code (1111 1111) occurs when [(AIN+)
– (AIN–)] = 1V and the minimum output code (0000 0000)
occurs when [(AIN+) – (AIN–)] = – 1V.
Each analog input can swing from ground to VDD but not
beyond. Therefore, the input common mode voltage can
range from 0.5V to 4.5V in differential mode and from 1V
to 4V in single-ended mode.
As an example, with AIN– connected to the VREF pin (2.5V)
the input range will be 1.5V to 3.5V (see Figure 8a). To
achieve other ranges the input may be capacitively coupled
to achieve a 2V span with virtually any common mode
voltage (see Figure 8b).
The 2V input span requires a 2.5V external reference be
connected to the VREF pin. The LT1460-2.5 micropower
precision series reference is recommended. To achieve
other input spans, the reference voltage (VREF) can vary
between 2V to 3V. The VREF pin can also be driven with a
DAC or other means. 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 analog inputs of the LTC1406 are easy to drive. The
inputs draw only one small current spike while charging
the sample-and-hold capacitors following a rising CLK edge.
ANALOG INPUT
1.5V TO 3.5V
AIN+
LTC1406
AIN–
ANALOG INPUT
2V SPAN
AIN+
LTC1406
AIN–
2.5V
VREF
1406 F08a
2.5V
VREF
1406 F08b
Figure 8a. DC Coupled
Figure 8b. AC Coupled
While CLK is low the analog inputs draw only a small leak-
age current. If the source impedance of the driving circuit
is low, then the LTC1406 inputs can be driven directly. As
source impedance increases, so will acquisition time. 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 25ns 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 (< 50Ω) 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 out-
put impedance at 50MHz must be less than 50Ω. The
second requirement is that the closed-loop bandwidth must
be greater than 70MHz to ensure adequate small-signal
settling for full throughput rate.
The following list is a summary of the op amps that are
suitable for driving the LTC1406. More detailed informa-
tion is available in the Linear Technology Databooks and
on the LinearViewTM CD-ROM.
LT®1223: 100MHz Video Current Feedback Amplifier. 6mA
supply current. ±5V to ±15V supplies. Low noise.
LT1227: 140MHz Video Current Feedback Amplifier. 10mA
supply current. ±5V to ±15V supplies. Low distortion.
Low noise.
LinearView is a trademark of Linear Technology Corporation.
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