ltc1411 Linear Technology Corporation, ltc1411 Datasheet - Page 10

• Manufacturer Part Number: ltc1411
• Description: Single Supply 14-bit 2.5msps Adc
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC1411
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 dB 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 74dB (12 effective bits). The
LTC1411 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 be-
yond Nyquist.
Driving the Analog Input
The differential analog inputs of the LTC1411 are easy to
drive. The inputs may be driven differentially or as a single-
ended input (i.e., the A
such as the REFOUT pin of the LTC1411 or an external
source). Figure 1 shows a simplified block diagram for the
analog inputs of the LTC1411. The A
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 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 circuits is low, then the LTC1411 inputs can be
driven directly. More acquisition time should be allowed
for a higher impedance source. Figure 5 shows the acqui-
sition time versus 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
10
U
IN
–
input is tied to a fixed DC voltage
U
W
IN
+
and A
U
IN
–
are
the sampling capacitor, choose an amplifier that has a low
output impedance (<100 ) 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 100 . The
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 settling time can be provided by increasing
the time between conversions.
The best choice for an op amp to drive the LTC1411 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 LTC1411. More detailed information is
available in the Linear Technology Databooks and on the
LinearView
LT
10mA supply current. 5V to 15V supplies. Low noise.
Good for AC applications.
LT1395: 400MHz Current Feedback Amplifier. Single 5V
or 5V supplies. Good for AC applications.
LT1800: 80MHz, 25V/ s Low Power Rail-to-Rail Input and
Output Precision Op Amp. Specified at 3V, 5V and 5V
supplies. Excellent DC performance.
LinearView is a trademark of Linear Technology Corporation.
®
1227: 140MHz Video Current Feedback Amplifier.
Figure 5. Acquisition Time vs Source Resistance
0.01
100
0.1
10
TM
1
1
CD-ROM.
10
SOURCE RESISTANCE ( )
100
1000
10000
1411 G16
100000
1411f