Datasheet

LTC2315-12
14
231512fa
For more information www.linear.com/LTC2315-12
APPLICATIONS INFORMATION
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
LTC2315-12. (More detailed information is available on
the Linear Technology website at www.linear.com.)
LT6230: 215MHz GBWP, –80dBc Distortion at 1MHz,
Unity-Gain Stable, Rail-to-Rail Input and Output, 3.5mA/
Amplifier, 1.1nV/√Hz.
LT6200: 165MHz GBWP, –85dBc Distortion at 1MHz, Unity-
Gain Stable, R-R In and Out, 15mA/Amplifier, 0.95nV/√Hz.
LT1818/LT1819: 400MHz GBWP, –85dBc Distortion at
5MHz, Unity-Gain Stable, 9mA/Amplifier, Single/Dual
Voltage Mode Operational Amplifier.
Input Drive Circuits
The analog input of the LTC2315-12 is designed to be driven
single-ended with respect to GND. A low impedance source
can directly drive the high impedance analog input of the
LTC2315-12 without gain error. A high impedance source
should be buffered to minimize settling time during acquisi
-
tion and to
optimize the distortion performance of the ADC.
For best performance, a buffer amplifier should be used
to drive the analog input of the LTC2315-12. The amplifier
provides low output impedance to allow for fast
settling
of the analog signal during the acquisition phase. It also
provides isolation between the signal source and the ADC
inputs which draw a small current spike during acquisition.
Input Filtering
The noise and distortion of the buffer amplifier and other
circuitry must be considered since they add to the ADC
noise and distortion. 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.
Large filter RC time constants slow down the settling at
the analog inputs. It is important that the overall RC time
constants be short enough to allow the analog inputs to
completely settle to >12-bit resolution within the minimum
acquisition time (t
ACQ-MIN
) of 40ns.
A simple 1-pole RC filter is sufficient for many applica-
tions. For example, Figure 10 shows a recommended
single-ended
buffered drive circuit using the LT1818 in
unity gain mode. The 47pF capacitor from A
IN
to ground
and 50Ω source resistor limits the input bandwidth to
68MHz. The 47pF capacitor also acts as a charge reservoir
for the input sample-and-hold and isolates the LT1818
from sampling glitch kick-back. The 50Ω source resistor
is used to
help stabilize the settling response of the drive
amplifier.
When choosing values of source resistance
and shunt capacitance, the drive amplifier data sheet
should be consulted and followed for optimum settling
response. If lower input bandwidths are desired, care
should be taken to optimize the settling response of the
driver amplifier with higher values of shunt capacitance
or series resistance. High quality capacitors and resistors
should be used in the RC filter since these components
can add distortion. NP0/C0G 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 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. High external source resistance,
combined with external shunt capacitance at Pin 4 and
13pF of input capacitance on the LTC2315-12 in sample
mode, will significantly reduce the internal 130MHz input
bandwidth and may increase the required acquisition time
beyond the minimum acquisition time (t
ACQ-MIN
) of 40ns.
47pF
50Ω
2315112 F10
A
IN
LTC2315-12
LT1818
GND
ANALOG IN
+
–
Figure 10. RC Input Filter