Datasheet
LTC2315-12
13
231512fa
For more information www.linear.com/LTC2315-12
Figure 8: LTC2315-12 Entering/Exiting Nap Mode
Figure 9: LTC2315-12 Entering/Exiting Sleep Mode
CS
SCK
Z
1 2
Z
SDO
HI-Z STATE
231512 F08
NAP MODE
HOLD STATIC HIGH or LOW HOLD STATIC HIGH or LOW
START t
ACQ
0 0
CS
SCK
1 2
Z Z
3 4
SDO
HI-Z STATE
231512 F09
NAP MODE SLEEP MODE
HOLD STATIC HIGH or LOW
START t
ACQ
V
REF
RECOVERY
0 0
t
WAIT
APPLICATIONS INFORMATION
Single-Ended Analog Input Drive
The analog input of the LTC2315-12 is easy to drive. The
input draws only one small current spike while charging
the sample-and-hold capacitor at the end of conversion.
During the conversion, the analog input draws only a small
leakage current. If the source impedance of the driving
circuit is low, then the input of the LTC2315-12 can be
driven directly. As the source impedance increases, so
will the acquisition time. For minimum acquisition time
with high source impedance, a buffer amplifier should be
used. The main requirement is that the amplifier driving
the analog input must settle after the small current spike
before the next conversion starts. Settling time must be less
than t
ACQ-MIN
(40ns) for full performance at the maximum
throughput rate. While choosing an input amplifier, also
keep in mind the amount of noise and harmonic distortion
the amplifier contributes.
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 100MHz, then the
output impedance at 100MHz must be less than 50Ω. The
second requirement is that the closed-loop bandwidth
must be greater than 100MHz to ensure adequate small
signal settling for full throughput rate. If slower op amps
are used, more time for settling can be provided by in
-
creasing the
time between conversions. The best choice
for
an op amp to drive the LTC2315-12 will depend on the
application. Generally, applications fall into two categories:
AC applications where dynamic specifications are most