ltc2355-12 Linear Technology Corporation, ltc2355-12 Datasheet - Page 9

• Manufacturer Part Number: ltc2355-12
• Description: Serial 12-bit, 3.5msps Sampling Adcs With Shutdown
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
DRIVING THE ANALOG INPUT
The differential analog inputs of the LTC2355-12/LTC2355-14
may be driven differentially or as a single-ended input (i.e., the
A
and A
signal that is common to both inputs of each input pair 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 LTC2355-12/LTC2355-14 inputs can
be driven directly. As source impedance increases, so will
acquisition time. For minimum acquisition time with high
source impedance, a buffer amplifier must be used. The main
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 39ns for full through-
put rate). Also keep in mind while choosing an input amplifier
the amount of noise and harmonic distortion added by the
amplifier.
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 sam-
pling capacitor, choose an amplifier that has a low output
impedance (<100Ω) 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 100Ω. 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 time for
settling can be provided by increasing the time between
conversions. The best choice for an op amp to drive the
LTC2355-12/LTC2355-14 will depend on the application.
Generally, applications fall into two categories: AC applica-
tions where dynamic specifications 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 LTC2355-12/LTC2355-14.
IN
–
input is grounded). Both differential analog inputs, A
IN
–
, are sampled at the same instant. Any unwanted
U
U
W
U
IN
+
(More detailed information is available in the Linear Technol-
ogy Databooks and on the LinearView
LTC1566-1: Low Noise 2.3MHz Continuous Time Low-Pass
Filter.
LT
2.7V to ±15V supplies. Very high A
520ns settling to 0.5LSB for a 4V swing. THD and noise are
–93dB to 40kHz and below 1LSB to 320kHz (A
into 1kΩ, V
applications (to 1/3 Nyquist) where rail-to-rail performance
is desired. Quad version is available as LT1631.
LT1632: Dual 45MHz Rail-to-Rail Voltage FB Amplifier. 2.7V
to ±15V supplies. Very high A
settling to 0.5LSB for a 4V swing. It is suitable for applica-
tions with a single 5V supply. THD and noise are
–93dB to 40kHz and below 1LSB to 800kHz (A
2V
applications where rail-to-rail performance is desired. Quad
version is available as LT1633.
LT1813: Dual 100MHz 750V/µs 3mA Voltage Feedback
Amplifier. 5V to ±5V supplies. Distortion is –86dB to 100kHz
and –77dB to 1MHz with ±5V supplies (2V
Excellent part for fast AC applications with ±5V supplies.
LT1801: 80MHz GBWP, –75dBc at 500kHz, 2mA/Amplifier,
8.5nV/√Hz.
LT1806/LT1807: 325MHz GBWP, –80dBc Distortion at 5MHz,
Unity-Gain Stable, R-R In and Out, 10mA/Amplifier, 3.5nV/√Hz.
LT1810: 180MHz GBWP, –90dBc Distortion at 5MHz,
Unity-Gain Stable, R-R In and Out, 15mA/Amplifier, 16nV/√Hz.
LT1818/LT1819: 400MHz, 2500V/µs,9mA, Single/Dual Volt-
age Mode Operational Amplifier.
LT6200: 165MHz GBWP, –85dBc Distortion at 1MHz, Unity-
Gain Stable, R-R In and Out, 15mA/Amplifier,
0.95nV/√Hz.
LT6203: 100MHz GBWP, –80dBc Distortion at 1MHz,
Unity-Gain Stable, R-R In and Out, 3mA/Amplifier,
1.9nV/√Hz.
LT6600-10: Amplifier/Filter Differential In/Out with 10MHz
Cutoff.
LinearView is a trademark of Linear Technology Corporation.
®
P-P
1630: Dual 30MHz Rail-to-Rail Voltage FB Amplifier.
into 1kΩ, V
LTC2355-12/LTC2355-14
S
= 5V), making the part excellent for AC
S
= 5V), making the part excellent for AC
VOL
, 1.5mV offset and 400ns
VOL
TM
, 500µV offset and
CD-ROM.)
P-P
V
into 500Ω).
= 1, 2V
V
= 1,
9
2355f
P-P