LTC2365/LTC2366
18
23656fb
For more information www.linear.com/LTC2365
SINGLE-ENDED ANALOG INPUT
Driving the Analog Input
The analog input of the LTC2365/LTC2366 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 LTC2365/LT2366 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
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
56ns for full 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 amplifier from charging the
sampling 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 LTC2365/LTC2366 will depend on the
application. Generally, applications fall into two categories:
AC applications 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
LTC2365/LTC2366. (More detailed information is available
on the Linear Technology website at www.linear.com.)
LTC1566-1: Low Noise 2.3MHz Continuous Time Lowpass
Filter.
LT
®
1630: Dual 30MHz Rail-to-Rail Voltage Feedback
Amplifier. 2.7V to ±15V supplies. Very high A
VOL
, 500µV
offset and 520ns settling to 0.5LSB for a 4V swing. THD
and noise are –93dB to 40kHz and below 1LSB to 320kHz
(A
V
= 1, 2V
P-P
into 1k, V
S
= 5V), making the part excellent
for AC applications (to 1/3 Nyquist) where rail-to-rail per-
formance is desired. Quad version is available as LT1631.
LT1632
: Dual 45MHz Rail-to-Rail Voltage Feedback Ampli-
fier. 2.7V to ±15
V supplies. Very high A
VOL
, 1.5mV offset
and 400ns settling to 0.5LSB for a 4V swing. It is suitable
for applications with a single 5V supply. THD and noise
are –93dB to 40kHz and below 1LSB to 800kHz (A
V
= 1,
2V
P-P
into 1k, V
S
= 5V), making the part excellent for AC
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
P-P
into 500).
Excellent part for fast AC applications with ±5V supplies.
LT1801: 180MHz GBWP, –75dBc at 500kHz, 2mA/Ampli-
fier, 8.5nV/√Hz.
LT1806/LT1807: 325MHz GBWP, –80dBc Distortion at
5MHz, Unity-Gain Stable, R-R In and Out, 10mA/Ampli-
fier, 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
Voltage 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.
applications inForMation
