ltc1407a-1 Linear Technology Corporation, ltc1407a-1 Datasheet - Page 12

• Manufacturer Part Number: ltc1407a-1
• Description: Serial 12-bit, 3msps Simultaneous Sampling Adcs With Shutdown
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC1407-1/LTC1407A-1
DRIVING THE ANALOG INPUT
The differential analog inputs of the LTC1407-1/
LTC1407A-1 are easy to drive. The inputs may be driven
differentially or as a single-ended input (i.e., the CH0
input is AC grounded at V
both differential analog input pairs, CH0
CH1
unwanted 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
LTC1407-1/LTC1407A-1 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 throughput 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 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
12
+
with CH1
–
, are sampled at the same instant. Any
U
U
CC
/2). All four analog inputs of
W
+
with CH0
U
–
and
–
increasing the time between conversions. The best choice
for an op amp to drive the LTC1407-1/LTC1407A-1 de-
pends 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 LTC1407-1/LTC1407A-1. (More detailed
information is available in the Linear Technology 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
2V
applications (to 1/3 Nyquist) where rail-to-rail perfor-
mance 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
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
2V
applications where rail-to-rail performance is desired.
Quad version is available as LT1633.
LT1801: 80MHz GBWP, –75dBc at 500kHz, 2mA/ampli-
fier, 8.5nV/√Hz.
LT1806/LT1807: 325MHz GBWP, –80dBc distortion at
5MHz, unity gain stable, rail-to-rail in and out,
10mA/amplifier, 3.5nV/√Hz.
LT1810: 180MHz GBWP, –90dBc distortion at 5MHz,
unity gain stable, rail-to-rail in and out, 15mA/amplifier,
16nV/√Hz.
LinearView is a trademark of Linear Technology Corporation.
®
P-P
P-P
1630: Dual 30MHz Rail-to-Rail Voltage FB Amplifier.
into 1kΩ, V
into 1kΩ, V
S
S
= 5V), making the part excellent for AC
= 5V), making the part excellent for AC
TM
CD-ROM.)
VOL
VOL
, 1.5mV offset and
, 500µV offset and
V
V
14071fa
= 1,
= 1,