LTC2362 Linear Technology, LTC2362 Datasheet - Page 16

• Manufacturer Part Number: LTC2362
• Description: (LTC2360 - LTC2362) 12-Bit Serial ADCs
• Manufacturer: Linear Technology

APPLICATIONS INFORMATION
SINGLE-ENDED ANALOG INPUT
Driving the Analog Input
The analog input of the LTC2360/LTC2361/LTC2362 is
easy to drive. The input draws only one small current
spike while charging the sample-and-hold capacitor with
the ADC going into track mode. 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 LTC2360/LTC2361/LTC2362 can be driven
directly. As source impedance increases, so will acquisi-
tion time. For minimum acquisition time with high source
impedance, a buffer amplifi er should be used. The main
requirement is that the amplifi er driving the analog input
must settle after the small current spike before the next
conversion starts (settling time must be less than t
for full throughput rate). While choosing an input ampli-
fi er, also keep in mind the amount of noise and harmonic
distortion the amplifi er contributes.
Choosing an Input Amplifi er
Choosing an input amplifi er is easy if a few requirements
are taken into consideration. First, to limit the magnitude
of the voltage spike seen by the amplifi er from charging
the sampling capacitor, choose an amplifi er that has a low
output impedance (<100Ω) at the closed-loop bandwidth
frequency. For example, if an amplifi er is used in a gain
of 1 and has a unity-gain bandwidth of 10MHz, then the
output impedance at 10MHz must be less than 100Ω. The
second requirement is that the closed-loop bandwidth must
be greater than 8MHz to ensure adequate small-signal
settling for full throughput rate. If slower op amps are
LTC2360/LTC2361/LTC2362
16
CONV
SDO
SCK
Figure 13. Minimize the Time When the Device Draws Power, While the Conversion Results are Available Right After Conversion
ACQUIRE
INPUT
t
ACQ
RECOMMENDED HIGH OR LOW
t
CONV
EXECUTE CONVERSION
1
B11
DATA TRANSFER
2
B10
ACQ
3
t
B9
THROUGHPUT
4
t
DATA
used, more time for settling can be provided by increasing
the time between conversions. The best choice for an op
amp to drive the LTC2360/LTC2361/LTC2362 will depend
on the application. Generally, applications fall into two
categories: AC applications where dynamic specifi cations
are most critical and time domain applications where DC
accuracy and settling time are most critical. The follow-
ing list is a summary of the op amps that are suitable for
driving the LTC2360/LTC2361/LTC2362. (More detailed
information is available on the Linear Technology website at
www.linear.com.)
LTC1566-1: Low Noise 2.3MHz Continuous Time Low-
pass Filter.
LT
2.7V to ±15V supplies. Very high AVOL, 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
1, 2V
AC applications (to 1/3 Nyquist) where rail-to-rail perfor-
mance is desired. Quad version is available as LT1631.
LTC6241: Dual 18MHz, Low Noise, Rail-to-Rail, CMOS
Voltage FB Amplifi er. 2.8V to 6V supplies. Very high AVOL
and 125μV offset. It is suitable for applications with a single
5V supply. Quad version is available as LTC6242.
LT1797: Unity-Gain Stable 10MHz, Rail-to-Rail Voltage
Feedback Amplifi er.
LT1801: 180MHz GBWP , –75dBc at 500kHz, 2mA/Ampli-
fi er, 8.5nV/√Hz.
LT6203: 100MHz GBWP , –80dBc Distortion at 1MHz, Unity-
Gain Stable, R-R In and Out, 3mA/Amplifi er, 1.9nV/√Hz.
9
B3
= t
®
1630: Dual 30MHz Rail-to-Rail Voltage FB Amplifi er.
ACQ
10
P-P
B2
+ 2 • t
11
into 1k, V
B1
CONV
12
+ t
B0
DATA
+ t
S
t
CONV
SLEEPMODE
= 5V), making the part excellent for
EXECUTING A DUMMY CONVERSION AND
PUT THE DEVICE INTO SLEEP MODE
RECOMMENDED HIGH OR LOW
Hi-Z STATE
SLEEP MODE
236012 F13
236012f
V
=