LTC1852 LINER [Linear Technology], LTC1852 Datasheet - Page 12

• Manufacturer Part Number: LTC1852
• Description: 8-Channel, 10-Bit/12-Bit, 400ksps, Low Power, Sampling ADCs
• Manufacturer: LINER [Linear Technology]

LTC1852/LTC1853
APPLICATIONS INFORMATION
The LTC1852/LTC1853 have a unique differential sample-
and-hold circuit that allows rail-to-rail inputs. The ADC will
always convert the difference of the “+” and “–” inputs
independent of the common mode voltage. The common
mode rejection holds up to high frequencies. The only
requirement is that both inputs can not exceed the AV
power supply voltage or ground. When a bipolar input
span is selected the “+” input can swing ± full scale rela-
tive to the “–” input but neither input can exceed AV
go below ground.
Integral nonlinearity errors (INL) and differential nonlin-
earity errors (DNL) are independent of the common mode
voltage, however, the bipolar offset will vary. The change
in bipolar offset is typically less than 0.1% of the common
mode voltage.
Some AC applications may have their performance limited
by distortion. Most circuits exhibit higher distortion when
signals approach the supply or ground. THD will degrade
as the inputs approach either power supply rail. Distor-
tion can be reduced by reducing the signal amplitude
and keeping the common mode voltage at approximately
midsupply.
Driving the Analog Inputs
The inputs of the LTC1852/LTC1853 are easy to drive. Each
of the analog inputs can be used as a single-ended input
relative to the input common pin (CH0-COM, CH1-COM,
etc.) or in pairs (CH0 and CH1, CH2 and CH3, CH4 and
CH5, CH6 and CH7) for differential inputs. Regardless
of the MUX confi guration, the “+” and “–” inputs are
sampled at the same instant. Any unwanted signal that is
common mode to both inputs will be reduced by the com-
mon 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 LTC1852/LTC1853 inputs can be
driven directly. As source impedance increases, so will
acquisition time. For minimum acquisition time with high
source impedance, a buffer amplifi er should be used. The
only requirement is that the amplifi er driving the analog
12
DD
DD
or
input(s) must settle after the small current spike before
the next conversion starts (settling time must be less than
150ns for full throughput rate).
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 50MHz, then the
output impedance at 50MHz should be less than 100Ω.
The second requirement is that the closed-loop bandwidth
must be greater than 10MHz to ensure adequate small-
signal settling for full throughput rate. The following list
is a summary of the op amps that are suitable for driving
the LTC1852/LTC1853, more detailed information is avail-
able in the Linear Technology Databooks, the LinearView
CD-ROM and on our web site at www.linear-tech.com.
LT
±15V supplies. 5mA supply current. Low distortion.
LT1363: 70MHz Voltage Feedback Amplifi er. ± 2.5V to ±15V
supplies. 7.5mA supply current. Low distortion.
LT1364/LT1365: Dual and Quad 70MHz Voltage Feedback
Amplifi ers. ±2.5V to ±15V supplies. 7.5mA supply current
per amplifi er. Low distortion.
LT1468/LT1469: Single and Dual 90MHz Voltage Feedback
Amplifi er. ± 5V to ±15V supplies. 7mA supply current per
amplifi er. Lowest noise and low distortion.
LT1630/LT1631: Dual and Quad 30MHz Rail-to-Rail Volt-
age Feedback Amplifi ers. Single 3V to ±15V supplies.
3.5mA supply current per amplifi er. Low noise and low
distortion.
LT1632/LT1633: Dual and Quad 45MHz Rail-to-Rail Voltage
Feedback Amplifi ers. Single 3V to ±15V supplies. 4.3mA
supply current per amplifi er. Low distortion.
LT1806/LT1807: Single and Dual 325MHz Rail-to-Rail
Voltage Feedback Amplifi er. Single 3V to ± 5V supplies.
13mA supply current. Lowest distortion.
LinearView is a trademark of Linear Technology Corporation.
®
1360: 50MHz Voltage Feedback Amplifi er. ± 2.5V to
18523fa
™