ltc1867lign-trpbf Linear Technology Corporation, ltc1867lign-trpbf Datasheet - Page 10

• Manufacturer Part Number: ltc1867lign-trpbf
• Description: Ltc1863 - 12-bit, 8-channel 200ksps Adcs
• Manufacturer: Linear Technology Corporation

LTC1863/LTC1867
APPLICATIO S I FOR ATIO
Driving the Analog Inputs
The analog inputs of the LTC1863/LTC1867 are easy to
drive. Each of the analog inputs can be used as a single-
ended input relative to the GND pin (CH0-GND, CH1-GND,
etc) or in pairs (CH0 and CH1, CH2 and CH3, CH4 and CH5,
CH6 and CH7) for differential inputs. In addition, CH7 can
act as a COM pin for both single-ended and differential
modes if the COM bit in the input word is high. Regardless
of the MUX configuration, the “+” and “–” inputs are
sampled at the same instant. Any unwanted signal that is
common mode to both inputs will be reduced by the
common mode rejection of the sample-and-hold circuit.
The inputs draw only one small current spike while charg-
ing the sample-and-hold capacitors during the acquire
mode. In conversion mode, the analog inputs draw only a
small leakage current. If the source impedance of the
driving circuit is low then the LTC1863/LTC1867 inputs
can be driven directly. More acquisition time should be
allowed for a higher impedance source.
The following list is a summary of the op amps that are
suitable for driving the LTC1863/LTC1867. More detailed
information is available in the Linear Technology data
books or Linear Technology website.
LT1007 - Low noise precision amplifier. 2.7mA supply
current 5V to 15V supplies. Gain bandwidth product
8MHz. DC applications.
LT1097 - Low cost, low power precision amplifier. 300 A
supply current. 5V to 15V supplies. Gain bandwidth
product 0.7MHz. DC applications.
LT1227 - 140MHz video current feedback amplifier. 10mA
supply current. 5V to 15V supplies. Low noise and low
distortion.
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LT1360 - 37MHz voltage feedback amplifier. 3.8mA sup-
ply current. 5V to 15V supplies. Good AC/DC specs.
LT1363 - 50MHz voltage feedback amplifier. 6.3mA sup-
ply current. Good AC/DC specs.
LT1364/LT1365 - Dual and quad 50MHz voltage feedback
amplifiers. 6.3mA supply current per amplifier. Good
AC/DC specs.
LT1468 - 90MHz, 22V/ s 16-bit accurate amplifier
LT1469 - Dual LT1468
Input Filtering
The noise and the distortion of the input amplifier and
other circuitry must be considered since they will add to
the LTC1863/LTC1867 noise and distortion. Noisy input
circuitry should be filtered prior to the analog inputs to
minimize noise. A simple 1-pole RC filter is sufficient for
many applications. For instance, Figure 1 shows a 50
source resistor and a 2000pF capacitor to ground on the
input will limit the input bandwidth to 1.6MHz. The source
impedance has to be kept low to avoid gain error and
degradation in the AC performance. The capacitor also
acts as a charge reservoir for the input sample-and-hold
and isolates the ADC input from sampling glitch sensitive
circuitry. High quality capacitors and resistors should be
used since these components can add distortion. NPO and
silver mica type dielectric capacitors have excellent linear-
ity. Carbon surface mount resistors can also generate
distortion from self heating and from damage that may
occur during soldering. Metal film surface mount resis-
tors are much less susceptible to both problems.
18637f