ltc1867l Linear Technology Corporation, ltc1867l Datasheet - Page 10

• Manufacturer Part Number: ltc1867l
• Description: Ltc1863l - Micropower, 3v, 12-bit, 8-channel 175ksps Adcs
• Manufacturer: Linear Technology Corporation

LTC1863L/LTC1867L
APPLICATIO S I FOR ATIO
Driving the Analog Inputs
The analog inputs of the LTC1863L/LTC1867L 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 LTC1863L/LTC1867L 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 LTC1863L/LTC1867L. More de-
tailed information is available in the Linear Technology
data books or Linear Technology website.
LT
LT1469: Dual LT1468
LT1490A/LT1491A: Dual/quad micropower amplifiers,
50µA/amplifier max, 500µV offset, common mode range
extends 44V above V
supplies.
LT1568: Very low noise, active RC filter building block,
cutoff frequency up to 10MHz, 2.7V to ±5V supplies.
10
Figure 1a. Optional RC Input Filtering for Single-Ended Input
®
1468: 90MHz, 22V/µs 16-bit accurate amplifier
ANALOG
INPUT
50Ω
U
–
independent of V
U
2000pF
10µF
CH0
GND
REFCOMP
W
LTC1863L/
LTC1867L
+
, 3V, 5V and ±15V
1863L7L F01a
U
LT1638/LT1639: Dual/quad 1.2MHz, 0.4V/µs amplifiers,
230µA per amplifier, 3V, 5V and ±15V supplies.
LT1881/LT1882: Dual and quad, 200pA bias current, rail-
to-rail output op amps, up to ±15V supplies.
LTC1992-2: Gain of 2 fully differential input/output am-
plifier/driver, 2.5mV offset, C
supplies.
LT1995: 30MHz, 1000V/µs gain selectable amplifier, pin
configurable as a difference amplifier, inverting and
noninverting amplifier, ±2.5V to ±15V supplies.
LTC6912: Dual programmable gain amplifiers with SPI
serial interface, 2mV offset, 2.7V to ±5V supplies.
LTC6915: Zero drift, instrumentation amplifier with SPI
programmable gain, 125dB CMRR, 0.1% gain accuracy,
10µV offset.
Input Filtering
The noise and the distortion of the input amplifier and
other circuitry must be considered since they will add to
the LTC1863L/LTC1867L noise and distortion. Noisy in-
put 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 50Ω
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
Figure 1b. Optional RC Input Filtering for Differential Inputs
DIFFERENTIAL
ANALOG
INPUTS
50Ω
50Ω
1000pF
1000pF
1000pF
10µF
LOAD
CH0
CH1
REFCOMP
stable, 2.7V to ±5V
LTC1863L/
LTC1867L
1863L7L F01b
1863l7lfa