LTC1867LIGN datasheet(10/16 Pages) LINER

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LTC1863L/LTC1867L

1863L7Lf

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.

®1468: 90MHz, 22V/µs 16-bit accurate amplifier

LT1469: Dual LT1468

LT1490A/LT1491A: Dual/quad micropower amplifiers,

50µA/amplifier max, 500µV offset, common mode range

extends 44V above V– independent of V+, 3V, 5V and ±15V

supplies.

LT1568: Very low noise, active RC filter building block,

cutoff frequency up to 10MHz, 2.7V to ±5V supplies.

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, CLOAD stable, 2.7V to ±5V

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

APPLICATIO S I FOR ATIO

1863L7L F01a

CH0

GND

LTC1863L/

LTC1867L

REFCOMP

2000pF

10µF

50Ω

ANALOG

INPUT

1000pF

1863L7L F01b

CH0

CH1

LTC1863L/

LTC1867L

REFCOMP

1000pF

10µF

50Ω

DIFFERENTIAL

ANALOG

INPUTS

Figure 1a. Optional RC Input Filtering for Single-Ended Input

Figure 1b. Optional RC Input Filtering for Differential Inputs

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