LTC1867IGN datasheet(10/16 Pages) LINER

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LTC1863/LTC1867

18637f

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

µA

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.

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/

µs 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

Ω

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.

APPLICATIO S I FOR ATIO

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