LT6201 datasheet(13/20 Pages) LINER

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LTC2314-14

231414f

For more information www.linear.com/2314-14

applicaTions inForMaTion

with high source impedance, a buffer amplifier should be

used. The main requirement is that the amplifier driving

the analog input must settle after the small current spike

beforethenextconversionstarts.Settlingtimemustbeless

than tACQ-MIN (40ns) for full performance at the maximum

throughput rate. While choosing an input amplifier, also

keep in mind the amount of noise and harmonic distortion

the amplifier contributes.

Choosing an Input Amplifier

Choosing an input amplifier is easy if a few requirements

are taken into consideration. First, to limit the magnitude

of the voltage spike seen by the amplifier from charging

the sampling capacitor, choose an amplifier that has a low

output impedance (<50Ω) at the closed-loop bandwidth

frequency. For example, if an amplifier is used in a gain

of 1 and has a unity-gain bandwidth of 100MHz, then the

output impedance at 100MHz must be less than 50Ω. The

second requirement is that the closed-loop bandwidth

must be greater than 100MHz to ensure adequate small

signal settling for full throughput rate. If slower op amps

are used, more time for settling can be provided by in-

creasing the time between conversions. The best choice

for an op amp to drive the LTC2314-14 will depend on the

application.Generally,applicationsfallintotwocategories:

AC applications where dynamic specifications are most

critical and time domain applications where DC accuracy

and settling time are most critical. The following list is a

summary of the op amps that are suitable for driving the

LTC2314-14. (More detailed information is available on

the Linear Technology website at www.linear.com.)

LT6230: 215MHz GBWP, –80dBc Distortion at 1MHz,

Unity-Gain Stable, Rail-to-Rail Input and Output, 3.5mA/

Amplifier, 1.1nV/√Hz.

LT6200:165MHzGBWP,–85dBcDistortionat1MHz,Unity-

Gain Stable, R-R In and Out, 15mA/Amplifier, 0.95nV/√Hz.

LT1818/1819:400MHzGBWP,–85dBcDistortionat5MHz,

Unity-Gain Stable, 9mA/Amplifier, Single/Dual Voltage

Mode Operational Amplifier.

Input Drive Circuits

TheanaloginputoftheLTC2314-14isdesignedtobedriven

single-ended with respect to GND. A low impedance source

can directly drive the high impedance analog input of the

LTC2314-14 without gain error. A high impedance source

shouldbebufferedtominimizesettlingtimeduringacquisi-

tion and to optimize the distortion performance of the ADC.

For best performance, a buffer amplifier should be used

to drive the analog input of the LTC2314-14. The amplifier

provides low output impedance to allow for fast settling

of the analog signal during the acquisition phase. It also

provides isolation between the signal source and the ADC

inputswhichdrawasmallcurrentspikeduringacquisition.

Input Filtering

The noise and distortion of the buffer amplifier and other

circuitry must be considered since they add to the ADC

noiseanddistortion.Noisyinputcircuitryshouldbefiltered

prior to the analog inputs to minimize noise. A simple

1-pole RC filter is sufficient for many applications.

Large filter RC time constants slow down the settling at

the analog inputs. It is important that the overall RC time

constants be short enough to allow the analog inputs to

completelysettleto>12-bitresolutionwithintheminimum

acquisition time (tACQ-MIN) of 40ns.

Asimple1-poleRCfilterissufficientformanyapplications.

For example, Figure 10 shows a recommended single-

ended buffered drive circuit using the LT1818 in unity gain

mode. The 47pF capacitor from AIN to ground and 50Ω

source resistor limits the input bandwidth to 68MHz. The

47pF capacitor also acts as a charge reservoir for the input

sample-and-hold and isolates the LT1818 from sampling

glitch kick-back. The 50Ω source resistor is used to help

stabilize the settling response of the drive amplifier. When

choosing values of source resistance and shunt capaci-

tance, the drive amplifier data sheet should be consulted

and followed for optimum settling response. If lower input

bandwidths are desired, care should be taken to optimize

the settling response of the driver amplifier with higher

values of shunt capacitance or series resistance. High

quality capacitors and resistors should be used in the RC

filtersincethesecomponentscanadddistortion.NP0/C0G

and silver mica type dielectric capacitors have excellent

linearity. Carbon surface mount resistors can generate

distortion from self heating and from damage that may

occurduringsoldering.Metalfilmsurfacemountresistors

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