LT6230 datasheet(13/22 Pages) LINER

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LTC2312-12

231212fa

For more information www.linear.com/LTC2312-12

applicaTions inForMaTion

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

LTC2312-12. (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

TheanaloginputoftheLTC2312-12isdesignedtobedriven

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

can directly drive the high impedance analog input of the

LTC2312-12 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 LTC2312-12. 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 600ns.

Asimple1-poleRCfilterissufficientformanyapplications.

For example, Figure 10 shows a recommended single-

ended buffered drive circuit using the LT1818 in unity

gain mode. The 470pF capacitor from AIN to ground and

50Ω source resistor limits the input bandwidth to 7MHz.

The 470pF 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

shuntcapacitance,thedriveamplifierdatasheetshouldbe

consulted and followed for optimum settling response. If

lower input bandwidths are desired, care should be taken

tooptimizethesettlingresponseofthedriveramplifierwith

higher values of shunt capacitance or series resistance.

High quality capacitors and resistors should be used in the

RC filter since these components can add distortion. NP0/

C0G and silver mica type dielectric capacitors have excel-

lentlinearity.Carbonsurfacemountresistorscangenerate

distortion from self heating and from damage that may

occurduringsoldering.Metalfilmsurfacemountresistors

are much less susceptible to both problems. When high

amplitude unwanted signals are close in frequency to the

desired signal frequency, a multiple pole filter is required.

High external source resistance, combined with external

shunt capacitance at Pin 4 will significantly reduce the

input bandwidth and may increase the required acquisi-

tion time beyond the minimum acquisition time (tACQ-MIN)

of 600ns.

Figure 10. RC Input Filter

470pF

231212 F10

AIN

LTC2312-12

LT1818

GND

ANALOG IN

–

ADC Reference

A low noise, low temperature drift reference is critical to

achieving the full data sheet performance of the ADC. The

LTC2312-12 provides an excellent internal reference with

aguaranteed20ppm/°Cmaximumtemperaturecoefficient.

Foraddedflexibility,anexternalreferencemayalsobeused.

The high speed, low noise internal reference buffer is used

only in the internal reference configuration. The reference

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