10

LTC1412

quencies above the converter’s Nyquist Frequency. The

noise floor stays very low at high frequencies; S/(N + D)

becomes dominated by distortion at frequencies far

beyond Nyquist.

Driving the Analog Input

The differential analog inputs of the LTC1412 are easy to

drive. The inputs may be driven differentially or as a single-

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

charging the sample-and-hold capacitors at the end of

conversion. During conversion, the analog inputs draw

only a small leakage current. If the source impedance of

the driving circuit is low then the LTC1412 inputs can be

driven directly. As source impedance increases so will

acquisition time (see Figure 6). For minimum acquisition

time, with high source impedance, a buffer amplifier must

be used. The only requirement is that the amplifier driving

the analog input(s) must settle after the small current

spike before the next conversion starts (settling time must

be 50ns for full throughput rate).

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

1 and has a unity-gain bandwidth of 50MHz, then the

output impedance at 50MHz should be less than 100

Ω.

The second requirement is that the closed-loop bandwidth

must be greater than 40MHz to ensure adequate small-

signal settling for full throughput rate. If slower op amps

are used, more settling time can be provided by increasing

the time between conversions.

The best choice for an op amp to drive the LTC1412 will

depend on the application. Generally applications fall into

two categories: AC applications where dynamic specifica-

tions 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 LTC1412. More detailed information is

available in the Linear Technology Databooks and on the

LinearView

LT

6mA supply current.

±5V to ±15V supplies. Low Noise.

Good for AC applications.

LT1227: 140MHz Video Current Feedback Amplifier. 10mA

supply current.

±5V to ±15V supplies. Low Noise. Best for

AC applications.

LT1229/LT1230: Dual and Quad 100MHz Current Feed-

back Amplifiers.

±2V to ±15V supplies. Low Noise. Good

AC specifications, 6mA supply current each amplifier.

LT1360: 50MHz Voltage Feedback Amplifier. 3.8mA sup-

ply current.

±5V to ±15V supplies. Good AC and DC

specifications. 70ns settling to 0.5LSB.

LT1363: 70MHz, 1000V/

µs Op Amps. 6.3mA supply cur-

rent. Good AC and DC specifications. 60ns settling to

0.5LSB.

LT1364/LT1365: Dual and Quad 70MHz, 1000V/

µs Op

Amps. 6.3mA supply current per amplifier. 60ns settling

to 0.5LSB.

Input Filtering

The noise and the distortion of the input amplifier and

other circuitry must be considered since they will add to

the LTC1412 noise and distortion. The small-signal band-

SOURCE RESISTANCE (

Ω)

10

0.01

0.1

1

10

100

1k

1412 F06

10k

100k

Figure 6. Acquisition Time vs Source Resistance

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 (<100

Ω) at the closed-loop bandwidth

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