Datasheet LTC1412 (Analog Devices) - 10

LTC1412
U U W U APPLICATIONS INFORMATION
quencies above the converter’s Nyquist Frequency. The frequency. For example, if an amplifier is used in a gain of noise floor stays very low at high frequencies; S/(N + D) 1 and has a unity-gain bandwidth of 50MHz, then the becomes dominated by distortion at frequencies far output impedance at 50MHz should be less than 100Ω. beyond Nyquist. The second requirement is that the closed-loop bandwidth must be greater than 40MHz to ensure adequate small-
Driving the Analog Input
signal settling for full throughput rate. If slower op amps The differential analog inputs of the LTC1412 are easy to are used, more settling time can be provided by increasing drive. The inputs may be driven differentially or as a single- the time between conversions. ended input (i.e., the A – + IN input is grounded). The AIN and The best choice for an op amp to drive the LTC1412 will A – IN inputs are sampled at the same instant. Any unwanted depend on the application. Generally applications fall into signal that is common mode to both inputs will be reduced two categories: AC applications where dynamic specifica- by the common mode rejection of the sample-and-hold tions are most critical and time domain applications where circuit. The inputs draw only one small current spike while DC accuracy and settling time are most critical. The charging the sample-and-hold capacitors at the end of following list is a summary of the op amps that are suitable conversion. During conversion, the analog inputs draw for driving the LTC1412. More detailed information is only a small leakage current. If the source impedance of available in the Linear Technology Databooks and on the the driving circuit is low then the LTC1412 inputs can be LinearViewTM CD-ROM. driven directly. As source impedance increases so will
LT®1223:
100MHz Video Current Feedback Amplifier. acquisition time (see Figure 6). For minimum acquisition 6mA supply current. ±5V to ±15V supplies. Low Noise. time, with high source impedance, a buffer amplifier must Good for AC applications. be used. The only requirement is that the amplifier driving the analog input(s) must settle after the small current
LT1227:
140MHz Video Current Feedback Amplifier. 10mA spike before the next conversion starts (settling time must supply current. ±5V to ±15V supplies. Low Noise. Best for be 50ns for full throughput rate). AC applications. 10
LT1229/LT1230:
Dual and Quad 100MHz Current Feed- back Amplifiers. ±2V to ±15V supplies. Low Noise. Good AC specifications, 6mA supply current each amplifier. µs) 1
LT1360:
50MHz Voltage Feedback Amplifier. 3.8mA sup- ply current. ±5V to ±15V supplies. Good AC and DC specifications. 70ns settling to 0.5LSB. 0.1 ACQUISITION TIME (
LT1363:
70MHz, 1000V/µs Op Amps. 6.3mA supply cur- rent. Good AC and DC specifications. 60ns settling to 0.01 0.5LSB. 10 100 1k 10k 100k SOURCE RESISTANCE (Ω)
LT1364/LT1365:
Dual and Quad 70MHz, 1000V/µs Op 1412 F06 Amps. 6.3mA supply current per amplifier. 60ns settling
Figure 6. Acquisition Time vs Source Resistance
to 0.5LSB.
Choosing an Input Amplifier Input Filtering
Choosing an input amplifier is easy if a few requirements The noise and the distortion of the input amplifier and are taken into consideration. First, to limit the magnitude other circuitry must be considered since they will add to of the voltage spike seen by the amplifier from charging the LTC1412 noise and distortion. The small-signal band- the sampling capacitor, choose an amplifier that has a low output impedance (<100Ω) at the closed-loop bandwidth LinearView is a trademark of Linear Technology Corporation. 10