Datasheet LTC1416 (Analog Devices) - 10

LTC1416
U U W U APPLICATIONS INFORMATION Driving the Analog Input
frequency. For example, if an amplifier is used in a gain of The differential analog inputs of the LTC1416 are easy to 1 and has a unity-gain bandwidth of 50MHz, then the drive. The inputs may be driven differentially or as a single- output impedance at 50MHz should be less than 100Ω. ended input (i.e., the A – + The second requirement is that the closed-loop bandwidth IN input is grounded). The AIN and A – must be greater than 10MHz to ensure adequate small- IN inputs are sampled at the same instant. Any un- wanted signal that is common mode to both inputs will be signal settling for full throughput rate. If slower op amps reduced by the common mode rejection of the sample- are used, more settling time can be provided by increasing and-hold circuit. The inputs draw only one small current the time between conversions. spike while charging the sample-and-hold capacitors at The best choice for an op amp to drive LTC1416 will the end of conversion. During conversion, the analog depend on the application. Generally, applications fall into inputs draw only a small leakage current. If the source two categories: AC applications where dynamic specifica- impedance of the driving circuit is low, then the LTC1416 tions are most critical and time domain applications where inputs can be driven directly. As source impedance DC accuracy and settling time are most critical. The increases so will acquisition time (see Figure 6). For following list is a summary of the op amps that are suitable minimum acquisition time, with high source impedance, a for driving the LTC1416. More detailed information is buffer amplifier should be used. The only requirement available in the Linear Technology Databooks and the is that the amplifier driving the analog input(s) must LinearViewTM CD-ROM. settle after the small current spike before the next
LT®1220:
30MHz unity-gain bandwidth voltage feedback conversion starts (settling time must be 400ns for full amplifier. ±5V to ±15V supplies, excellent DC specifica- throughput rate). tions. 10
LT1223:
100MHz video current feedback amplifier. 6mA supply current, ±5V to ±15V supplies, low distortion at frequencies above 400kHz, low noise, good for AC appli- µs) cations. 1
LT1227:
140MHz video current feedback amplifier. 10mA supply current, ±5V to ±15V supplies, lowest distortion at 0.1 frequencies above 400kHz, low noise, best for AC applica- ACQUISITION TIME ( tions.
LT1229/LT1230:
Dual and quad 100MHz current feedback 0.01 amplifiers. 10 100 1k 10k 100k ±2V to ±15V supplies, low noise, good AC SOURCE RESISTANCE (Ω) specs, 6mA supply current each amplifier. 1416 F06
LT1360:
50MHz voltage feedback amplifier. 3.8mA supply
Figure 6. Acquisition Time vs Source Resistance
current, good AC and DC specs, ±5V to ±15V supplies.
LT1363:
70MHz, 1000V/µs op amps. 6.3mA supply cur-
Choosing an Input Amplifier
rent, good AC and DC specs. Choosing an input amplifier is easy if a few requirements
LT1364/LT1365:
Dual and quad 70MHz, 100V/µs op amps. are taken into consideration. First, to limit the magnitude 6.3mA supply current per amplifier. of the voltage spike seen by the amplifier from charging 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