Datasheet LTC1402 (Analog Devices) - 10

LTC1402
U U W U APPLICATIONS INFORMATION DRIVING THE ANALOG INPUT
second requirement is that the closed-loop bandwidth must be greater than 40MHz to ensure adequate small- The differential analog inputs of the LTC1402 are easy to signal settling for full throughput rate. If slower op amps drive. The inputs may be driven differentially or as a single- – + are used, more time for settling can be provided by ended input (i.e., the AIN input is grounded). The AIN and – increasing the time between conversions. The best choice AIN inputs are sampled at the same instant. Any unwanted for an op amp to drive the LTC1402 will depend on the signal that is common to both inputs will be reduced by the application. Generally, applications fall into two catego- common mode rejection of the sample-and-hold circuit. ries: AC applications where dynamic specifications are The inputs draw only one small current spike while charg- most critical, and time domain applications where DC ing the sample-and-hold capacitors at the end of conver- accuracy and settling time are most critical. The following sion. During conversion, the analog inputs draw only a list is a summary of the op amps that are suitable for small leakage current. If the source impedance of the driving the LTC1402. More detailed information is avail- driving circuit is low, then the LTC1402 inputs can be able in the Linear Technology Databooks and on the driven directly. As source impedance increases, so will LinearViewTM CD-ROM. acquisition time (see Figure 1). For minimum acquisition time with high source impedance, a buffer amplifier must
LT®1206:
60MHz Current Feedback Amplifier with Shut- be used. The only requirement is that the amplifier driving down Pin (Amplifier Draws 200µA While in Shutdown). the analog input(s) must settle after the small current ±5V to ±15V supplies. Distortion is –80dB to 1MHz spike before the next conversion starts (settling time must (2VP-P into 30Ω). Good for AC applications. Dual avail- be 50ns for full throughput rate). able with shutdown as LT1207. Output swings to within 2VBE of the supply rails. 1500 1400
LT1223:
100MHz Video Current Feedback Amplifier. 6mA 1300 ±5V 1200 supply current. ±5V to ±15V supplies. Low distortion at 1100 1000 frequencies above 400kHz. Low noise. Good for AC appli- 900 cations. 800 5V 700
LT1227:
140MHz Video Current Feedback Amplifier. 10mA 600 500 supply current; has shutdown pin (draws 120µA while in ACQUISITION TIME (ns) 400 300 shutdown). ±5V to ±15V supplies. Lowest distortion 200 (– 92dB) at frequencies above 400kHz. Low noise. Best for 100 0 AC applications. 10 100 1k 10k 100k SOURCE RESISTANCE (Ω)
LT1229/LT1230:
Dual and Quad 100MHz Current Feed- 1402 F01 back Amplifiers. ±2V to ±15V supplies. Low noise. Good
Figure 1. Acquisition Time vs Source Resistance
AC specifications, 6mA supply current each amplifier.
in Bipolar and Unipolar Modes LT1360:
50MHz Voltage Feedback Amplifier. 3.8mA sup- ply current. ±5V to ±15V supplies. Good AC and DC
CHOOSING AN INPUT AMPLIFIER
specifications. 70ns settling to 0.5LSB. Choosing an input amplifier is easy if a few requirements
LT1363:
70MHz, 1000V/µs Op Amps. 6.3mA supply cur- are taken into consideration. First, to limit the magnitude rent. Good AC and DC specifications. 60ns settling to of the voltage spike seen by the amplifier from charging 0.5LSB. the sampling capacitor, choose an amplifier that has a low output impedance (< 100Ω) at the closed-loop bandwidth
LT1364/LT1365:
Dual and Quad 70MHz, 1000V/µs Op frequency. For example, if an amplifier is used in a gain of Amps. 6.3mA supply current per amplifier. 60ns settling 1 and has a unity-gain bandwidth of 50MHz, then the to 0.5LSB. output impedance at 50MHz must be less than 100Ω. The LinearView is a trademark of Linear Technology Corporation. 10