Datasheet LTC1406 (Analog Devices) - 10

LTC1406
U U W U APPLICATIONS INFORMATION
70 ANALOG INPUT A + ANALOG INPUT + IN A 1.5V TO 3.5V IN 2V SPAN 60 LTC1406 LTC1406 – – 50 AIN AIN 40 2.5V VREF 2.5V VREF 30 1406 F08a 1406 F08b 20
Figure 8a. DC Coupled Figure 8b. AC Coupled
COMMON MODE REJECTION (dB) 10 0 100k 1M 10M 100M While CLK is low the analog inputs draw only a small leak- INPUT FREQUENCY (Hz) age current. If the source impedance of the driving circuit 1406 G08 is low, then the LTC1406 inputs can be driven directly. As
Figure 7. Common Mode Rejection vs Input Frequency
source impedance increases, so will acquisition time. For minimum acquisition time with high source impedance, a buffer amplifier should be used. The only requirement is single input providing a ±1V bipolar input range centered that the amplifier driving the analog input(s) must settle around A – + IN . Likewise, AIN can be tied to a fixed voltage after the small current spike before the next conversion and A – IN used as the single input. In any configuration the starts (settling time must be 25ns for full throughput rate). maximum output code (1111 1111) occurs when [(A + IN ) – (A – IN )] = 1V and the minimum output code (0000 0000)
Choosing an Input Amplifier
occurs when [(A + – IN ) – (AIN )] = – 1V. Choosing an input amplifier is easy if a few requirements Each analog input can swing from ground to VDD but not are taken into consideration. First, to limit the magnitude beyond. Therefore, the input common mode voltage can of the voltage spike seen by the amplifier from charging range from 0.5V to 4.5V in differential mode and from 1V the sampling capacitor, choose an amplifier that has a low to 4V in single-ended mode. output impedance (< 50Ω) at the closed-loop bandwidth As an example, with A – frequency. For example, if an amplifier is used in a gain of IN connected to the VREF pin (2.5V) the input range will be 1.5V to 3.5V (see Figure 8a). To 1 and has a unity-gain bandwidth of 50MHz, then the out- achieve other ranges the input may be capacitively coupled put impedance at 50MHz must be less than 50Ω. The to achieve a 2V span with virtually any common mode second requirement is that the closed-loop bandwidth must voltage (see Figure 8b). be greater than 70MHz to ensure adequate small-signal settling for full throughput rate. The 2V input span requires a 2.5V external reference be connected to the V The following list is a summary of the op amps that are REF pin. The LT1460-2.5 micropower precision series reference is recommended. To achieve suitable for driving the LTC1406. More detailed informa- other input spans, the reference voltage (V tion is available in the Linear Technology Databooks and REF) can vary between 2V to 3V. The V on the LinearViewTM CD-ROM. REF pin can also be driven with a DAC or other means. This is useful in applications where
LT®1223:
100MHz Video Current Feedback Amplifier. 6mA the peak input signal amplitude may vary. The input span supply current. ±5V to ±15V supplies. Low noise. of the ADC can then be adjusted to match the peak input signal, maximizing the signal-to-noise ratio.
LT1227:
140MHz Video Current Feedback Amplifier. 10mA supply current. ±5V to ±15V supplies. Low distortion. The analog inputs of the LTC1406 are easy to drive. The Low noise. inputs draw only one small current spike while charging the sample-and-hold capacitors following a rising CLK edge. LinearView is a trademark of Linear Technology Corporation. 10