Datasheet LTC1411 (Analog Devices) - 10

LTC1411
U U W U APPLICATIO S I FOR ATIO Peak Harmonic or Spurious Noise
100 The peak harmonic or spurious noise is the largest spec- tral component excluding the input signal and DC. This 10 µs) value is expressed in dB relative to the RMS value of a full- scale input signal. 1
Full-Power and Full-Linear Bandwidth
ACQUISITION TIME ( 0.1 The full-power bandwidth is that input frequency at which the amplitude of the reconstructed fundamental is re- 0.01 duced by 3db for a full-scale input signal. 1 10 100 1000 10000 100000 SOURCE RESISTANCE (Ω) The full-linear bandwidth is the input frequency at which 1411 G16 the S/(N + D) has dropped to 74dB (12 effective bits). The
Figure 5. Acquisition Time vs Source Resistance
LTC1411 has been designed to optimize input bandwidth, allowing the ADC to undersample input signals with fre- the sampling capacitor, choose an amplifier that has a low quencies above the converter’s Nyquist frequency. The output impedance (<100Ω) at the closed-loop bandwidth noise floor stays very low at high frequencies; S/(N + D) frequency. For example, if an amplifier is used in a gain of becomes dominated by distortion at frequencies far be- 1 and has a unity-gain bandwidth of 50MHz, then the yond Nyquist. output impedance at 50MHz must be less than 100Ω. The second requirement is that the closed-loop bandwidth
Driving the Analog Input
must be greater than 40MHz to ensure adequate small- signal settling for full throughput rate. If slower op amps The differential analog inputs of the LTC1411 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 AIN input is tied to a fixed DC voltage such as the REFOUT pin of the LTC1411 or an external The best choice for an op amp to drive the LTC1411 will source). Figure 1 shows a simplified block diagram for the depend on the application. Generally applications fall into analog inputs of the LTC1411. The A + – IN and AIN are two categories: AC applications where dynamic specifica- sampled at the same instant. Any unwanted signal that is tions are most critical and time domain applications where common mode to both inputs will be reduced by the DC accuracy and settling time are most critical. The common mode rejection of the sample-and-hold circuit. following list is a summary of the op amps that are suitable The inputs draw only one small current spike while charg- for driving the LTC1411. More detailed information is ing the sample-and-hold capacitors at the end of conver- available in the Linear Technology Databooks and on the sion. During conversion, the analog inputs draw only a LinearViewTM CD-ROM. small leakage current. If the source impedance of the
LT®1227:
140MHz Video Current Feedback Amplifier. driving circuits is low, then the LTC1411 inputs can be 10mA supply current. ±5V to ±15V supplies. Low noise. driven directly. More acquisition time should be allowed Good for AC applications. for a higher impedance source. Figure 5 shows the acqui- sition time versus source resistance.
LT1395:
400MHz Current Feedback Amplifier. Single 5V or ±5V supplies. Good for AC applications.
Choosing an Input Amplifier LT1800:
80MHz, 25V/µs Low Power Rail-to-Rail Input and Choosing an input amplifier is easy if a few requirements Output Precision Op Amp. Specified at 3V, 5V and ±5V are taken into consideration. First, to limit the magnitude supplies. Excellent DC performance. of the voltage spike seen by the amplifier from charging LinearView is a trademark of Linear Technology Corporation. 1411f 10