Datasheet LTC1409 (Analog Devices) - 10

LTC1409
U U W U APPLICATIONS INFORMATION
0 fSAMPLE = 800kHz fIN1 = 88.19580078kHz –20 fIN2 = 111.9995117kHz –40 –60 fb – fa fa + fb 2fa + fb fa + 2fb 3fa AMPLITUDE (dB) –80 2fa – fb 2fb – fa 2fb 2fa 3fb –100 –120 0 50k 100k 150k 200k 250k 300k 350k 400k FREQUENCY (Hz) LTC1409 • F05
Figure 5. Intermodulation Distortion Plot
value is expressed in decibels relative to the RMS value of minimum acquisition time, with high source impedance, a full-scale input signal. a buffer amplifier should be used. The only requirement is that the amplifier driving the analog input(s) must
Full Power and Full Linear Bandwidth
settle after the small current spike before the next conver- The full power bandwidth is that input frequency at which sion starts (settling time must be 150ns for full through- the amplitude of the reconstructed fundamental is put rate). reduced by 3dB for a full-scale input signal. 10 The full linear bandwidth is the input frequency at which the S/(N + D) has dropped to 68dB (11 effective bits). The µs) 1 LTC1409 has been designed to optimize input bandwidth, allowing the ADC to undersample input signals with fre- quencies above the converter’s Nyquist Frequency. The noise floor stays very low at high frequencies; S/(N + D) 0.1 ACQUISITION TIME ( becomes dominated by distortion at frequencies far beyond Nyquist. 0.010.01 0.1 1 10 100
Driving the Analog Input
SOURCE RESISTANCE (kΩ) LTC1409 • F06 The differential analog inputs of the LTC1409 are easy to drive. The inputs may be driven differentially or as a
Figure 6. Acquisition Time vs Source Resistance
single-ended input (i.e., the –AIN input is grounded). The
Choosing an Input Amplifier
+AIN and –AIN inputs are sampled at the same instant. Any unwanted signal that is common mode to both Choosing an input amplifier is easy if a few requirements inputs will be reduced by the common mode rejection of are taken into consideration. First, to limit the magnitude the sample-and-hold circuit. The inputs draw only one of the voltage spike seen by the amplifier from charging small current spike while charging the sample-and-hold the sampling capacitor, choose an amplifier that has a capacitors at the end of conversion. During conversion low output impedance (< 100Ω) at the closed-loop band- the analog inputs draw only a small leakage current. If the width frequency. For example, if an amplifier is used in a source impedance of the driving circuit is low then the gain of 1 and has a unity-gain bandwidth of 50MHz, then LTC1409 inputs can be driven directly. As source imped- the output impedance at 50MHz should be less than ance increases so will acquisition time (see Figure 6). For 100Ω. The second requirement is that the closed-loop 10