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LTC1409IG Datasheet(PDF) 10 Page - Linear Technology |
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LTC1409IG Datasheet(HTML) 10 Page - Linear Technology |
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10 / 20 page  10 LTC1409 APPLICATIONS INFORMATION FREQUENCY (Hz) 0 0 –20 –40 –60 –80 –100 –120 100k 200k 300k 400k LTC1409 • F05 50k 150k 250k 350k fb – fa 2fb – fa 2fa – fb 2fa 2fb 3fb fa + 2fb 3fa 2fa + fb fa + fb fSAMPLE = 800kHz fIN1 = 88.19580078kHz fIN2 = 111.9995117kHz Figure 5. Intermodulation Distortion Plot value is expressed in decibels relative to the RMS value of a full-scale input signal. Full Power and Full Linear Bandwidth The full power bandwidth is that input frequency at which the amplitude of the reconstructed fundamental is reduced by 3dB for a full-scale input signal. The full linear bandwidth is the input frequency at which the S/(N + D) has dropped to 68dB (11 effective bits). The 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) becomes dominated by distortion at frequencies far beyond Nyquist. Driving the Analog Input The differential analog inputs of the LTC1409 are easy to drive. The inputs may be driven differentially or as a single-ended input (i.e., the –AIN input is grounded). The +AIN and –AIN inputs are sampled at the same instant. Any unwanted signal that is common mode to both inputs will be reduced by the common mode rejection of the sample-and-hold circuit. The inputs draw only one small current spike while charging the sample-and-hold capacitors at the end of conversion. During conversion the analog inputs draw only a small leakage current. If the source impedance of the driving circuit is low then the LTC1409 inputs can be driven directly. As source imped- ance increases so will acquisition time (see Figure 6). For minimum acquisition time, with high source impedance, a buffer amplifier should be used. The only requirement is that the amplifier driving the analog input(s) must settle after the small current spike before the next conver- sion starts (settling time must be 150ns for full through- put rate). Choosing an Input Amplifier Choosing an input amplifier is easy if a few requirements are taken into consideration. First, to limit the magnitude of the voltage spike seen by the amplifier from charging the sampling capacitor, choose an amplifier that has a low output impedance (< 100 Ω) at the closed-loop band- width frequency. For example, if an amplifier is used in a gain of 1 and has a unity-gain bandwidth of 50MHz, then the output impedance at 50MHz should be less than 100 Ω. The second requirement is that the closed-loop SOURCE RESISTANCE (k Ω) 0.01 1 LTC1409 • F06 0.1 0.01 0.1 110 100 10 Figure 6. Acquisition Time vs Source Resistance |
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