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LTC1415IG Datasheet(PDF) 10 Page - Linear Technology |
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LTC1415IG Datasheet(HTML) 10 Page - Linear Technology |
10 / 24 page 10 LTC1415 the presence of another sinusoidal input at a different frequency. If two pure sine waves of frequencies fa and fb are applied to the ADC input, nonlinearities in the ADC transfer func- tion can create distortion products at the sum and differ- ence frequencies of mfa + – nfb, where m and n = 0, 1, 2, 3, etc. For example, the 2nd order IMD terms include (fa + fb). If the two input sine waves are equal in magni- tude, the value (in decibels) of the 2nd order IMD products can be expressed by the following formula: IMD fa fb Log + () = 20 Amplitude at (fa + fb) Amplitude at fa Peak Harmonic or Spurious Noise The peak harmonic or spurious noise is the largest spec- tral component excluding the input signal and DC. This 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 LTC1415 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 LTC1415 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 inputsaresampledatthesameinstant.Anyunwanted 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 APPLICATIONS INFORMATION only a small leakage current. If the source impedance of the driving circuit is low, then the LTC1415 inputs can be driven directly. As source impedance 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 conversion starts (settling time must be 150ns for full throughput rate). SOURCE RESISTANCE (k Ω) 0.01 1 1415 F06 0.1 0.01 0.1 110 100 10 Figure 6. Acquisition Time vs Source Resistance 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 bandwidth must be greater than 20MHz to ensure adequate small-signal settling for full throughput rate. If slower op amps are used, more settling time can be provided by increasing the time between conversions. The best choice for an op amp to drive the LTC1415 will depend on the application. Generally applications fall into two categories: AC applications where dynamic specifi- cations are most critical and time domain applications where DC accuracy and settling time are most critical. |
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