Datasheet LTC1415 (Analog Devices) - 10

LTC1415
U U W U APPLICATIONS INFORMATION
the presence of another sinusoidal input at a different only a small leakage current. If the source impedance of the frequency. driving circuit is low, then the LTC1415 inputs can be driven directly. As source impedance increases so will If two pure sine waves of frequencies fa and fb are applied acquisition time (see Figure 6). For minimum acquisition to the ADC input, nonlinearities in the ADC transfer func- time with high source impedance, a buffer amplifier should tion can create distortion products at the sum and differ- be used. The only requirement is that the amplifier driving ence frequencies of mfa + – nfb, where m and n = 0, 1, 2, the analog input(s) must settle after the small current spike 3, etc. For example, the 2nd order IMD terms include before the next conversion starts (settling time must be (fa + fb). If the two input sine waves are equal in magni- 150ns for full throughput rate). tude, the value (in decibels) of the 2nd order IMD products can be expressed by the following formula: 10 Amplitude at (fa + fb) IMD(fa + fb) = Log 20 Amplitude at fa µs) 1
Peak Harmonic or Spurious Noise
The peak harmonic or spurious noise is the largest spec- tral component excluding the input signal and DC. This 0.1 ACQUISITION TIME ( value is expressed in decibels relative to the RMS value of a full-scale input signal. 0.010.01 0.1 1 10 100
Full-Power and Full-Linear Bandwidth
SOURCE RESISTANCE (kΩ) The full-power bandwidth is that input frequency at which 1415 F06 the amplitude of the reconstructed fundamental is
Figure 6. Acquisition Time vs Source Resistance
reduced by 3dB for a full-scale input signal.
Choosing an Input Amplifier
The full-linear bandwidth is the input frequency at which the S/(N + D) has dropped to 68dB (11 effective bits). The Choosing an input amplifier is easy if a few requirements LTC1415 has been designed to optimize input bandwidth, are taken into consideration. First, to limit the magnitude allowing the ADC to undersample input signals with fre- of the voltage spike seen by the amplifier from charging quencies above the converter’s Nyquist Frequency. The the sampling capacitor, choose an amplifier that has a noise floor stays very low at high frequencies; S/(N + D) low output impedance (< 100Ω) at the closed-loop band- becomes dominated by distortion at frequencies far width frequency. For example, if an amplifier is used in a beyond Nyquist. gain of +1 and has a unity-gain bandwidth of 50MHz, then the output impedance at 50MHz should be less than
Driving the Analog Input
100Ω. The second requirement is that the closed-loop The differential analog inputs of the LTC1415 are easy to bandwidth must be greater than 20MHz to ensure drive. The inputs may be driven differentially or as a single- adequate small-signal settling for full throughput rate. If ended input (i.e., the –A slower op amps are used, more settling time can be IN input is grounded). The +AIN and –A provided by increasing the time between conversions. IN inputs are sampled at the same instant. Any unwanted signal that is common mode to both inputs will be reduced The best choice for an op amp to drive the LTC1415 will by the common mode rejection of the sample-and-hold depend on the application. Generally applications fall into circuit. The inputs draw only one small current spike while two categories: AC applications where dynamic specifi- charging the sample-and-hold capacitors at the end of cations are most critical and time domain applications conversion. During conversion the analog inputs draw where DC accuracy and settling time are most critical. 10