Datasheet LTC1414 (Analog Devices) - 9

LTC1414
U U W U APPLICATIONS INFORMATION
 The full-linear bandwidth is the input frequency at which amplitude at f  the S/(N + D) has dropped to 74dB (12 effective bits). The 20log ( ±f ) a b IMD f ( ±f )  a b =  amplitude at f  LTC1414 has been designed to optimize input bandwidth, a   allowing the ADC to undersample input signals with fre- quencies above the converter’s Nyquist frequency. The 0 f noise floor stays very low at high frequencies; S/(N + D) SAMPLE = 2.2MHz –20 fIN1 = 80.566kHz becomes dominated by distortion at frequencies far be- fIN2 = 97.753kHz yond Nyquist. –40
Driving the Analog Input
–60 The differential analog inputs of the LTC1414 are easy to AMPLITUDE (dB) –80 drive. The inputs may be driven differentially or as a single- –100 ended input (i.e., the A – + IN input is grounded). The A IN and A – IN inputs are sampled at the same instant. Any –120 0 200 400 600 800 1000 unwanted signal that is common mode to both inputs will FREQUENCY (kHz) be reduced by the common mode rejection of the sample- 1414 F05a and-hold circuit. The inputs draw only one small current
Figure 5a. Intermodulation Distortion Plot
spike while charging the sample-and-hold capacitors at
with Inputs at 80kHz and 97kHz
the end of conversion. During conversion, the analog 0 inputs draw only a small leakage current. If the source fSAMPLE = 2.2MHz f impedance of the driving circuit is low then the LTC1414 –20 IN1 = 970.019kHz fIN2 = 1.492MHz inputs can be driven directly. As source impedance –40 increases so will acquisition time (see Figure 6). For minimum acquisition time, with high source impedance, a –60 buffer amplifier should be used. The only requirement is AMPLITUDE (dB) –80 that the amplifier driving the analog input(s) must settle after the small current spike before the next conversion –100 starts (settling time must be 70ns for full throughput rate). –120 0 200 400 600 800 1000 10 FREQUENCY (kHz) 1414 F05b
Figure 5b. Intermodulation Distortion Plot
µs)
with Input Signals of 1MHz and 1.5MHz
1
Peak Harmonic or Spurious Noise
The peak harmonic or spurious noise is the largest spec- 0.1 tral component excluding the input signal and DC. This ACQUISITION TIME ( value is expressed in dB relative to the RMS value of a full- scale input signal. 0.01 10 100 1k 10k 100k
Full-Power and Full-Linear Bandwidth
SOURCE RESISTANCE (Ω) 1414 FO6 The full-power bandwidth is that input frequency at which
Figure 6. Acquisition Time vs Source Resistance
the amplitude of the reconstructed fundamental is re- duced by 3db for a full-scale input signal. 9