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AD9240 データシート(PDF) 11 Page - Analog Devices |
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AD9240 データシート(HTML) 11 Page - Analog Devices |
11 / 25 page AD9240 REV. –10– Figure 27 compares the AD9240’s THD vs. frequency perfor- mance for a 2 V input span with a common-mode voltage of 1 V and 2.5 V. Note the difference in the amount of degrada- tion in THD performance as the input frequency increases. Similarly, note how the THD performance at lower frequencies becomes less sensitive to the common-mode voltage. As the input frequency approaches dc, the distortion will be domi- nated by static nonlinearities such as INL and DNL. It is important to note that these dc static nonlinearities are inde- pendent of any RON modulation. –50 –80 0.1 1 20 –70 –60 –90 10 FREQUENCY – MHz VCM = 1.0V VCM = 2.5V Figure 27. THD vs. Frequency for VCM = 2.5 V and 1.0 V (AIN = –0.5 dB, Input Span = 2.0 V p-p) Due to the high degree of symmetry within the SHA topology, a significant improvement in distortion performance for differen- tial input signals with frequencies up to and beyond Nyquist can be realized. This inherent symmetry provides excellent cancella- tion of both common-mode distortion and noise. Also, the required input signal voltage span is reduced a factor of two which further reduces the degree of RON modulation and its effects on distortion. The optimum noise and dc linearity performance for either differential or single-ended inputs is achieved with the largest input signal voltage span (i.e., 5 V input span) and matched input impedance for VINA and VINB. Note that only a slight degradation in dc linearity performance exists between the 2 V and 5 V input span as specified in the AD9240 DC SPECIFICATIONS. Referring to Figure 24, the differential SHA is implemented using a switched-capacitor topology. Hence, its input imped- ance and its subsequent effects on the input drive source should be understood to maximize the converter’s performance. The combination of the pin capacitance, CPIN, parasitic capacitance CPAR, and the sampling capacitance, CS, is typically less than 16 pF. When the SHA goes into track mode, the input source must charge or discharge the voltage stored on CS to the new input voltage. This action of charging and discharging CS which is approximately 4 pF, averaged over a period of time and for a given sampling frequency, FS, makes the input impedance ap- pear to have a benign resistive component (i.e., 83 k Ω at F S = 10 MSPS). However, if this action is analyzed within a sam- pling period (i.e., T = <1/FS), the input impedance is dynamic due to the instantaneous requirement of charging and discharg- ing CS. A series resistor inserted between the input drive source and the SHA input as shown in Figure 28 provides effective isolation. 10 F VINA VINB SENSE AD9240 0.1 F RS* VCC VEE RS* VREF REFCOM *OPTIONAL SERIES RESISTOR Figure 28. Series Resistor Isolates Switched-Capacitor SHA Input from Op Amp. Matching Resistors Improve SNR Performance The optimum size of this resistor is dependent on several fac- tors, which include the AD9240 sampling rate, the selected op amp and the particular application. In most applications, a 30 Ω to 50 Ω resistor is sufficient; however, some applications may require a larger resistor value to reduce the noise band- width or possibly limit the fault current in an overvoltage condition. Other applications may require a larger resistor value as part of an antialiasing filter. In any case, since the THD performance is dependent on the series resistance and the above mentioned factors, optimizing this resistor value for a given application is encouraged. A slight improvement in SNR performance and dc offset performance is achieved by matching the input resistance con- nected to VINA and VINB. The degree of improvement is de- pendent on the resistor value and the sampling rate. For series resistor values greater than 100 Ω, the use of a matching resis- tor is encouraged. The noise or small-signal bandwidth of the AD9240 is the same as its full-power bandwidth. For noise sensitive applications, the excessive bandwidth may be detrimental and the addition of a series resistor and/or shunt capacitor can help limit the wide- band noise at the A/D’s input by forming a low-pass filter. Note, however, that the combination of this series resistance with the equivalent input capacitance of the AD9240 should be evalu- ated for those time-domain applications that are sensitive to the input signal’s absolute settling time. In applications where har- monic distortion is not a primary concern, the series resistance may be selected in combination with the SHA’s nominal 16 pF of input capacitance to set the filter’s 3 dB cutoff frequency. A better method of reducing the noise bandwidth, while possi- bly establishing a real pole for an antialiasing filter, is to add some additional shunt capacitance between the input (i.e., VINA and/or VINB) and analog ground. Since this additional shunt capacitance combines with the equivalent input capaci- tance of the AD9240, a lower series resistance can be selected to establish the filter’s cutoff frequency while not degrading the distortion performance of the device. The shunt capacitance also acts as a charge reservoir, sinking or sourcing the additional charge required by the hold capacitor, CH, further reducing current transients seen at the op amp’s output. The effect of this increased capacitive load on the op amp driv- ing the AD9240 should be evaluated. To optimize performance when noise is the primary consideration, increase the shunt capacitance as much as the transient response of the input signal will allow. Increasing the capacitance too much may adversely affect the op amp’s settling time, frequency response and distor- tion performance. B |
同様の部品番号 - AD9240_17 |
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同様の説明 - AD9240_17 |
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