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AD9243AS Arkusz danych(PDF) 11 Page - Analog Devices |
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AD9243AS Arkusz danych(HTML) 11 Page - Analog Devices |
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11 / 25 page  AD9243 REV. A –10– 10 F VINA VINB SENSE AD9243 0.1 F RS* VCC VEE RS* VREF REFCOM *OPTIONAL SERIES RESISTOR Figure 25. 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 factors which include the AD9243 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 re- quire a larger resistor value to reduce the noise bandwidth or possibly limit the fault current in an overvoltage condition. Other applications may require a larger resistor value as part of an anti-aliasing 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 perfor- mance is achieved by matching the input resistance connected to VINA and VINB. The degree of improvement is dependent on the resistor value and the sampling rate. For series resistor values greater than 100 Ω, the use of a matching resistor is encouraged. The noise or small-signal bandwidth of the AD9243 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 AD9243 should be evaluated for those time-domain applications that are sensitive to the input signal’s absolute settling time. In applications where harmonic distortion is not a primary concern, the series resis- tance 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 AD9243, 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 like a charge reservoir, sinking or sourcing the addi- tional charge required by the hold capacitor, CH, further reduc- ing current transients seen at the op amp’s output. The effect of this increased capacitive load on the op amp driv- ing the AD9243 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 dis- tortion performance. Table I. Analog Input Configuration Summary Input Input Input Range (V) Figure Connection Coupling Span (V) VINA1 VINB1 # Comments Single-Ended DC 2 0 to 2 1 32, 33 Best for stepped input response applications, suboptimum THD and noise performance, requires ±5 V op amp. 2 × VREF 0 to VREF 32, 33 Same as above but with improved noise performance due to 2 × VREF increase in dynamic range. Headroom/settling time require- ments of ±5 op amp should be evaluated. 5 0 to 5 2.5 32, 33 Optimum noise performance, excellent THD performance. Requires op amp with VCC > +5 V due to insufficient headroom @ 5 V. 2 × VREF 2.5 – VREF 2.5 39 Optimum THD performance with VREF = 1, noise performance to improves while THD performance degrades as VREF increases 2.5 + VREF to 2.5 V. Single supply operation (i.e., +5 V) for many op amps. Single-Ended AC 2 or 0 to 1 or 1 or VREF 34 Suboptimum ac performance due to input common-mode 2 × VREF 0 to 2 × VREF level not biased at optimum midsupply level (i.e., 2.5 V). 5 0 to 5 2.5 34 Optimum noise performance, excellent THD performance. 2 × VREF 2.5 – VREF 2.5 35 Flexible input range, Optimum THD performance with to VREF = 1. Noise performance improves while THD perfor- 2.5 + VREF mance degrades as VREF increases to 2.5 V. Differential AC or 2 2 to 3 3 to 2 29–31 Optimum full-scale THD and SFDR performance well be- DC yond the A/Ds Nyquist frequency. 2 × VREF 2.5 – VREF/2 2.5 + VREF/2 29–31 Same as 2 V to 3 V input range with the exception that full-scale to to THD and SFDR performance can be traded off for better noise 2.5 + VREF/2 2.5 – VREF/2 performance. 5 1.75 to 3.25 3.25 to 1.75 29–31 Widest dynamic range (i.e., ENOBs) due to Optimum Noise performance. NOTE 1VINA and VINB can be interchanged if signal inversion is required. |
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Podobny opis - AD9243AS |
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