 |
Electronic Components Datasheet Search |
|
|
Electronic Components Datasheet Search |
|
AD9225ARSZ Datasheet(PDF) 10 Page - Analog Devices |
|
||||||||||||||||||||||||||||||
AD9225ARSZ Datasheet(HTML) 10 Page - Analog Devices |
|
|
10 / 26 page  AD9225 –9– 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 by a half, 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., 4 V input span) and matched input impedance for VINA and VINB. Only a slight degrada- tion in dc linearity performance exists between the 2 V and 4 V input spans. Referring to Figure 3, the differential SHA is implemented using a switched capacitor topology. Its input impedance and its switching effects on the input drive source should be considered in order to maximize the converter’s performance. The combi- nation of the pin capacitance, CPIN, parasitic capacitance, CPAR, and the sampling capacitance, CS, is typically less than 5 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, averaged over a period of time and for a given sampling frequency, fS, makes the input impedance appear to have a benign resistive component. However, if this action is analyzed within a sampling period (i.e., T = 1/fS), the input impedance is dynamic and therefore certain precautions on the input drive source should be observed. The resistive component to the input impedance can be com- puted by calculating the average charge that gets drawn by CH from the input drive source. It can be shown that if CS is al- lowed to fully charge up to the input voltage before switches QS1 are opened, then the average current into the input would be the same as it would if there were a resistor of 1/(CS fS) Ohms connected between the inputs. This means that the input im- pedance is inversely proportional to the converter’s sample rate. Since CS is only 5 pF, this resistive component is typically much larger than that of the drive source (i.e., 8 k W at fS = 25 MSPS). The SHA’s input impedance over a sampling period appears as a dynamic input impedance to the input drive source. When the SHA goes into the track mode, the input source ideally should provide the charging current through RON of switch QS1 in an exponential manner. The requirement of exponential charging means that the most common input source, an op amp, must exhibit a source impedance that is both low and resistive up to and beyond the sampling frequency. The output impedance of an op amp can be modeled with a series inductor and resistor. When a capacitive load is switched onto the output of the op amp, the output will momentarily drop due to its effective output impedance. As the output recov- ers, ringing may occur. To remedy the situation, a series resistor can be inserted between the op amp and the SHA input as shown in Figure 4. The series resistance helps isolate the op amp from the switched capacitor load. 10 F VINA VINB SENSE AD9225 0.1 F RS VCC VEE RS VREF REFCOM Figure 4. 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 ADC sampling rate, the selected op amp, and the particular application. In most applications, a 30 W to 100 W resistor is sufficient. However, some applica- tions may require a larger resistor value to reduce the noise bandwidth or possibly to limit the fault current in an overvolt- age 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. The source impedance driving VINA and VINB should be matched. Failure to provide that matching will result in degra- dation of the AD9225’s superb SNR, THD, and SFDR. For noise sensitive applications, the very high bandwidth of the AD9225 may be detrimental. The addition of a series resistor and/or shunt capacitor can help limit the wideband noise at the ADC’s input by forming a low-pass filter. Note, however, that the combination of this series resistance with the equivalent input capacitance of the AD9225 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 resistance may be selected in combination with the SHA’s nominal 10 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 AD9225, 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, and 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 AD9225 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. Rev. C |
Similar Part No. - AD9225ARSZ |
|
Similar Description - AD9225ARSZ |
|
|
|
Link URL |
| Privacy Policy |
| ALLDATASHEET.COM |
| Does ALLDATASHEET help your business so far? [ DONATE ] |
About Alldatasheet | Advertisement | Datasheet Upload | Contact us | Privacy Policy | Link Exchange | Manufacturer List All Rights Reserved©Alldatasheet.com |
| Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
|
Family Site : ic2ic.com |
icmetro.com |
English ▼
English
Chinese
German
Japanese
Russian
Korean
Spanish
French
Italian
Portuguese
Polish
Vietnamese
Indian
Mexican
British
New Zealand
