Motore di ricerca datesheet componenti elettronici |
|
AD9220AR Scheda tecnica(PDF) 11 Page - Analog Devices |
|
AD9220AR Scheda tecnica(HTML) 11 Page - Analog Devices |
11 / 32 page REV. E AD9221/AD9223/AD9220 –11– Referring to Figure 5, the differential SHA is implemented using a switched-capacitor topology. Therefore, its input impedance and its subsequent effects on the input drive source should be understood to maximize the converter’s performance. The com- bination of the pin capacitance, CPIN, parasitic capacitance, CPAR, and 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, 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 there- fore 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 allowed to fully charge up to the input voltage before switches QS1 are opened, then the average current into the input is the same as if there were a resistor of 1/(CS fS) ohms connected between the inputs. This means that the input impedance is inversely pro- portional to the converter’s sample rate. Since CS is only 4 pF, this resistive component is typically much larger than that of the drive source (i.e., 25 k Ω at f S = 10 MSPS). If one considers the SHA’s input impedance over a sampling period, it appears as a dynamic input impedance to the input drive source. When the SHA goes into the track mode, the input source should ideally 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 7. The series resistance helps isolate the op amp from the switched-capacitor load. 10 F VINA VINB SENSE AD9221/AD9223/ AD9220 0.1 F RS VCC VEE RS VREF REFCOM Figure 7. 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 AD9221/AD9223/AD9220 sampling rate, the selected op amp, and the particular application. In most applica- tions, a 30 Ω to 50 Ω resistor is sufficient. However, some applications may require 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 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 of 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. Figure 8 shows a plot for THD performance versus RSERIES for the AD9221/AD9223/AD9220 at their respective sampling rate and Nyquist frequency. The Nyquist frequency typically repre- sents the worst case scenario for an ADC. In this case, a high speed, high performance amplifier (AD8047) was used as the buffer op amp. Although not shown, the AD9221/AD9223/AD9220 exhibits a slight increase in SNR (i.e. 1 dB to 1.5 dB) as the resistance is increased from 0 k Ω to 2.56 kΩ due to its bandlimiting effect on wideband noise. Conversely, it exhibits slight decrease in SNR (i.e., 0.5 dB to 2 dB) if VINA and VINB do not have a matched input resistance. RSERIES – –45 –55 –85 1 10k 10 100 1k –65 –75 AD9220 AD9223 AD9221 Figure 8. THD vs. RSERIES (fIN = fS / 2, AIN = –0.5 dB, Input Span = 2 V p-p, VCM = 2.5 V) Figure 8 shows that a small RSERIES between 30 Ω and 50 Ω provides the optimum THD performance for the AD9220. Lower values of RSERIES are acceptable for the AD9223 and AD9221 as their lower sampling rates provide a longer transient recovery period for the AD8047. Note that op amps with lower bandwidths will typically have a longer transient recovery period and therefore require a slightly higher value of RSERIES and/or lower sampling rate to achieve the optimum THD performance. As the value of RSERIES increases, a corresponding increase in distortion is noted. This is due to its interaction with the SHA’s parasitic capacitor, CPAR, which has a signal dependency. Thus, the resulting R-C time constant is signal dependent and conse- quently a source of distortion. The noise or small-signal bandwidth of the AD9221/AD9223/ AD9220 is the same as their full-power bandwidth as shown in Figure 2. For noise sensitive applications, the excessive bandwidth may be detrimental and the addition of a series resistor and/or |
Codice articolo simile - AD9220AR |
|
Descrizione simile - AD9220AR |
|
|
Link URL |
Privacy Policy |
ALLDATASHEETIT.COM |
Lei ha avuto il aiuto da alldatasheet? [ DONATE ] |
Di alldatasheet | Richest di pubblicita | contatti | Privacy Policy | scambio Link | Ricerca produttore 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 |