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AD9225ARS Arkusz danych(PDF) 11 Page - Analog Devices |
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AD9225ARS Arkusz danych(HTML) 11 Page - Analog Devices |
11 / 26 page –10– AD9225 REFERENCE OPERATION The AD9225 contains an on-board band gap reference that provides a pin strappable option to generate either a 1 V or 2 V output. With the addition of two external resistors, the user can generate reference voltages other than 1 V and 2 V. Another alternative is to use an external reference for designs requiring enhanced accuracy and/or drift performance. See Table II for a summary of the pin strapping options for the AD9225 refer- ence configurations. Figure 5 shows a simplified model of the internal voltage reference of the AD9225. A pin strappable reference amplifier buffers a 1 V fixed reference. The output from the reference amplifier, A1, appears on the VREF pin. The voltage on the VREF pin determines the full-scale input span of the ADC. This input span equals Full-Scale Input Span = 2 ¥ VREF The voltage appearing at the VREF pin as well as the state of the internal reference amplifier, A1, are determined by the volt- age appearing at the SENSE pin. The logic circuitry contains two comparators that monitor the voltage at the SENSE pin. The comparator with the lowest set point (approximately 0.3 V) controls the position of the switch within the feedback path of A1. If the SENSE pin is tied to AVSS (AGND), the switch is connected to the internal resistor network thus providing a VREF of 2.0 V. If the SENSE pin is tied to the VREF pin via a short or resistor, the switch will connect to the SENSE pin. This short will provide a VREF of 1.0 V. An external resistor network will provide an alternative VREF between 1.0 V and 2.0 V. The other comparator controls internal circuitry that will disable the reference amplifier if the SENSE pin is tied AVDD. Disabling the reference amplifier allows the VREF pin to be driven by an external voltage reference. A2 5k 5k 5k 5k LOGIC DISABLE A2 6.25k LOGIC A1 DISABLE A1 1V TO A/D AD9225 CAPT CAPB VREF SENSE REFCOM 6.25k Figure 5. Equivalent Reference Circuit The actual reference voltages used by the internal circuitry of the AD9225 appears on the CAPT and CAPB pins. For proper operation when using the internal or an external reference, it is necessary to add a capacitor network to decouple these pins. Figure 6 shows the recommended decoupling network. This capacitive network performs the following three functions: (1) along with the reference amplifier, A2, it provides a low source impedance over a large frequency range to drive the ADC inter- nal circuitry, (2) it provides the necessary compensation for A2, and (3) it bandlimits the noise contribution from the reference. The turn-on time of the reference voltage appearing between CAPT and CAPB is approximately 15 ms and should be evalu- ated in any power-down mode of operation. 0.1 F 10 F 0.1 F 0.1 F CAPT CAPB AD9225 Figure 6. Recommended CAPT/CAPB Decoupling Network The ADC’s input span may be varied dynamically by changing the differential reference voltage appearing across CAPT and CAPB symmetrically around 2.5 V (i.e., midsupply). To change the refer- ence at speeds beyond the capabilities of A2, it will be necessary to drive CAPT and CAPB with two high speed, low noise amplifiers. In this case, both internal amplifiers (i.e., A1 and A2) must be disabled by connecting SENSE to AVDD, connecting VREF to AVSS and removing the capacitive decoupling network. The exter- nal voltages applied to CAPT and CAPB must be 2.0 V + Input Span/4 and 2.0 V – Input Span/4, respectively, in which the input span can be varied between 2 V and 4 V. Note that those samples within the pipeline ADC during any reference transition will be corrupted and should be discarded. DRIVING THE ANALOG INPUTS The AD9225 has a highly flexible input structure allowing it to interface with single ended or differential input interface cir- cuitry. The applications shown in this section and the Reference Configurations section along with the information presented in the Input and Reference Overview give examples of single- ended and differential operation. Refer to Tables I and II for a list of the different possible input and reference configurations and their associated figures in the data sheet. The optimum mode of operation, analog input range, and asso- ciated interface circuitry will be determined by the particular applications performance requirements as well as power supply options. For example, a dc-coupled single-ended input would be appropriate for most data acquisition and imaging applica- tions. Many communication applications, which require a dc-coupled input for proper demodulation, can take advantage of the excellent single-ended distortion performance of the AD9225. The input span should be configured so the system’s performance objectives and the headroom requirements of the driving op amp are simultaneously met. Rev. C |
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