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전자부품 데이터시트 검색엔진 |
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전자부품 데이터시트 검색엔진 |
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AD9224ARS 데이터시트(PDF) 10 Page - Analog Devices |
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AD9224ARS 데이터시트(HTML) 10 Page - Analog Devices |
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10 / 24 page  AD9224 –10– REV. A REFERENCE OPERATION The AD9224 contains an onboard bandgap 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 AD9224 refer- ence configurations. Figure 16 shows a simplified model of the internal voltage reference of the AD9224. A pin strappable reference ampli- fier buffers a 1 V fixed reference. The output from the refer- ence amplifier, A1, appears on the VREF pin. The voltage on the VREF pin determines the full-scale input span of the A/D. 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 voltage appearing at the SENSE pin. The logic circuitry con- tains two comparators which 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 provid- ing 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 resis- tor 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 6.25k DISABLE A1 1V TO A/D AD9224 CAPT CAPB VREF SENSE REFCOM Figure 16. Equivalent Reference Circuit The actual reference voltages used by the internal circuitry of the AD9224 appear 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 17 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 A/D 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 AD9224 Figure 17. Recommended CAPT/CAPB Decoupling Network The A/D’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 reference 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 decou- pling network. The external 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 A/D dur- ing any reference transition will be corrupted and should be discarded. |
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