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AD9241AS データシート(PDF) 11 Page - Analog Devices |
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AD9241AS データシート(HTML) 11 Page - Analog Devices |
11 / 24 page AD9241 REV. 0 –11– REFERENCE OPERATION The AD9241 contains an onboard bandgap reference that pro- vides a pin-strappable option to generate either a 1 V or 2.5 V output. With the addition of two external resistors, the user can generate reference voltages other than 1 V and 2.5 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 AD9241 reference configurations. Figure 26 shows a simplified model of the internal voltage refer- ence of the AD9241. A pin-strappable reference amplifier buff- ers 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 A/D. This input span equals, Full-Scale Input Span = 2 × VREF A2 5k Ω 5k Ω 5k Ω 5k Ω LOGIC DISABLE A2 7.5k Ω LOGIC A1 5k Ω DISABLE A1 1V TO A/D AD9241 CAPT CAPB VREF SENSE REFCOM Figure 26. Equivalent Reference Circuit The voltage appearing at the VREF pin, and the state of the internal reference amplifier, A1, are determined by the voltage 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 REFCOM, the switch is connected to the internal resistor network thus providing a VREF of 2.5 V. If the SENSE pin is tied to the VREF pin via a short or resistor, the switch is connected to the SENSE pin. A short will provide a VREF of 1.0 V while an external resistor network will provide an alternative VREF between 1.0 V and 2.5 V. The second comparator controls internal circuitry that will disable the reference amplifier if the SENSE pin is tied to AVDD. Disabling the reference amplifier allows the VREF pin to be driven by an external voltage reference. The actual reference voltages used by the internal circuitry of the AD9241 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 27 shows the recommended decoupling network. This capacitive network performs the following three functions: (1) in conjunction with the reference amplifier, A2, it provides a low source impedance over a large frequency range to drive the A/D internal circuitry, (2) it provides the necessary compensation for A2, and (3) it bandlimits the noise contribution from the refer- ence. The turn-on time of the reference voltage appearing be- tween CAPT and CAPB is approximately 15 ms and should be evaluated in any power-down mode of operation. 0.1µF 10µF 0.1µF 0.1µF CAPT CAPB AD9241 Figure 27. 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 and VREF to REFCOM, and the capacitive decoupling network removed. The external voltages applied to CAPT and CAPB must be 2.5 V + Input Span/4 and 2.5 V – Input Span/4, respec- tively where the input span can be varied between 2 V and 5 V. Note that those samples within the pipeline A/D during any reference transition will be corrupted and should be discarded. Table II. Reference Configuration Summary Reference Input Span (VINA–VINB) Operating Mode (V p-p) Required VREF (V) Connect To INTERNAL 2 1 SENSE VREF INTERNAL 5 2.5 SENSE REFCOM INTERNAL 2 ≤ SPAN ≤ 5 AND 1 ≤ VREF ≤ 2.5 AND R1 VREF AND SENSE SPAN = 2 × VREF VREF = (1 + R1/R2) R2 SENSE AND REFCOM EXTERNAL 2 ≤ SPAN ≤ 51 ≤ VREF ≤ 2.5 SENSE AVDD (NONDYNAMIC) VREF EXT. REF. EXTERNAL 2 ≤ SPAN ≤ 5 CAPT and CAPB SENSE AVDD (DYNAMIC) Externally Driven VREF REFCOM EXT. REF. 1 CAPT EXT. REF. 2 CAPB |
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同様の説明 - AD9241AS |
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