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AD9754ARU Datenblatt(PDF) 11 Page - Analog Devices |
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AD9754ARU Datenblatt(HTML) 11 Page - Analog Devices |
11 / 24 page AD9754 –11– REV. A input impedance of REFIO is approximately 1 M Ω, a simple, low cost R-2R ladder DAC configured in the voltage mode topology may be used to control the gain. This circuit is shown in Figure 19 using the AD7524 and an external 1.2 V reference, the AD1580. The second method may be used in a dual-supply system in which the common-mode voltage of REFIO is fixed, and IREF is varied by an external voltage, VGC, applied to RSET via an ampli- fier. An example of this method is shown in Figure 25 in which the internal reference is used to set the common-mode voltage of the control amplifier to 1.20 V. The external voltage, VGC, is referenced to ACOM and should not exceed 1.2 V. The value of RSET is such that IREFMAX and IREFMIN do not exceed 62.5 µA and 625 µA, respectively. The associated equations in Figure 20 can be used to determine the value of RSET. 150pF +1.2V REF AVDD REFLO CURRENT SOURCE ARRAY AVDD REFIO FS ADJ RSET AD9754 IREF VGC 1 F IREF = (1.2 – VGC)/RSET WITH VGC VREFIO AND 62.5 A IREF 625A Figure 20. Dual-Supply Gain Control Circuit ANALOG OUTPUTS The AD9754 produces two complementary current outputs, IOUTA and IOUTB, which may be configured for single-end or differential operation. IOUTA and IOUTB can be converted into complementary single-ended voltage outputs, VOUTA and VOUTB, via a load resistor, RLOAD, as described in the DAC Transfer Function section by Equations 5 through 8. The differential voltage, VDIFF, existing between VOUTA and VOUTB can also be converted to a single-ended voltage via a transformer or differential amplifier configuration. Figure 21 shows the equivalent analog output circuit of the AD9754 consisting of a parallel combination of PMOS differen- tial current switches associated with each segmented current source. The output impedance of IOUTA and IOUTB is deter- mined by the equivalent parallel combination of the PMOS switches and is typically 100 k Ω in parallel with 5 pF. Due to the nature of a PMOS device, the output impedance is also slightly dependent on the output voltage (i.e., VOUTA and VOUTB) and, to a lesser extent, the analog supply voltage, AVDD, and full-scale current, IOUTFS. Although the output impedance’s signal dependency can be a source of dc nonlinearity and ac linearity (i.e., distortion), its effects can be limited if certain precautions are noted. AD9754 AVDD IOUTA IOUTB RLOAD RLOAD Figure 21. Equivalent Analog Output Circuit IOUTA and IOUTB also have a negative and positive voltage compliance range. The negative output compliance range of –1.0 V is set by the breakdown limits of the CMOS process. Operation beyond this maximum limit may result in a break- down of the output stage and affect the reliability of the AD9754. The positive output compliance range is slightly dependent on the full-scale output current, IOUTFS. It degrades slightly from its nominal 1.25 V for an IOUTFS = 20 mA to 1.00 V for an IOUTFS = 2 mA. Operation beyond the positive compliance range will induce clipping of the output signal which severely degrades the AD9754’s linearity and distortion performance. For applications requiring the optimum dc linearity, IOUTA and/or IOUTB should be maintained at a virtual ground via an I-V op amp configuration. Maintaining IOUTA and/or IOUTB at a virtual ground keeps the output impedance of the AD9754 fixed, significantly reducing its effect on linearity. However, it does not necessarily lead to the optimum distortion perfor- mance due to limitations of the I-V op amp. Note that the INL/DNL specifications for the AD9754 are measured in this manner using IOUTA. In addition, these dc linearity specifications remain virtually unaffected over the specified power supply range of +4.5 V to +5.5 V. Operating the AD9754 with reduced voltage output swings at IOUTA and IOUTB in a differential or single-ended output configuration reduces the signal dependency of its output impedance thus enhancing distortion performance. Although the voltage compliance range of IOUTA and IOUTB extends from –1.0 V to +1.25 V, optimum distortion performance is achieved when the maximum full-scale signal at IOUTA and IOUTB does not exceed approximately 0.5 V. A properly se- lected transformer with a grounded center-tap will allow the AD9754 to provide the required power and voltage levels to different loads while maintaining reduced voltage swings at IOUTA and IOUTB. DC-coupled applications requiring a differential or single-ended output configuration should size RLOAD accordingly. Refer to Applying the AD9754 section for examples of various output configurations. |
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