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AD9764 Arkusz danych(PDF) 9 Page - Analog Devices |
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AD9764 Arkusz danych(HTML) 9 Page - Analog Devices |
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9 / 22 page  REV. B AD9764 –9– FUNCTIONAL DESCRIPTION Figure 21 shows a simplified block diagram of the AD9764. The AD9764 consists of a large PMOS current source array that is capable of providing up to 20 mA of total current. The array is divided into 31 equal currents that make up the five most significant bits (MSBs). The next four bits or middle bits consist of 15 equal current sources whose value is 1/16th of an MSB current source. The remaining LSBs are binary weighted frac- tions of the middle bits current sources. Implementing the middle and lower bits with current sources, instead of an R-2R ladder, enhances its dynamic performance for multitone or low amplitude signals and helps maintain the DAC’s high output impedance (i.e., >100 k Ω). All of these current sources are switched to one or the other of the two output nodes (i.e., IOUTA or IOUTB) via PMOS differen- tial current switches. The switches are based on a new architec- ture that drastically improves distortion performance. This new switch architecture reduces various timing errors and provides matching complementary drive signals to the inputs of the dif- ferential current switches. The analog and digital sections of the AD9764 have separate power supply inputs (i.e., AVDD and DVDD) that can operate independently over a 2.7 volt to 5.5 volt range. The digital section, which is capable of operating up to a 125 MSPS clock rate, consists of edge-triggered latches and segment decoding logic circuitry. The analog section includes the PMOS current sources, the associated differential switches, a 1.20 V bandgap voltage reference and a reference control amplifier. The full-scale output current is regulated by the reference con- trol amplifier and can be set from 2 mA to 20 mA via an exter- nal resistor, RSET. The external resistor, in combination with both the reference control amplifier and voltage reference VREFIO, sets the reference current IREF, which is mirrored over to the segmented current sources with the proper scaling factor. The full-scale current, IOUTFS, is 32 times the value of IREF. DAC TRANSFER FUNCTION The AD9764 provides complementary current outputs, IOUTA and IOUTB. IOUTA will provide a near full-scale current output, IOUTFS, when all bits are high (i.e., DAC CODE = 16383) while IOUTB, the complementary output, provides no current. The current output appearing at IOUTA and IOUTB is a function of both the input code and IOUTFS and can be expressed as: IOUTA = (DAC CODE/16384) × I OUTFS (1) IOUTB = (16383 – DAC CODE)/16384 × I OUTFS (2) where DAC CODE = 0 to 16383 (i.e., Decimal Representation). As mentioned previously, IOUTFS is a function of the reference current IREF, which is nominally set by a reference voltage VREFIO and external resistor RSET. It can be expressed as: IOUTFS = 32 × I REF (3) where IREF = VREFIO/RSET (4) The two current outputs will typically drive a resistive load directly or via a transformer. If dc coupling is required, IOUTA and IOUTB should be directly connected to matching resistive loads, RLOAD, that are tied to analog common, ACOM. Note that RLOAD may represent the equivalent load resistance seen by IOUTA or IOUTB as would be the case in a doubly terminated 50 Ω or 75 Ω cable. The single-ended voltage output appearing at the IOUTA and IOUTB nodes is simply: VOUTA = IOUTA × R LOAD (5) VOUTB = IOUTB × R LOAD (6) Note that the full-scale value of VOUTA and VOUTB should not exceed the specified output compliance range to maintain speci- fied distortion and linearity performance. The differential voltage, VDIFF, appearing across IOUTA and IOUTB is: VDIFF = (IOUTA – IOUTB) × R LOAD (7) Substituting the values of IOUTA, IOUTB and IREF; VDIFF can be expressed as: VDIFF = {(2 DAC CODE – 16383)/16384} × VDIFF = {(32 RLOAD/RSET) × V REFIO (8) These last two equations highlight some of the advantages of operating the AD9764 differentially. First, the differential op- eration will help cancel common-mode error sources associated with IOUTA and IOUTB such as noise, distortion and dc offsets. Second, the differential code-dependent current and subsequent voltage, VDIFF, is twice the value of the single-ended voltage output (i.e., VOUTA or VOUTB), thus providing twice the signal power to the load. Note that the gain drift temperature performance for a single- ended (VOUTA and VOUTB) or differential output (VDIFF) of the AD9764 can be enhanced by selecting temperature tracking resistors for RLOAD and RSET due to their ratiometric relation- ship as shown in Equation 8. DIGITAL DATA INPUTS (DB13–DB0) 50pF COMP1 +1.20V REF AVDD ACOM REFLO COMP2 PMOS CURRENT SOURCE ARRAY 0.1 F +5V SEGMENTED SWITCHES FOR DB13–DB5 LSB SWITCHES REFIO FS ADJ DVDD DCOM CLOCK +5V RSET 2k 0.1 F IOUTA IOUTB 0.1 F AD9764 SLEEP IREF VREFIO CLOCK IOUTB IOUTA RLOAD 50 VOUTB VOUTA RLOAD 50 VDIFF = VOUTA – VOUTB LATCHES Figure 21. Functional Block Diagram |
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