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AD8801 Arkusz danych(PDF) 5 Page - Analog Devices |
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AD8801 Arkusz danych(HTML) 5 Page - Analog Devices |
5 / 16 page AD8801/AD8803 REV. A –5– For example, when VREFH = +5 V and VREFL = 0 V the follow- ing output voltages will be generated for the following codes: DVOX Output State (VREFH = +5 V, VREFL = 0 V) 255 4.98 V Full-Scale 128 2.50 V Half-Scale (Midscale Reset Value) 1 0.02 V 1 LSB 0 0.00 V Zero-Scale REFERENCE INPUTS (VREFH, VREFL) The reference input pins set the output voltage range of all eight DACs. In the case of the AD8801 only the VREFH pin is avail- able to establish a user designed full-scale output voltage. The external reference voltage can be any value between 0 and VDD but must not exceed the VDD supply voltage. In the case of the AD8803, which has access to the VREFL which establishes the zero-scale output voltage, any voltage can be applied between 0 V and VDD. VREFL can be smaller or larger in voltage than VREFH since the DAC design uses fully bidirectional switches as shown in Figure 3. The input resistance to the DAC has a code dependent variation that has a nominal worst case measured at 55H, which is approximately 2 kΩ. When VREFH is greater than VREFL, the REFL reference must be able to sink current out of the DAC ladder, while the REFH reference is sourcing current into the DAC ladder. The DAC design minimizes reference glitch current maintaining minimum interference between DAC channels during code changes. DAC OUTPUTS (O1–O8) The eight DAC outputs present a constant output resistance of approximately 5 k Ω independent of code setting. The distribu- tion of ROUT from DAC to DAC typically matches within ±1%. However, device to device matching is process lot dependent having a ±20% variation. The change in R OUT with temperature has a 500 ppm/ °C temperature coefficient. During power shut- down all eight outputs are open circuited. DAC REG #1 EN ADDR DEC DAC DAC REG #8 D10 D9 D8 D7 SER REG D D0 .. . .. . .. . DAC 1 AD8801/AD8803 D7 D0 DAC 8 D7 D0 8 R R VDD VREFH O1 O2 O3 O4 O5 O6 O7 O8 CS CLK SDI SHDN GND RS VREFL . .. .. . . .. (AD8801 ONLY) (AD8803 ONLY) Figure 4. Block Diagram DIGITAL INTERFACING The AD8801/AD8803 contains a standard three-wire serial in- put control interface. The three inputs are clock (CLK), CS and serial data input (SDI). The positive-edge sensitive CLK input requires clean transitions to avoid clocking incorrect data into the serial input register. Standard logic families work well. If mechanical switches are used for product evaluation, they should be debounced by a flip-flop or other suitable means. Fig- ure 4 block diagram shows more detail of the internal digital cir- cuitry. When CS is taken active low, the clock can load data into the serial register on each positive clock edge, see Table II. Table II. Input Logic Control Truth Table CS CLK Register Activity 1 X No effect. 0 P Shifts Serial Register one bit loading the next bit in from the SDI pin. P X Data is transferred from the serial register to the decoded DAC register. See Figure 5. NOTE: P = positive edge, X = don’t care. The data setup and data hold times in the specification table determine the data valid time requirements. The last 11 bits of the data word entered into the serial register are held when CS returns high. At the same time CS goes high it gates the address decoder which enables one of the eight positive edge triggered DAC registers, see Figure 5 detail. ... DAC 1 DAC 2 DAC 8 ADDR DECODE SERIAL REGISTER CS CLK SDI Figure 5. Equivalent Control Logic The target DAC register is loaded with the last eight bits of the se- rial data word completing one DAC update. Eight separate 11-bit data words must be clocked in to change all eight output settings. All digital inputs are protected with a series input resistor and parallel Zener ESD structure shown in Figure 6. This applies to digital input pins CS, SDI, RS, SHDN, CLK. LOGIC 100 Ω Figure 6. Equivalent ESD Protection Circuit Digital inputs can be driven by voltages exceeding the AD8801/ AD8803 VDD value. This allows 5 V logic to interface directly to the part when it is operated at 3 V. |
Podobny numer części - AD8801 |
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Podobny opis - AD8801 |
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