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DAC7744EB Datasheet(PDF) 21 Page - Texas Instruments |
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DAC7744EB Datasheet(HTML) 21 Page - Texas Instruments |
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21 / 29 page  21 ® DAC7744 DIGITAL INTERFACE Table I shows the basic control logic for the DAC7744. Note that each DAC register is edge triggered and not level triggered. When the LOADDACS signal is transitioned to HIGH, the digital word currently in the DAC register is latched. The first set of registers (the input registers) are triggered via the A0, A1, R/W, and CS inputs. Only one of these registers is transparent at any given time. The double-buffered architecture is designed mainly so that each DAC input register can be written to at any time and then all DAC voltages updated simultaneously by the rising edge of LOADDACS. It also allows a DAC input register to be written to at any point then the DAC output voltages can be synchronously changed via a trigger signal connected to LOADDACS. DIGITAL TIMING Figure 11 and Table II provide detailed timing for the digital interface of the DAC7744. DIGITAL INPUT CODING The DAC7744 input data is in Straight Binary format. The output voltage is given by Equation 1. where N is the digital input code. This equation does not include the effects of offset (zero scale) or gain (full scale) errors. INPUT DAC A1 A0 R/W CS RST RSTSEL LOADDACS REGISTER REGISTER MODE DAC LLLL X X X Write Hold Write Input A L H L L X X X Write Hold Write Input B H L L L X X X Write Hold Write Input C H H L L X X X Write Hold Write Input D L L H L X X X Read Hold Read Input A L H H L X X X Read Hold Read Input B H L H L X X X Read Hold Read Input C H H H L X X X Read Hold Read Input D XXX H X X ↑ Hold Write Update All X X X H X X H Hold Hold Hold All XXXX ↑ L X Reset to Zero Reset to Zero All XXXX ↑ H X Reset to Midscale Reset to Midscale All TABLE I. DAC7744 Logic Truth Table. DIGITALLY-PROGRAMMABLE CURRENT SOURCE The DAC7744 offers a unique set of features that allows a wide range of flexibility in designing applications circuits such as programmable current sources. The DAC7744 offers both a differential reference input as well as an open-loop configuration around the output amplifier. The open-loop configuration around the output amplifier allows transistor to be placed within the loop to implement a digitally- programmable, uni-directional current source. The availabil- ity of a differential reference also allows programmability for both the full-scale and zero-scale currents. The output current is calculated as: Figure 12 shows a DAC7744 in a 4-to-20mA current output configuration. The output current can be determined by Equation 3: At full scale, the output current is 16mA plus the 4mA for the zero current. At zero scale, the output current is the offset current of 4mA (1V/250 Ω). (1) VV L VH V L N OUT REF REF REF =+ () –• , 65 536 (3) I VV N V OUT = + 51 250 65 536 1 250 – • , ΩΩ (2) I VH V L R N VL R OUT REF REF SENSE REF SENSE = + () – • , / 65 536 |
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