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AD7834AR Datasheet(PDF) 10 Page - Analog Devices |
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AD7834AR Datasheet(HTML) 10 Page - Analog Devices |
10 / 16 page REV. A –10– AD7834/AD7835 Table IV. Code Table for Unipolar Operation Binary Number in DAC Latch Analog Output MSB LSB (VOUT) 11 1111 1111 1111 VREF (16383/16384) V 10 0000 0000 0000 VREF (8192/16384) V 01 1111 1111 1111 VREF (8191/16384) V 00 0000 0000 0001 VREF (1/16384) V 00 0000 0000 0000 0 V NOTE VREF = VREF(+); VREF(–) = 0 V for unipolar operation. For VREF(+) = +5 V, 1 LSB = +5 V/2 14 = +5 V/16384 = 305 µV. Bipolar Configuration Figure 14 shows the AD7834/AD7835 set up for ±5 V opera- tion. The AD588 provides precision ±5 V tracking outputs which are fed to the VREF(+) and VREF(–) inputs of the AD7834/ AD7835. The code table for bipolar operation of the AD7834/ AD7835 is shown in Table V. +15V +5V VOUT (–5 TO +5V) VCC 6 3 4 C1 1µF AGND DGND VDD VOUT VREF(+) VREF(–) VSS –15V *ADDITIONAL PINS OMITTED FOR CLARITY R2 100k Ω AD7834/ AD7835 * SIGNAL GND 2 14 15 16 12 8 13 11 10 5 9 7 R3 100k Ω R1 39k Ω 1 AD588 Figure 14. Bipolar ±5 V Operation Table V. Code Table for Bipolar Operation Binary Number in DAC Latch Analog Output MSB LSB (VOUT) 11 1111 1111 1111 VREF(–) + VREF (16383/16384) V 10 0000 0000 0001 VREF(–) + VREF (8193/16384) V 10 0000 0000 0000 VREF(–) + VREF (8192/16384) V 01 1111 1111 1111 VREF(–) + VREF (8191/16384) V 00 0000 0000 0001 VREF(–) + VREF (1/16384) V 00 0000 0000 0000 VREF(–) V NOTE VREF = (VREF(+) – VREF(–)). For VREF(+) = +5 V, and VREF(–) = –5 V, 1 LSB = 10 V/2 14 = 10 V/16384 = 610 µV. In Figure 14, full-scale and bipolar zero adjustments are pro- vided by varying the gain and balance on the AD588. R2 varies the gain on the AD588 while R3 adjusts the offset of both the +5 V and –5 V outputs together with respect to ground. For bipolar-zero adjustment, the DAC is loaded with 1000 . . . 0000 and R3 is adjusted until VOUT = 0 V. Full scale is adjusted by loading the DAC with all 1s and adjusting R2 un- til VOUT = 5(8191/8192) V = 4.99939 V. When bipolar-zero and full-scale adjustment are not needed, R2 and R3 can be omitted. Pin 12 on the AD588 should be con- nected to Pin 11 and Pin 5 should be left floating. When 14-bit transfers are being used, the DAC output voltages, VOUT1–VOUT4, can be updated to reflect new data in the DAC input registers in one of two ways. The first method normally keeps LDAC high and only pulses LDAC low momentarily to update all DAC latches simultaneously with the contents of their respective input registers. The second method ties LDAC low and channel updating occurs on a per channel basis after new data is loaded to an input register. In order to avoid the DAC output going to an intermediate value during a 2-byte transfer, LDAC should not be tied low permanently, but should be held high until the 2 bytes are writ- ten to the input register. When the selected input register has been loaded with the 2 bytes, LDAC should then be pulsed low to update the DAC latch and, hence, perform the digital-to- analog conversion. In many applications, it may be acceptable to allow the DAC output to go to an intermediate value during a 2-byte transfer. In such applications, LDAC can be tied low, thus using one less control line. The actual DAC input register that is being written to is deter- mined by the logic levels present on the devices address lines, as shown in Table III. Table III. AD7835—Address Line Truth Table A2 A1 A0 DAC Selected 0 0 0 DAC 1 0 0 1 DAC 2 0 1 0 DAC 3 0 1 1 DAC 4 1 X X All DACs Selected Unipolar Configuration Figure 13 shows the AD7834/AD7835 in the unipolar binary circuit configuration. The VREF(+) input of the DAC is driven by the AD586, a +5 V reference. VREF(–) is tied to ground. Table IV gives the code table for unipolar operation of the AD7834/AD7835. +15V +5V VOUT (0 TO +5V) VCC 2 6 8 5 4 SIGNAL GND C1 1nF AGND DGND VDD VOUT VREF(+) VREF(–) VSS –15V *ADDITIONAL PINS OMITTED FOR CLARITY R1 10k Ω AD7834/ AD7835 * AD586 SIGNAL GND Figure 13. Unipolar +5 V Operation Offset and gain may be adjusted in Figure 13 as follows: To ad- just offset, disconnect the VREF(–) input from 0 V, load the DAC with all 0s and adjust the VREF(–) voltage until VOUT = 0 V. For gain adjustment, the AD7834/AD7835 should be loaded with all 1s and R1 adjusted until VOUT = 5 V(16383/16384) = 4.999695. Many circuits will not require these offset and gain adjustments. In these circuits R1 can be omitted. Pin 5 of the AD586 may be left open circuit and Pin 2 (VREF(–)) of the AD7834/AD7835 tied to 0 V. |
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