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DAC1006 Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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DAC1006 Datasheet(HTML) 9 Page - National Semiconductor (TI) |
9 / 22 page shown in Figure 4 where the sign of the reference voltage has been changed to provide a positive output voltage Note that the output current IOUT1 now flows through the RFB pin 512 Providing a Bipolar Output Voltage with the DAC in the Current Switching Mode The addition of a second op amp to the circuit of Figure 4 can be used to generate a bipolar output voltage from a fixed reference voltage Figure 5 This in effect gives sign significance to the MSB of the digital input word to allow two quadrant multiplication of the reference voltage The polarity of the reference can also be reversed to realize the full four- quadrant multiplication The applied digital word is offset binary which includes a code to output zero volts without the need of a large valued resistor common to existing bipolar multiplying DAC circuits Offset binary code can be derived from 2’s complement data (most common for signed processor arithmetic) by in- verting the state of the MSB in either software or hardware After doing this the output then responds in accordance to the following expression VOeVREFc D 512 where VREF can be positive or negative and D is the signed decimal equivalent of the 2’s complement processor data (b512sDsa511 or 1000000000sDs0111111111) If the applied digital input is interpreted as the decimal equivalent of a true binary word VOUT can be found by VOeVREF Db512 512 J 0sDs1023 With this configuration only the offset voltage of amplifier 1 need be nulled to preserve linearity of the DAC The offset voltage error of the second op amp has no effect on lineari- ty It presents a constant output voltage error and should be nulled only if absolute accuracy is needed Another advan- tage of this configuration is that the values of the external resistors required do not have to match the value of the internal DAC resistors they need only to match and temper- ature track each other A thin film 4 resistor network available from Beckman Instru- ments Inc (part no 694-3-R10K-D) is ideally suited for this application Two of the four available 10 kX resistor can be paralleled to form R in Figure 5 and the other two can be used separately as the resistors labeled 2R Operation is summarized in the table below Applied 2’s Comp 2’s Comp Applied True Binary VOUT (Decimal) (Binary) Digital Input (Decimal) a VREF b VREF a 511 0111111111 1111111111 1023 VREFb1 LSB b lVREFla1 LSB a 256 0100000000 1100000000 768 VREF 2 b lVREFl 2 0 0000000000 1000000000 512 0 0 b 1 1111111111 0111111111 511 b 1 LSB a 1 LSB b 256 1100000000 0100000000 256 b VREF 2 a lVREFl 2 b 512 1000000000 0000000000 0 b VREF a lVREFl with 1 LSBel VREF l 512 FIGURE 4 Providing a Unipolar Output Voltage TLH5688 – 11 FIGURE 5 Providing a Bipolar Output Voltage with the DAC in the Current Switching Mode 9 |
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