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LTC1590I Datasheet(PDF) 8 Page - Linear Technology |
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LTC1590I Datasheet(HTML) 8 Page - Linear Technology |
8 / 12 page 8 LTC1590 Unipolar 2-Quadrant Multiplying Mode (VOUT = 0V to –VREF) The LTC1590 can be used with a dual op amp to provide a dual 2-quadrant multiplying DAC as shown in Figure 2. The unipolar DAC transfer function is shown in Table 1. The 33pF feedback capacitor is recommended to compen- sate for the pole caused by the internal feedback resistor and the OUT1 output capacitance. For high speed op amps this feedback capacitor is required for stability, and a smaller value, 8pF to 15pF, may be desired to get the fastest transient response and shortest settling time. A larger feedback capacitor can be used to reduce wideband noise, glitch impulse and distortion for lower frequency signals. A pole is introduced in the DAC transfer function at approximately (CFB)(RFB). For example, a 100pF feed- back capacitor will typically give a pole at: 145 1 2100 11 kHz pF k = ()( ) πΩ APPLICATIONS INFORMATION Figure 3. Bipolar Operation (4-Quadrant Multiplication) –15V 5V 0.1 µF VCC VREF LTC1590 DAC A OR DAC B RFB DGND AGND VREF –10V TO 10V OUT1 OUT2 VOUT –VREF TO VREF 1590 F03 15V 1/2 LT1112 R3 20k R1 10k R2 20k 15V –15V 1/2 LT1112 33pF Table 1. Unipolar Binary Code Table DIGITAL INPUT BINARY NUMBER ANALOG OUTPUT IN DAC REGISTER VOUT MSB LSB 1111 1111 1111 – VREF (4095/4096) 1000 0000 0000 – VREF (2048/4096) = – VREF/2 0000 0000 0001 – VREF (1/4096) 0000 0000 0000 0V Bipolar 4-Quadrant Multiplying Mode (VOUT = –VREF to +VREF) The circuit of Figure 3 can be used to provide a dual 4-quadrant multiplying DAC. This circuit starts with the unipolar application circuit and adds three resistors and an op amp. These extra devices provide a gain of – 2 from the unipolar output to the bipolar output, plus an offset of (–1)(VREF) to produce the transfer function shown in Table 2. A pack of matched 20k resistors, with two resistors in parallel forming the 10k resistor, is recommended. Table 2. Bipolar Offset Binary Code Table DIGITAL INPUT BINARY NUMBER ANALOG OUTPUT IN DAC REGISTER VOUT MSB LSB 1111 1111 1111 +VREF (2047/2048) 1000 0000 0001 +VREF (1/2048) 1000 0000 0000 0V 0111 1111 1111 – VREF (1/2048) 0000 0000 0000 – VREF (2048/2048) = – VREF VCC VREF LTC1590 DAC A OR DAC B RFB DGND AGND 5V VREF –10V TO 10V OUT2 OUT1 33pF VOUT 0V TO –VREF 1590 F02 0.1 µF 1/2 LT1112 15V –15V Figure 2. Unipolar Operation (2-Quadrant Multiplication) |
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