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AD5231 Datasheet(PDF) 23 Page - Analog Devices |
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AD5231 Datasheet(HTML) 23 Page - Analog Devices |
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23 / 28 page  Data Sheet AD5231 Rev. D | Page 23 of 28 APPLICATIONS BIPOLAR OPERATION FROM DUAL SUPPLIES The AD5231 can be operated from dual supplies ±2.5 V, which enables control of ground referenced ac signals or bipolar operation. AC signals as high as VDD/VSS can be applied directly across Terminal A to Terminal B with output taken from Terminal W. See Figure 46 for a typical circuit connection. ±2.5V p-p AD5231 VSS GND SDI CLK SS SCLK MOSI GND µC ±1.25V p-p VDD VDD +2.5V –2.5V CS D = MIDSCALE A W B Figure 46. Bipolar Operation from Dual Supplies HIGH VOLTAGE OPERATION The digital potentiometer can be placed directly in the feedback or input path of an op amp for gain control, provided that the voltage across Terminals A–B, Terminals W–A, or Terminals W–B does not exceed |5 V|. When high voltage gain is needed, users should set a fixed gain in an op amp operated at a higher voltage and let the digital potentiometer control the adjustable input. Figure 47 shows a simple implementation. R 2R 5V AD5231 A W B 15V V+ V– VO 0V TO 15V A1 – + CC 2.2pF Figure 47. 15 V Voltage Span Control BIPOLAR PROGRAMMABLE GAIN AMPLIFIER There are several ways to achieve bipolar gain. Figure 48 shows one versatile implementation. Digital potentiometer U1 sets the adjustment range; the wiper voltage VW2 can, therefore, be programmed between Vi and −KVi at a given U2 setting. For linear adjustment, configure A2 as a noninverting amplifier and the transfer function becomes − + × × + = K K D R1 R2 V V 2 I O ) 1 ( 1024 1 (4) where: K is the ratio of RWB/RWA that is set by U1. D is the decimal equivalent of the input code. V+ V– OP2177 AD5231 V+ V– OP2177 AD5231 Vi A W B –KVi A B W VDD VO VSS R1 R2 A U2 A2 U1 CC 2.2pF VDD VSS Figure 48. Bipolar Programmable Gain Amplifier In the simpler (and much more usual) case where K = 1, a pair of matched resistors can replace U1. Equation 4 can be simplified to − × + = 1 1024 2 1 2 D R1 R2 V V I O (5) Table 20 shows the result of adjusting D with A2 configured as a unity gain, a gain of 2, and a gain of 10. The result is a bipolar amplifier with linearly programmable gain and 1024-step resolution. Table 20. Result of Bipolar Gain Amplifier D R1 = ∞, R2 = 0 R1 = R2 R2 = 9 × R1 0 −1 −2 −10 256 −0.5 −1 −5 512 0 0 0 768 0.5 1 5 1023 0.992 1.984 9.92 10-BIT BIPOLAR DAC If the circuit in Figure 48 is changed with the input taken from a voltage reference and A2 configured as a buffer, a 10-bit bipolar DAC can be realized. Compared to the conventional DAC, this circuit offers comparable resolution but not the precision because of the wiper resistance effects. Degradation of the nonlinearity and temperature coefficient is prominent near both ends of the adjustment range. On the other hand, this circuit offers a unique nonvolatile memory feature that in some cases outweighs any shortfall in precision. The output of this circuit is REF O V D V × − = 1 1024 2 2 (6) |
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