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AD7845KR Datasheet(PDF) 8 Page - Analog Devices |
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AD7845KR Datasheet(HTML) 8 Page - Analog Devices |
8 / 12 page AD7845 REV. B –8– APPLI CATIONS CIRCUITS PROGRAMMABLE GAIN AMPLIFIER (PGA) The AD7845 performs a PGA function when connected as in Figure 15. In this configuration, the R-2R ladder is connected in the amplifier feedback loop. RFB is the amplifier input resis- tor. As the code decreases, the R-2R ladder resistance increases and so the gain increases. VOUT = –VIN × R DAC D × 1 R FB , D = 0 to 4095 4096 = –VIN × R DAC D × 1 R DAC = –V IN D , since RFB = RDAC Figure 15. AD7845 Connected as PGA As the programmed gain increases, the error and noise also increase. For this reason, the maximum gain should be limited to 256. Table III shows gain versus code. Note that instead of using RFB as the input resistor, it is also possible to use combinations of the other application resistors, RA, RB and RC. For instance, if RB is used instead of RFB, the gain range for the same codes of Table II now goes from l/2 to 128. Table III. Gain and Error vs. Input Code for Figure 15 Digital Inputs Gain Error (%) 1111 1111 1111 4096/4095 ≈ 10.04 1000 0000 0000 2 0.07 0100 0000 0000 4 0.13 0010 0000 0000 8 0.26 0001 0000 0000 16 0.51 0000 1000 0000 32 1.02 0000 0100 0000 64 2.0 0000 0010 0000 128 4.0 0000 0001 0000 256 8.0 PROGRAMMABLE CURRENT SOURCES The AD7845 is ideal for designing programmable current sources using a minimum of external components. Figures 16 and 17 are examples. The circuit of Figure 16 drives a program- mable current IL into a load referenced to a negative supply. Figure 17 shows the circuit for sinking a programmable current, IL. The same set of circuit equations apply for both diagrams. IL = I3 = I2 + I1 I1 = D ×|V IN | R DAC , D = 0 to 4095 4096 I2 = 1 R1 D ×|V IN | R DAC RFB = D ×|V IN | R1 , since RFB = RDAC IL = D ×|V IN | R1 + D ×|V IN | R DAC = D ×|V IN | R1 × 1 + R1 R DAC Note that by making R1 much smaller than RDAC, the circuit becomes insensitive to both the absolute value of RDAC and its temperature variations. Now, the only resistor determining load current IL is the sense resistor R1. If R1 = 100 Ω, then the programming range is 0 mA to 100 mA, and the resolution is 0.024 mA. Figure 16. Programmable Current Source |
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