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AD7837SQ Datasheet(PDF) 9 Page - Analog Devices |
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AD7837SQ Datasheet(HTML) 9 Page - Analog Devices |
9 / 12 page AD7837/AD7847 REV. C –9– APPLICATIONS PROGRAMMABLE GAIN AMPLIFIER (PGA) The dual DAC/amplifier combination along with access to RFB make the AD7837 ideal as a programmable gain amplifier. In this application, the DAC functions as a programmable resistor in the amplifier feedback loop. This type of configuration is shown in Figure 17 and is suitable for ac gain control. The circuit con- sists of two PGAs in series. Use of a dual configuration provides greater accuracy over a wider dynamic range than a single PGA solution. The overall system gain is the product of the individual gain stages. The effective gains for each stage are controlled by the DAC codes. As the code decreases, the effective DAC resistance increases, and so the gain also increases. DAC B AGNDA VOUTB VREFB VIN RFBB AD7837 VOUT RFBA AGNDB DAC A VOUTA VREFA Figure 17. Dual PGA Circuit The transfer function is given by VOUT VIN = REQA RFBA × REQB RFBB (1) where REQA, REQB are the effective DAC resistances controlled by the digital input code: REQ = 212 RIN N (2) where RIN is the DAC input resistance and is equal to RFB and N = DAC input code in decimal. The transfer function in (1) thus simplifies to VOUT VIN = 212 N A × 212 NB (3) where NA = DAC A input code in decimal and NB = DAC B input code in decimal. NA, NB may be programmed between 1 and (2 12–1). The zero code is not allowed as it results in an open loop amplifier response. To minimize errors, the digital codes NA and NB should be chosen to be equal to or as close as possible to each other to achieve the required gain. BIPOLAR OPERATION (4-QUADRANT MULTIPLICATION) Figure 16 shows the AD7837/AD7847 connected for bipolar operation. The coding is offset binary as shown in Table IV. When VIN is an ac signal, the circuit performs 4-quadrant multi- plication. To maintain the gain error specifications, resistors R1, R2 and R3 should be ratio matched to 0.01%. Note that on the AD7847 the feedback resistor RFB is internally connected to VOUT. DAC A AGNDA VOUTA VREFA VIN DGND VSS RFBA * VDD VDD AD7837 AD7847 *INTERNALLY CONNECTED ON AD7847 R3 10k R1 20k AD711 R2 20k VOUT VSS Figure 16. Bipolar Offset Binary Operation Table IV. Bipolar Code Table DAC Latch Contents MSB LSB Analog Output, VOUT 1111 1111 1111 +V IN × 2047 2048 1000 0000 0001 +V IN × 1 2048 1000 0000 0000 0 V 0111 1111 1111 –VIN × 1 2048 0000 0000 0000 –VIN × 2048 2048 = –V IN Note 1 LSB = V IN 2048 . |
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