Electronic Components Datasheet Search 

AD7982 Datasheet(PDF) 17 Page  Analog Devices 

AD7982 Datasheet(HTML) 17 Page  Analog Devices 
17 / 24 page AD8475 Rev. B  Page 17 of 24 THEORY OF OPERATION OVERVIEW The AD8475 is a fully differential amplifier, with integrated laser trimmed resistors, that provides precision attenuating gains of 0.4 and 0.8. The internal differential amplifier of the AD8475 differs from conventional operational amplifiers in that it has two outputs whose voltages are equal in magnitude, but move in opposite directions (180° out of phase). An additional input, VOCM, sets the output commonmode voltage. Like an opera tional amplifier, it relies on high openloop gain and negative feedback to force the output nodes to the desired voltages. The AD8475 is designed to greatly simplify singleendedtodifferential conversion, commonmode level shifting and precision attenua tion of large signals so that they are compatible with low voltage, differential input ADCs. 1kΩ 1kΩ 1.25kΩ 1.25kΩ AD8475 –IN 0.8x –IN 0.4x +VS VOCM +OUT +IN 0.8x +IN 0.4x –VS NC –OUT 1.25kΩ 1.25kΩ Figure 49. Block Diagram CIRCUIT INFORMATION The AD8475 amplifier uses a voltage feedback topology; therefore, the amplifier exhibits a nominally constant gain bandwidth product. Like a voltage feedback operational amplifier, the AD8475 also has high input impedance at its internal input terminals (the summing nodes of the internal amplifier) and low output impedance. The AD8475 employs two feedback loops, one each to control the differential and commonmode output voltages. The differen tial feedback loop, which is fixed with precision laser trimmed onchip resistors, controls the differential output voltage. Output CommonMode Voltage (VOCM) The internal commonmode feedback controls the common mode output voltage. This architecture makes it easy to set the output commonmode level to any arbitrary value independent of the input voltage. The output commonmode voltage is forced by the internal commonmode feedback loop to be equal to the voltage applied to the VOCM input. The VOCM pin can be left unconnected, and the output commonmode voltage selfbiases to midsupply by the internal feedback control. Due to the internal commonmode feedback loop and the fully differential topology of the amplifier, the AD8475 outputs are precisely balanced over a wide frequency range. This means that the amplifier’s differential outputs are very close to the ideal of being identical in amplitude and exactly 180° out of phase. DC PRECISION The dc precision of the AD8475 is highly dependent on the accuracy of its internal resistors. Using superposition to analyze the circuit shown in Figure 50, the following equation shows the relationship between the input and output voltages of the amplifier: () () () () N P dm OUT N P cm OUT N P N P dm IN N P cm IN R R V R R V R R R R V R R V + + + − = + + + − 2 2 1 2 2 1 , , , , where, RGP RFP R P = , RGN RFN R N = N P dm IN V V V − = , ) ( 2 1 , N P cm IN V V V + = The differential closed loop gain of the amplifier is N P N P N P dm IN dm OUT R R R R R R V V + + + + = 2 2 , , and the common rejection of the amplifier is ( ) N P N P cm IN dm OUT R R R R V V + + − = 2 2 , , RFP RFN RGP RGN VON VOP VOCM VP VN Figure 50. Functional Circuit Diagram of the AD8475 at a Given Gain The preceding equations show that the gain accuracy and the commonmode rejection (CMRR) of the AD8475 are deter mined primarily by the matching of the feedback networks (resistor ratios). If the two networks are perfectly matched, that is, if RP and RN equal RF/RG, then the resistor network does not generate any CMRR errors and the differential closed loop gain of the amplifier reduces to RG RF v v dm IN dm OUT = , , The AD8475’s integrated resistors are precision waferlaser trimmed to guarantee a minimum CMRR of 86dB (50μV/V), and gain error of less that 0.05%. To achieve equivalent precision and performance using a discrete solution, resistors must be matched to 0.01% or better. 
