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AD830 Datasheet(PDF) 11 Page - Analog Devices |
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AD830 Datasheet(HTML) 11 Page - Analog Devices |
11 / 20 page AD830 Rev. C | Page 11 of 20 THEORY OF OPERATION TRADITIONAL DIFFERENTIAL AMPLIFICATION In the past, when differential amplification was needed to reject common-mode signals superimposed with a desired signal, most often the solution used was the classic op amp based difference amplifier shown in Figure 24. The basic function VO = V1 − V2 is simply achieved, but the overall performance is poor and the circuit possesses many serious problems that make it difficult to realize a robust design with moderate to high levels of performance. V1 VOUT V2 R1 R2 R3 R4 ONLY IF R1 = R2 = R3 = R4 DOES VOUT = V1 – V2 Figure 24. Op Amp Based Difference Amplifier PROBLEMS WITH THE OP AMP BASED APPROACH • Low common-mode rejection ratio (CMRR) • Low impedance inputs • CMRR highly sensitive to the value of source R • Different input impedance for the + and − input • Poor high frequency CMRR • Requires very highly matched resistors, R1 to R4, to achieve high CMRR • Halves the bandwidth of the op amp • High power dissipation in the resistors for large common- mode voltage AD830 FOR DIFFERENTIAL AMPLIFICATION The AD830 amplifier was specifically developed to solve the listed problems with the discrete difference amplifier approach. Its topology, discussed in detail in the Understanding the AD830 Topology section, by design acts as a difference amplifier. The circuit of Figure 25 shows how simply the AD830 is configured to produce the difference of the two signals, V1 and V2, in which the applied differential signal is exactly reproduced at the output relative to a separate output common. Any common- mode voltage present at the input is removed by the AD830. V1 VOUT IY IX V2 A = 1 V I → V I → VOUT = V1 – V2 Figure 25. AD830 as a Difference Amplifier ADVANTAGEOUS PROPERTIES OF THE AD830 • High common-mode rejection ratio (CMRR) • High impedance inputs • Symmetrical dynamic response for +1 and −1 Gain • Low sensitivity to the value of source R • Equal input impedance for the + and − input • Excellent high frequency CMRR • No halving of the bandwidth • Constant power distortion versus common-mode voltage • Highly matched resistors not needed UNDERSTANDING THE AD830 TOPOLOGY The AD830 represents Analog Devices first amplifier product to embody a powerful alternative amplifier topology. Referred to as active feedback, the topology used in the AD830 provides inherent advantages in the handling of differential signals, differing system commons, level shifting, and low distortion, high frequency amplification. In addition, it makes possible the implementation of many functions not realizable with single op amp circuits or superior to op amp based equivalent circuits. With this in mind, it is important to understand the internal structure of the AD830. The topology, reduced to its elemental form, is shown in Figure 26. Nonideal effects, such as nonlinearity, bias currents, and limited full scale, are omitted from this model for simplicity but are discussed later. The key feature of this topology is the use of two, identical voltage-to-current converters, GM, that make up input and feedback signal interfaces. They are labeled with inputs VX and VY, respectively. These voltage-to-current converters possess fully differential inputs, high linearity, high input impedance, and wide voltage range operation. This enables the part to handle large amplitude differential signals; it also provides high common-mode rejection, low distortion, and negligible loading on the source. The label, GM, is meant to convey that the transconductance is a large signal quantity, unlike in the front end of most op amps. The two GM stage current outputs, IX and IY, sum together at a high impedance node, which is characterized by an equivalent resistance and capacitance connected to an ac common. A unity voltage gain stage follows the high impedance node to provide buffering from loads. Relative to either input, the open-loop gain, AOL, is set by the transconductance, GM, working into the resistance, RP; AOL = GM × RP. The unity gain frequency, ω0 dB, for the open- loop gain is established by the transconductance, GM, working into the capacitance, CC; ω0 dB = GM/CC. The open-loop description of the AD830 is shown below for completeness. |
Número de pieza similar - AD830_10 |
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Descripción similar - AD830_10 |
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