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LMX2531 Datasheet(PDF) 7 Page - Texas Instruments |
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LMX2531 Datasheet(HTML) 7 Page - Texas Instruments |
7 / 14 page 6 SIGNAL PATH designer of a CFB amplifier. For a Voltage Feedback (VFB) amplifier, RB is set equal to the effective impedance seen at the inverting input to cancel errors caused by input-bias current. In the case of high RB values, CB may be required to reduce high-frequency noise due to the amplifier input-noise current flowing through RB. In the inverting-amplifier configuration, the ampli- fier inputs are held at a fixed virtual ground while in the non-inverting configuration, the inputs see the full input-signal swing. Consequently, the inverting-amplifier-input stage sees a much smaller voltage on its inputs which reduces any distortion introduced by the input stage. In order to obtain the full performance, a differen- tial-input ADC must be driven differentially. Figure 6a shows an integrated amplifier with differential outputs and differential inputs. Capable of either AC- or DC-coupled operation, the amplifier gain is set by four external resistors per the equation given in the diagram. The amplifier can also be driven by a single-ended source, as in a single-to-differential amplifier, with the unused amplifier input grounded as shown in Figure 6b. Resistors RF and RG should be well matched and strict symmetry should be observed in PC- board layout. This ensures optimum output balance, necessary for low distortion, and good Common Mode Rejection Ratio (CMRR) from this circuit. The output common-mode level is set by the VCM common-mode control, independently of the input common-mode level, which is ideal for level-shifting the amplifier input signal to match the required ADC-input common-mode level. The fully-differential amplifier can be thought of as two forward-amplification channels plus a third feed- back amplifier which senses the output common mode of the two forward channels and servos it to the level of the VCM pin. The VCM common-mode feedback loop forces the two outputs to be equal and opposite, thereby controlling the amplifier- output balance even when only one input channel is driven and the other input is grounded, as in the single-to-differential amplifier application. Setting the output common-mode voltage with the VCM pin also affects the amplifier-input common-mode voltage. It is essential to stay within Figure 6a. Fully differential amplifier VOUT RF RG VIN = - RF RG AV VCM RG RF VO Source B2 = R G + R M RG + RF + R M ) B1 = R G RG + RF ( ) AV = 2(1-B 1) B1 + B 2 ( ) RIN = 2R G + R M(1-B 2) 1 + B2 ( To match circuit input impedance RIN to source impedance RS, set R T//R IN = R S, with R M = RS//R T then AV, RIN RT RF RF RS VIN VCM RM RG RG ( ) Figure 6b. Single-ended input with differential output Selecting Amplifiers, ADCs, and Clocks for High-Performance Signal Paths RB RF RG RS (Optional) CB (Optional) Source ( ) = // + // - VOUT to ADC RF RG AV RG RG RT RS RT VIN RT Setting RS =RT //RG , gives AV = - RF 2 RG Figure 5b. Inverting single-ended amplifier SignalPathDesigner.indd 6 SignalPathDesigner.indd 6 9/5/07 3:24:34 PM 9/5/07 3:24:34 PM |
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