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ADA4939-2YCPZ-R7 Datasheet(PDF) 19 Page - Analog Devices |
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ADA4939-2YCPZ-R7 Datasheet(HTML) 19 Page - Analog Devices |
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19 / 24 page  ADA4939-1/ADA4939-2 Rev. 0 | Page 19 of 24 Mismatched feedback networks also result in a degradation of the ability of the circuit to reject input common-mode signals, much the same as for a four-resistor difference amplifier made from a conventional op amp. As a practical summarization of the above issues, resistors of 1% tolerance produce a worst-case input CMRR of approximately 40 dB, a worst-case differential-mode output offset of 25 mV due to a 2.5 V VOCM input, negligible VOCM noise contribution, and no significant degradation in output balance error. CALCULATING THE INPUT IMPEDANCE FOR AN APPLICATION CIRCUIT The effective input impedance of a circuit depends on whether the amplifier is being driven by a single-ended or differential signal source. For balanced differential input signals, as shown in Figure 44, the input impedance (RIN, dm) between the inputs (+DIN and −DIN) is simply RIN, dm = 2 × RG. +VS ADA4939 +IN –IN RF RF +DIN –DIN VOCM RG RG VOUT, dm Figure 44. ADA4939 Configured for Balanced (Differential) Inputs For an unbalanced, single-ended input signal (see Figure 45), the input impedance is () ⎟ ⎟ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎜ ⎜ ⎝ ⎛ + × − = F G F G SE IN R R R R R 2 1 , ADA4939 RL VOUT, dm +VS –VS RG RG RF RF VOCM RIN, SE Figure 45. ADA4939 with Unbalanced (Single-Ended) Input The input impedance of the circuit is effectively higher than it would be for a conventional op amp connected as an inverter because a fraction of the differential output voltage appears at the inputs as a common-mode signal, partially bootstrapping the voltage across the input resistor RG. The common-mode voltage at the amplifier input terminals can be easily determined by noting that the voltage at the inverting input is equal to the noninverting output voltage divided down by the voltage divider formed by RF and RG in the lower loop. This voltage is present at both input terminals due to negative voltage feedback and is in phase with the input signal, thus reducing the effective voltage across RG in the upper loop and partially bootstrapping RG. Terminating a Single-Ended Input This section deals with how to properly terminate a single- ended input to the ADA4939 with a gain of 2, RF = 400 Ω, and RG = 200 Ω. An example using an input source with a terminated output voltage of 1 V p-p and source resistance of 50 Ω illustrates the four simple steps that must be followed. Note that because the terminated output voltage of the source is 1 V p-p, the open circuit output voltage of the source is 2 V p-p. The source shown in Figure 46 indicates this open-circuit voltage. 1. The input impedance must be calculated using the formula Ω 300 ) 400 200 ( 2 400 1 200 ) ( 2 1 = ⎟ ⎟ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎜ ⎜ ⎝ ⎛ + × − = ⎟ ⎟ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎜ ⎜ ⎝ ⎛ + × − = F G F G IN R R R R R RS 50Ω VS 2V p-p RIN 300Ω ADA4939 RL VOUT, dm +VS –VS RG 200Ω RG 200Ω RF 400Ω RF 400Ω VOCM Figure 46. Calculating Single-Ended Input Impedance RIN |
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