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OPA3691 Datasheet(PDF) 20 Page - Texas Instruments |
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OPA3691 Datasheet(HTML) 20 Page - Texas Instruments |
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20 / 33 page  OPA2674 SBOS270A − AUGUST 2003 − REVISED MAY 2006 www.ti.com 20 inductance can have a major effect on circuit performance. A SPICE model for the OPA2674 is available through the TI web site (www.ti.com). This model does a good job of predicting small-signal AC and transient performance under a wide variety of operating conditions, but does not do as well in predicting the harmonic distortion or dG/dP characteristics. This model does not attempt to distinguish between the package types in small-signal AC performance, nor does it attempt to simulate channel-to- channel coupling. OPERATING SUGGESTIONS SETTING RESISTOR VALUES TO OPTIMIZE BANDWIDTH A current-feedback op amp such as the OPA2674 can hold an almost constant bandwidth over signal gain settings with the proper adjustment of the external resistor values, which are shown in the Typical Characteristics; the small-signal bandwidth decreases only slightly with increasing gain. These characteristic curves also show that the feedback resistor is changed for each gain setting. The resistor values on the inverting side of the circuit for a current-feedback op amp can be treated as frequency response compensation elements, whereas the ratios set the signal gain. Figure 10 shows the small-signal frequency response analysis circuit for the OPA2674. V O R G V I R I Z (S) IERR α R F I ERR Figure 10. Current-Feedback Transfer Function Analysis Circuit The key elements of this current-feedback op amp model are: α = buffer gain from the noninverting input to the inverting input RI = buffer output impedance IERR = feedback error current signal Z(s) = frequency dependent open-loop transimpe- dance gain from IERR to VO NG + NoiseGain + 1 ) R F R G The buffer gain is typically very close to 1.00 and is normal- ly neglected from signal gain considerations. This gain, however, sets the CMRR for a single op amp differential amplifier configuration. For a buffer gain of α < 1.0, the CMRR = −20 • log(1 − α)dB. RI, the buffer output impedance, is a critical portion of the bandwidth control equation. The OPA2674 inverting output impedance is typically 22 Ω. A current-feedback op amp senses an error current in the inverting node (as opposed to a differential input error voltage for a voltage-feedback op amp) and passes this on to the output through an internal frequency dependent transimpedance gain. The Typical Characteristics show this open-loop transimpedance response, which is analogous to the open-loop voltage gain curve for a voltage-feedback op amp. Developing the transfer function for the circuit of Figure 10 gives Equation 14: V O V I + a 1 ) R F R G 1 ) R F) R I 1 ) R F R G Z(s) + a NG 1 ) R F) R I NG Z(s) This is written in a loop-gain analysis format, where the errors arising from a non-infinite open-loop gain are shown in the denominator. If Z(s) were infinite over all frequen- cies, the denominator of Equation 14 reduces to 1 and the ideal desired signal gain shown in the numerator is achieved. The fraction in the denominator of Equation 14 determines the frequency response. Equation 15 shows this as the loop-gain equation: Z(s) R F ) R I NG + LoopGain If 20 log(RF + NG × RI) is drawn on top of the open-loop transimpedance plot, the difference between the two would be the loop gain at a given frequency. Eventually, Z(s) rolls off to equal the denominator of Equation 15, at which point the loop gain has reduced to 1 (and the curves have intersected). This point of equality is where the amplifier closed-loop frequency response given by Equation 14 starts to roll off, and is exactly analogous to the frequency at which the noise gain equals the open-loop voltage gain for a voltage-feedback op amp. The difference here is that the total impedance in the denominator of Equation 15 may be controlled somewhat separately from the desired signal gain (or NG). The OPA2674 is internally compensated to give a maximally flat frequency response for RF = 402Ω at NG = 4 on ±6V (14) (15) |
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