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OPA2134UAE4 Datasheet(PDF) 9 Page - Burr-Brown (TI) |
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OPA2134UAE4 Datasheet(HTML) 9 Page - Burr-Brown (TI) |
9 / 14 page 9 OPA134/2134/4134 ® V OUT V IN R 1 If R S > 2kΩ or R1 II R2 > 2kΩ R S = R1 II R2 R 2 OPA134 configuration alters the feedback factor or noise gain of the circuit. The closed-loop gain is unchanged, but the feedback available for error correction is reduced by a factor of 101, thus extending the resolution by 101. Note that the input signal and load applied to the op amp are the same as with conventional feedback without R3. The value of R3 should be kept small to minimize its effect on the distortion mea- surements. Validity of this technique can be verified by duplicating measurements at high gain and/or high frequency where the distortion is within the measurement capability of the test equipment. Measurements for this data sheet were made with an Audio Precision distortion/noise analyzer which greatly simplifies such repetitive measurements. The mea- surement technique can, however, be performed with manual distortion measurement instruments. SOURCE IMPEDANCE AND DISTORTION For lowest distortion with a source or feedback network which has an impedance greater than 2k Ω, the impedance seen by the positive and negative inputs in noninverting applications should be matched. The p-channel JFETs in the FET input stage exhibit a varying input capacitance with applied common-mode input voltage. In inverting configu- rations the input does not vary with input voltage since the inverting input is held at virtual ground. However, in noninverting applications the inputs do vary, and the gate- to-source voltage is not constant. The effect is increased distortion due to the varying capacitance for unmatched source impedances greater than 2k Ω. To maintain low distortion, match unbalanced source im- pedance with appropriate values in the feedback network as shown in Figure 3. Of course, the unbalanced impedance may be from gain-setting resistors in the feedback path. If the parallel combination of R1 and R2 is greater than 2kΩ, a matching impedance on the noninverting input should be used. As always, resistor values should be minimized to reduce the effects of thermal noise. FIGURE 3. Impedance Matching for Maintaining Low Distortion in Non-Inverting Circuits. NOISE PERFORMANCE Circuit noise is determined by the thermal noise of external resistors and op amp noise. Op amp noise is described by two parameters—noise voltage and noise current. The total noise is quantified by the equation: With low source impedance, the current noise term is insignificant and voltage noise dominates the noise perfor- mance. At high source impedance, the current noise term becomes the dominant contributor. Low noise bipolar op amps such as the OPA27 and OPA37 provide very low voltage noise at the expense of a higher current noise. However, OPA134 series op amps are unique in providing very low voltage noise and very low current noise. This provides optimum noise performance over a wide range of sources, including reactive source imped- ances, refer to the typical curve, “Voltage Noise vs Source Resistance.” Above 2k Ω source resistance, the op amp contributes little additional noise—the voltage and current terms in the total noise equation become insignificant and the source resistance term dominates. Below 2k Ω, op amp voltage noise dominates over the resistor noise, but com- pares favorably with other audio op amps such as OP176. PHASE REVERSAL PROTECTION OPA134 series op amps are free from output phase-reversal problems. Many audio op amps, such as OP176, exhibit phase-reversal of the output when the input common-mode voltage range is exceeded. This can occur in voltage-fol- lower circuits, causing serious problems in control loop applications. OPA134 series op amps are free from this undesirable behavior even with inputs of 10V beyond the input common-mode range. POWER DISSIPATION OPA134 series op amps are capable of driving 600 Ω loads with power supply voltage up to ±18V. Internal power dissipation is increased when operating at high supply voltages. Copper leadframe construction used in OPA134 series op amps improves heat dissipation compared to con- ventional materials. Circuit board layout can also help minimize junction temperature rise. Wide copper traces help dissipate the heat by acting as an additional heat sink. Temperature rise can be further minimized by soldering the devices to the circuit board rather than using a socket. OUTPUT CURRENT LIMIT Output current is limited by internal circuitry to approxi- mately ±40mA at 25°C. The limit current decreases with increasing temperature as shown in the typical performance curve “Short-Circuit Current vs Temperature.” V total i R e kTR nn S n s () ( ) =+ + 2 2 4 |
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