OP275OP27516V–20V–++15V1k 0.1 FOUTPUTR(TO SCOPE)+V+LD3D41k 2N4416DUT1/2 OP260AJ–+V–D1D21 F–0.1 FRF2k 10k –+–IC210k 16V–20V+RG 5V222 2N2222A750 1N414815k SCHOTTKY DIODES D1–D4 ARE HEWLETT-PACKARD HP5082-2835 IC1 IS 1/2 OP260AJ–15VIC2 IS PMI OP41EJ Figure 11. OP275’s Settling Time Test Fixture +15VCFB10 F +0.1R FFB–28– 1/2V1RSCSCINOUTV3 OP275OUTV+IN+R4L0.1 F2k 10 F+ Figure 14. Compensating the Feedback Pole Attention to Source Impedances Minimizes Distortion–15V Since the OP275 is a very low distortion amplifier, careful atten- Figure 12. Unity Gain Follower tion should be given to source impedances seen by both inputs. As with many FET-type amplifiers, the p-channel JFETs in the +15V OP275’s input stage exhibit a gate-to-source capacitance that var- 10 ies with the applied input voltage. In an inverting configuration, F+ the inverting input is held at a virtual ground and, as such, does 0.1 F not vary with input voltage. Thus, since the gate-to-source voltage 10pF is constant, there is no distortion due to input capacitance modu- lation. In noninverting applications, however, the gate-to-source VIN4.99k 4.99k voltage is not constant. The resulting capacitance modulation 28– can cause distortion above 1 kHz if the input impedance is 1/21V greater than 2 k and unbalanced. OP275OUT3 +42k RRGF2.49k 10 F0.1 F+–OP275VR–15VOUTS*VIN+ Figure 13. Unity Gain Inverter * RS = RG//RF IF RG//RF > 2k FOR MINIMUM DISTORTION In inverting and noninverting applications, the feedback resis- tance forms a pole with the source resistance and capacitance Figure 15. Balanced Input Impedance to Minimize (RS and CS) and the OP275’s input capacitance (CIN), as shown Distortion in Noninverting Amplifier Circuits in Figure 14. With RS and RF in the kilohm range, this pole Figure 15 shows some guidelines for maximizing the distortion can create excess phase shift and even oscillation. A small performance of the OP275 in noninverting applications. The best capacitor, CFB, in parallel and RFB eliminates this problem. way to prevent unwanted distortion is to ensure that the parallel By setting RS (CS + CIN) = RFBCFB, the effect of the feedback combination of the feedback and gain setting resistors (R pole is completely removed. F and RG) is less than 2 k. Keeping the values of these resistors small has the added benefits of reducing the thermal noise of the circuit –8– REV. C REV. C –9– Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION PIN CONNECTIONS SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS ORDERING GUIDE Typical Performance Characteristics APPLICATIONS Circuit Protection Total Harmonic Distortion Noise Noise Testing Input Overcurrent Protection Output Voltage Phase Reversal Overload or Overdrive Recovery Measuring Settling Time Driving Capacitive Loads High Speed, Low Noise Differential Line Driver A 3-Pole, 40 kHz Low-Pass Filter OP275 SPICE Model OUTLINE DIMENSIONS Revision History
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