Datasheet OP285 (Analog Devices) - 9

OP285 +15V 10 F + R3 2k 0.1 F 2 R9 V 1 50 O1 10pF 3 A2 R11 4.99k V 1k IN R1 4.99k R7 2k 2k 2 – R4 8 VIN 3 2k V O2 – VO1 = VIN 1/2 1 VOUT 1 OP285 P1 3 2 A1 + 10k 4 2k R5 2.49k 2k R6 0.1 F R2 2k 2k R12 6 1k R10 V 10 F 7 50 O2 + 5 A3 R8 2k –15V A1 = 1/2OP285
Figure 9. Unity-Gain Inverter
A2, A3 = 1/2 OP285 GAIN = SET R2, R4, R5 = R1 AND R, R7, R8 = R2
In inverting and noninverting applications, the feedback resis- tance forms a pole with the source resistance and capacitance Figure 11. High-Speed, Low-Noise Differential Line Driver (RS and CS) and the OP285’s input capacitance (CIN), as shown in Figure 10. With RS and RF in the kilohm range, this
Low Phase Error Amplifier
pole can create excess phase shift and even oscillation. A small The simple amplifier configuration of Figure 12 uses the OP285 capacitor, CFB, in parallel with RFB eliminates this problem. By and resistors to reduce phase error substantially over a wide setting RS (CS + CIN) = RFBCFB, the effect of the feedback pole frequency range when compared to conventional amplifier designs. is completely removed. This technique relies on the matched frequency characteristics of the two amplifiers in the OP285. Each amplifier in the circuit
CFB
has the same feedback network which produces a circuit gain of 10. Since the two amplifiers are set to the same gain and are
RFB
matched due to the monolithic construction of the OP285, they will exhibit identical frequency response. Recall from feedback theory that a pole of a feedback network becomes a zero in the
VOUT
loop gain response. By using this technique, the dominant pole
RS CS CIN
of the amplifier in the feedback loop compensates for the domi- nant pole of the main amplifier,
R2
Figure 10. Compensating the Feedback Pole
4.99k R1 549 2 1 High-Speed, Low-Noise Differential Line Driver 3 A1 R5
The circuit of Figure 11 is a unique line driver widely used in
549
industrial applications. With ± 18 V supplies, the line driver can
R4
deliver a differential signal of 30 V p-p into a 2.5 kΩ load. The
6 4.99 7
high slew rate and wide bandwidth of the OP285 combine to
A2 5 VOUT V
yield a full power bandwidth of 130 kHz while the low noise
IN R3 A1, A2 = 1/2 OP285
front end produces a referred-to-input noise voltage spectral
499
density of 10 nV/√Hz. The design is a transformerless, balanced Figure 12. Cancellation of A2’s Dominant Pole by A1 transmission system where output common-mode rejection of noise is of paramount importance. Like the transformer-based design, either output can be shorted to ground for unbalanced line driver applications without changing the circuit gain of 1. Other circuit gains can be set according to the equation in the diagram. This allows the design to be easily set to noninverting, inverting, or differential operation. REV. C –9–