OP285 Datasheet, PDF (9/16 Pages) Analog Devices – Dual 9 MHz Precision Operational Amplifier

+15V
10␮F
+
0.1␮F
10pF
4.99k⍀
V_IN
4.99k⍀
2– ₈
1/2
1
V_OUT
3
OP285
+
4
2k⍀
2.49k⍀
0.1␮F
10␮F
+
–15V
Figure 9. Unity-Gain Inverter
In inverting and noninverting applications, the feedback resis-
tance forms a pole with the source resistance and capacitance
(R_S and C_S) and the OP285’s input capacitance (C_IN), as
shown in Figure 10. With R_S and R_F in the kilohm range, this
pole can create excess phase shift and even oscillation. A small
capacitor, C_FB, in parallel with R_FB eliminates this problem. By
setting R_S (C_S + C_IN) = R_FBC_FB, the effect of the feedback pole
is completely removed.
C_FB
R_FB
R_S
C_S
C_IN
V_OUT
Figure 10. Compensating the Feedback Pole
High-Speed, Low-Noise Differential Line Driver
The circuit of Figure 11 is a unique line driver widely used in
industrial applications. With ± 18 V supplies, the line driver can
deliver a differential signal of 30 V p-p into a 2.5 kΩ load. The
high slew rate and wide bandwidth of the OP285 combine to
yield a full power bandwidth of 130 kHz while the low noise
front end produces a referred-to-input noise voltage spectral
density of 10 nV/√Hz. The design is a transformerless, balanced
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.
OP285
R3
2k⍀
2
1
3 ^A2
R9
50⍀
R1
2k⍀
V_IN 3
1
2 ^A1
R2
2k⍀
R4
2k⍀
R7
2k⍀
R5
2k⍀ _R6
2k⍀
6
5 ^A3
R10
7
50⍀
R8
2k⍀
V_O1
R11
1k⍀
P1
10k⍀
V_O2 – V_O1 = V_IN
R12
1k⍀
V_O2
A1 = 1/2OP285
A2, A3 = 1/2 OP285
GAIN = SET R2, R4, R5 = R1 AND R, R7, R8 = R2
Figure 11. High-Speed, Low-Noise Differential Line Driver
Low Phase Error Amplifier
The simple amplifier configuration of Figure 12 uses the OP285
and resistors to reduce phase error substantially over a wide
frequency range when compared to conventional amplifier designs.
This technique relies on the matched frequency characteristics
of the two amplifiers in the OP285. Each amplifier in the circuit
has the same feedback network which produces a circuit gain of
10. Since the two amplifiers are set to the same gain and are
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
loop gain response. By using this technique, the dominant pole
of the amplifier in the feedback loop compensates for the domi-
nant pole of the main amplifier,
R1
549⍀
R2
4.99k⍀
2
1
3 ^A1
R5
549⍀
R4
6
7 ^4.99⍀
V_IN
5 ^A2
V_OUT
R3
A1, A2 = 1/2 OP285
499⍀
Figure 12. Cancellation of A2’s Dominant Pole by A1
REV. A
–9–