OP275 Datasheet, PDF (8/12 Pages) Analog Devices – Dual Bipolar/JFET, Audio Operational Amplifier

OP275
Noise Testing
For audio applications the noise density is usually the most im-
portant noise parameter. For characterization the OP275 is
tested using an Audio Precision, System One. The input signal
to the Audio Precision must be amplified enough to measure it
accurately. For the OP275 the noise is gained by approximately
1020 using the circuit shown in Figure 7. Any readings on the
Audio Precision must then be divided by the gain. In imple-
menting this test fixture, good supply bypassing is essential.
100Ω
909Ω
this effect from occurring in noninverting applications. For these
applications, the fix is a simple one and is illustrated in Figure 9.
A 3.92 kΩ resistor in series with the noninverting input of the
OP275 cures the problem.
R_FB*
V_IN
R_S
3.92kΩ
*R_FB IS OPTIONAL
V_OUT
R_L
2kΩ
A
OP275
B
100Ω
909Ω
OP37
OP37
909Ω
100Ω
4.42kΩ
490Ω
OUTPUT
Figure 7. Noise Test Fixture
Input Overcurrent Protection
The maximum input differential voltage that can be applied to
the OP275 is determined by a pair of internal Zener diodes con-
nected across its inputs. They limit the maximum differential in-
put voltage to ± 7.5 V. This is to prevent emitter-base junction
breakdown from occurring in the input stage of the OP275
when very large differential voltages are applied. However, in or-
der to preserve the OP275’s low input noise voltage, internal re-
sistances in series with the inputs were not used to limit the
current in the clamp diodes. In small signal applications, this is
not an issue; however, in applications where large differential
voltages can be inadvertently applied to the device, large tran-
sient currents can flow through these diodes. Although these di-
odes have been designed to carry a current of ± 5 mA, external
resistors as shown in Figure 8 should be used in the event that
the OP275’s differential voltage were to exceed ± 7.5 V.
1.4kΩ ₂
–
6
OP275
1.4kΩ ₃
+
Figure 8. Input Overcurrent Protection
Output Voltage Phase Reversal
Since the OP275’s input stage combines bipolar transistors for
low noise and p-channel JFETs for high speed performance, the
output voltage of the OP275 may exhibit phase reversal if either
of its inputs exceed its negative common-mode input voltage.
This might occur in very severe industrial applications where a
sensor, or system, fault might apply very large voltages on the
inputs of the OP275. Even though the input voltage range of the
OP275 is ± 10.5 V, an input voltage of approximately –13.5 V
will cause output voltage phase reversal. In inverting amplifier
configurations, the OP275’s internal 7.5 V input clamping di-
odes will prevent phase reversal; however, they will not prevent
Figure 9. Output Voltage Phase Reversal Fix
Overload, or Overdrive, Recovery
Overload, or overdrive, recovery time of an operational amplifier
is the time required for the output voltage to recover to a rated
output voltage from a saturated condition. This recovery time is
important in applications where the amplifier must recover
quickly after a large abnormal transient event. The circuit
shown in Figure 10 was used to evaluate the OP275’s overload
recovery time. The OP275 takes approximately 1.2 µs to recover
to V_OUT = +10 V and approximately 1.5 µs to recover to V_OUT =
–10 V.
R1
R2
1kΩ
10kΩ
V_IN
4V p-p
@100Hz
2
1
3 ^A1
V_OUT
R_S
909Ω
R_L
2.43kΩ
A1 = 1/2 OP275
Figure 10.␣ Overload Recovery Time Test Circuit
Measuring Settling Time
The design of OP275 combines high slew rate and wide gain-
bandwidth product to produce a fast-settling (t_S < 1 µs) ampli-
fier for 8- and 12-bit applications. The test circuit designed to
measure the settling time of the OP275 is shown in Figure 11.
This test method has advantages over false-sum node tech-
niques in that the actual output of the amplifier is measured, in-
stead of an error voltage at the sum node. Common-mode
settling effects are exercised in this circuit in addition to the slew
rate and bandwidth effects measured by the false-sum-node
method. Of course, a reasonably flat-top pulse is required as the
stimulus.
The output waveform of the OP275 under test is clamped by
Schottky diodes and buffered by the JFET source follower. The
signal is amplified by a factor of ten by the OP260 and then
Schottky-clamped at the output to prevent overloading the
oscilloscope’s input amplifier. The OP41 is configured as a fast
integrator which provides overall dc offset nulling.
High Speed Operation
As with most high speed amplifiers, care should be taken with
supply decoupling, lead dress, and component placement. Rec-
ommended circuit configurations for inverting and noninverting
applications are shown in Figures 12 and Figure 13.
–8–
REV. A