OP249 Datasheet, PDF (11/17 Pages) Analog Devices – Dual, Precision JFET High Speed Operational Amplifier

The OP249 was carefully designed to provide symmetrically
matched slew characteristics in both the negative and positive
directions, even when driving a large output load.
An amplifier’s slewing limitation determines the maximum
frequency at which a sinusoidal output can be obtained without
significant distortion. It is, however, important to note that the
nonsymmetric stewing typical of previously available JFET
amplifiers adds a higher series of harmonic energy content to
the resulting response—and an additional dc output compo-
nent. Examples of potential problems of nonsymmetric
slewing behavior could be in audio amplifier applications,
where a natural low distortion sound quality is desired, and in
servo or signal processing systems where a net dc offset cannot
be tolerated. The linear and symmetric stewing feature of the
OP249 makes it an ideal choice for applications that will exceed
the full-power bandwidth range of the amplifier.
Figure 42. Small-Signal Transient Response, A_V = +1,
Z_L = 2 kΩʈ100 pF, No Compensation, V_S = ±15 V
As with most JFET-input amplifiers, the output of the OP249
may undergo phase inversion if either input exceeds the speci-
fied input voltage range. Phase inversion will not damage the
amplifier, nor will it cause an internal latch-up condition.
Supply decoupling should be used to overcome inductance and
resistance associated with supply lines to the amplifier. A 0.1 µF
and a 10 µF capacitor should be placed between each supply
pin and ground.
OPEN-LOOP GAIN LINEARITY
The OP249 has both an extremely high open-loop gain of
1 kV/mV minimum and constant gain linearity. This feature of
the OP249 enhances its dc precision, and provides superb accu-
racy in high closed-loop gain applications. Figure 43 illustrates
the typical open-loop gain linearity—high gain accuracy is as-
sured, even when driving a 600 Ω load.
OFFSET VOLTAGE ADJUSTMENT
The inherent low offset voltage of the OP249 will make offset
adjustments unnecessary in most applications. However, where
a lower offset error is required, balancing can be performed with
simple external circuitry, as illustrated in Figures 44 and 45.
OP249
VERTICAL 50␮V/DIV
INPUT VARIATION
HORIZONTAL 5V/DIV
OUTPUT CHARGE
Figure 43. Open-Loop Gain Linearity. Variation in Open-
Loop Gain Results in Errors in High Closed-Loop Gain
Circuits. R_L = 600 Ω, V_S = ±15 V
R4
+V
R5
50k⍀
R3
V_IN
R1
200k⍀
R2
31⍀
1/2
OP249
V_OUT
V_OS ADJUST RANGE = ؎V
R2
R1
–V
Figure 44. Offset Adjust for Inverting Amplifier
Configuration
+V
R3
50k⍀
–V
V_IN
R1
200k⍀
R4
R2
33⍀
R5
1/2
OP249
V_OUT
V_OS ADJUST RANGE = ؎V
R2
R1
GAIN =
V_OUT
V_IN
=
1
+
R5
R4 + R2
1 + ^R5 IF R2 << R4
R4
Figure 45. Offset Adjust for Noninverting Amplifier
Configuration
In Figure 44, the offset adjustment is made by supplying a small
voltage at the noninverting input of the amplifier. Resistors R1
and R2 attenuates the pot voltage, providing a ± 2.5 mV (with
V_S = ± 15 V) adjustment range, referred to the input. Figure 45
illustrates offset adjust for the noninverting amplifier configura-
tion, also providing a ± 2.5 mV adjustment range. As indicated
in the equations in Figure 45, if R4 is not much greater than R2,
there will be a resulting closed-loop gain error that must be
accounted for.
REV. C
–11–