OPA2695 Datasheet, PDF (30/40 Pages) Texas Instruments – Dual, Ultra-Wideband, Current-Feedback OPERATIONAL AMPLIFIER with Disable

OPA2695
SBOS354 – APRIL 2008..................................................................................................................................................................................................... www.ti.com
voltage is higher than the 1.8nV/√Hz specification for
the op amp voltage noise alone. This increased value
reflects the noise added to the output by the inverting
current noise times the feedback resistor. If the
feedback resistor is reduced in high-gain
configurations (as suggested previously), the total
input-referred voltage noise given by Equation 9
approaches the 1.8nV/√Hz of the op amp itself. For
example, going to a gain of +20 (using R_F = 200Ω)
gives a total input-referred noise of 2.0nV/√Hz.
For a more complete discussion of op amp noise
calculation, see TI Application Note, SBOA066, Noise
Analysis for High Speed Op Amps, available through
the TI web site at www.ti.com.
DC ACCURACY AND OFFSET CONTROL
A current-feedback op amp such as the OPA2695
provides exceptional bandwidth in high gains, giving
fast pulse settling but only moderate dc accuracy.
The typical specifications show an input offset voltage
comparable to high-speed voltage-feedback
amplifiers; however, the two input bias currents are
somewhat higher and are unmatched. Although bias
current cancellation techniques are very effective with
most voltage-feedback op amps, they do not
generally reduce the output dc offset for wideband
current-feedback op amps. Because the two input
bias currents are unrelated in both magnitude and
polarity, matching the source impedance looking out
of each input to reduce the respective error
contribution to the output is ineffective. Evaluating the
configuration of Figure 80, using a worst-case +25°C
input offset voltage and the two input bias currents,
gives a worst-case output offset range equal to:
±(NG × V_OS) + (I_BN ×R_S/2 × NG) ± (I_BI × R_F)
where NG = noninverting signal gain.
= ±(8 × 3.5mV) ± (30µA × 25Ω × 8) ± (402Ω ×
60µA)
= ±28mV ± 8mV ± 24mV
= ±60mV
A fine-scale output offset null, or dc operating point
adjustment, is often required. Numerous techniques
are available for introducing dc offset control into an
op amp circuit. Most simple adjustment techniques do
not correct for temperature drift.
POWER SHUTDOWN OPERATION
(QFN-16 Package Only)
The OPA2695IRGT provides an optional power
shutdown feature that can be used to reduce system
power. If the V_DIS control pin is left unconnected, the
OPA2695IRGT operates normally. This shutdown is
intended only as a power-saving feature. Forward
path isolation is very good for small signals. Large
signal isolation is not ensured. Using this feature to
multiplex two or more outputs together is not
recommended. Large signals applied to the shutdown
output stages can turn on parasitic devices,
degrading signal linearity for the desired channel.
Turn-on time is very quick from the shutdown
condition, typically less than 60ns. Turn-off time
strongly depends on the external circuit configuration,
but is typically 200ns for the circuit of Figure 68.
To shut down, the control pin must be asserted low.
This logic control is referenced to the positive supply,
as shown in the simplified circuit of Figure 79.
+V_S
8kW
Q1
V_DIS
17kW
I_S
Control
120kW
-V_S
Figure 79. Simplified Shutdown Circuit
In normal operation, base current to Q1 is provided
through the 120kΩ resistor, while the emitter current
through the 8kΩ resistor sets up a voltage drop that is
inadequate to turn on the two diodes in the Q1
emitter. As V_DIS is pulled low, additional current is
pulled through the 8kΩ resistor, eventually turning on
these two diodes (≈ 180µA). At this point, any further
current pulled out of V_DIS goes through those diodes
holding the emitter-base voltage of Q1 at
approximately 0V. This limitation shuts off the
collector current out of Q1, turning the amplifier off.
The supply current in the shutdown mode is only that
required to operate the circuit of Figure 79.
When disabled, the output and input nodes go to a
high-impedance state. If the OPA2695IRGT is
operating in a gain of +1V/V, this configuration shows
a very high impedance (3pF || 1MΩ) at the output and
exceptional signal isolation. If operating at a gain
greater than +1V/V, the total feedback network
resistance (R_F + R_G) appears as the impedance
looking back into the output, but the circuit continues
to show very high forward and reverse isolation. If
configured as an inverting amplifier, the input and
output are connected through the feedback network
resistance (R_F + R_G), giving relatively poor input to
output isolation.
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