Datasheet
DC ACCURACY AND OFFSET CONTROL
17kW
120kW
8kW
I
Control
S
-V
S
+V
S
V
DIS
Q1
POWER SHUTDOWN OPERATION
OPA2695
SBOS354A – APRIL 2008 – REVISED AUGUST 2008 ......................................................................................................................................................
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voltage is higher than the 1.8nV/ √ Hz specification for signal isolation is not ensured. Using this feature to
the op amp voltage noise alone. This increased value multiplex two or more outputs together is not
reflects the noise added to the output by the inverting recommended. Large signals applied to the shutdown
current noise times the feedback resistor. If the output stages can turn on parasitic devices,
feedback resistor is reduced in high-gain degrading signal linearity for the desired channel.
configurations (as suggested previously), the total
Turn-on time is very quick from the shutdown
input-referred voltage noise given by Equation 9
condition, typically less than 60ns. Turn-off time
approaches the 1.8nV/ √ Hz of the op amp itself. For
strongly depends on the external circuit configuration,
example, going to a gain of +20 (using R
F
= 200 Ω )
but is typically 200ns for the circuit of Figure 68 .
gives a total input-referred noise of 2.0nV/ √ Hz.
To shut down, the control pin must be asserted low.
For a more complete discussion of op amp noise
This logic control is referenced to the positive supply,
calculation, see TI Application Note, SBOA066 , Noise
as shown in the simplified circuit of Figure 79 .
Analysis for High Speed Op Amps, available through
the TI web site at www.ti.com .
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
Figure 79. Simplified Shutdown Circuit
polarity, matching the source impedance looking out
of each input to reduce the respective error
In normal operation, base current to Q1 is provided
contribution to the output is ineffective. Evaluating the
through the 120k Ω resistor, while the emitter current
configuration of Figure 80 , using a worst-case +25 ° C
through the 8k Ω resistor sets up a voltage drop that is
input offset voltage and the two input bias currents,
inadequate to turn on the two diodes in the Q1
gives a worst-case output offset range equal to:
emitter. As V
DIS
is pulled low, additional current is
± (NG × V
OS
) + (I
BN
× R
S
/2 × NG) ± (I
BI
× R
F
)
pulled through the 8k Ω resistor, eventually turning on
these two diodes ( ≈ 180 µ A). At this point, any further
where NG = noninverting signal gain.
current pulled out of V
DIS
goes through those diodes
= ± (8 × 3.5mV) ± (30 µ A × 25 Ω × 8) ± (402 Ω ×
holding the emitter-base voltage of Q1 at
60 µ A)
approximately 0V. This limitation shuts off the
= ± 28mV ± 8mV ± 24mV
collector current out of Q1, turning the amplifier off.
= ± 60mV
The supply current in the shutdown mode is only that
required to operate the circuit of Figure 79 .
A fine-scale output offset null, or dc operating point
adjustment, is often required. Numerous techniques
When disabled, the output and input nodes go to a
are available for introducing dc offset control into an
high-impedance state. If the OPA2695IRGT is
op amp circuit. Most simple adjustment techniques do
operating in a gain of +1V/V, this configuration shows
not correct for temperature drift.
a very high impedance (3pF || 1M Ω ) at the output and
exceptional signal isolation. If operating at a gain
greater than +1V/V, the total feedback network
(QFN-16 Package Only)
resistance (R
F
+ R
G
) appears as the impedance
looking back into the output, but the circuit continues
The OPA2695IRGT provides an optional power
to show very high forward and reverse isolation. If
shutdown feature that can be used to reduce system
configured as an inverting amplifier, the input and
power. If the V
DIS
control pin is left unconnected, the
output are connected through the feedback network
OPA2695IRGT operates normally. This shutdown is
resistance (R
F
+ R
G
), giving relatively poor input to
intended only as a power-saving feature. Forward
output isolation.
path isolation is very good for small signals. Large
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