OPA3695_14 Datasheet, PDF (19/39 Pages) Texas Instruments – Triple, Ultra-Wideband, Current-Feedback OPERATIONAL AMPLIFIER with Disable

OPA3695
www.ti.com ............................................................................................................................................... SBOS355A – APRIL 2008 – REVISED SEPTEMBER 2008
NOISE PERFORMANCE
The OPA3695 offers an excellent balance between
voltage and current noise terms to achieve a low
output noise under a variety of operating conditions.
The input noise voltage (1.8nV/√Hz) is very low for a
unity-gain stable amplifier. This low input voltage
noise was achieved at the price of higher
noninverting input current noise (18pA/√Hz). As long
as the ac source impedance looking out of the
noninverting input is less than 100Ω, this current
noise does not contribute significantly to the total
output noise. The op amp input voltage noise and the
two input current noise terms combine to give low
output noise using the OPA3695. Figure 43 shows
the op amp noise analysis model with all of the noise
terms included. In this model, all noise terms are
taken to be noise voltage or current density terms in
either nV/√Hz or pA/√Hz.
E_NI
1/3
OPA3695
E_O
R_S
I_BN
DC ACCURACY AND OFFSET CONTROL
A current-feedback op amp such as the OPA3695
provides exceptional bandwidth and slew rate, giving
fast pulse settling but only moderate dc accuracy.
The Electrical Characteristics show an input offset
voltage comparable to high-speed voltage-feedback
amplifiers. However, the two input bias currents are
somewhat higher and are unmatched. Whereas 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 error contributions to the
output is ineffective. Evaluating the configuration of
Figure 35 using a gain of +2V/V, using worst-case
+25°C input offset voltage, and the two input bias
currents, gives a worst-case output offset range equal
to:
V_OS = ±(NG ´ V_OS) ± (I_BN ´ R_S/2 ´ NG) ± (I_BI ´ R_F)
= ±(2 ´ 3.5mV) ± (30mA ´ 25W ´ 2) ± (60mA ´ 604W)
= ±7mV ± 1.5mV ± 36.2mV
= ±44.7mV
E_RS
4kTR_S
4kT
R_G
R_F
R_G
I_BI
4kTR_F
4kT = 1.6E -20J
at 290°K
Figure 43. Op Amp Noise Model
The total output spot noise voltage can be computed
as the square root of the sum of all squared output
noise voltage contributors. Equation 1 shows the
general form for the output noise voltage using the
terms shown in Figure 43.
E_O =
E_NI² + (I_BNR_S)² + 4kTR_S NG² + (I_BIR_F)² + 4kTR_FNG
(1)
Dividing this expression through by noise gain (NG =
1 + R_F/R_G) gives the equivalent input-referred spot
noise voltage at the noninverting input, as shown in
Equation 2.
E_N =
E_NI² + (I_BNR_S)² + 4kTR_S +
I_BIR_F
NG
2
+
4kTR_F
NG
(2)
where NG = noninverting signal gain.
Minimizing the resistance seen by the noninverting
input also minimizes the output dc error. For
improved dc precision in a wideband low-gain
amplifier, consider the OPA842 where a bipolar input
is acceptable (low source resistance) or the OPA656
where a JFET input is required.
DISABLE OPERATION
The OPA3695 provides an optional disable feature
that can be used to reduce system power. If the V_DIS
control pin is left unconnected, the OPA3695
operates normally. This shutdown is intended only as
a power-savings feature. Forward path isolation when
disabled is very good for small signals when
configured for low gains. However, large-signal
isolation is not ensured because of the ±1.2V
limitation between the inverting node and the
noninverting node. Failure to properly account for this
voltage may cause undesirable responses in the
output signal when multiplexed. Configuring the
amplifier for high gains helps minimize this impact,
but it is not ensured; proper analysis should be done
by the designer.
Copyright © 2008, Texas Instruments Incorporated
Product Folder Link(s): OPA3695
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