Datasheet

NOISE PERFORMANCE DC ACCURACY AND OFFSET CONTROL
= (NG V ) (I R /2 NG) (I R )± ´
OS BN S BI F
± ´ ´ ± ´
± ´ ± m ´ W ´ ± m ´ W
± ± ±
±
= (2 3.5mV) (30 A 25 2) (60 A 604 )
= 7mV 1.5mV 36.2mV
= 44.7mV
V
OS
4kT
R
G
R
G
R
F
R
S
1/3
OPA3695
I
BI
E
O
I
BN
4kT=1.6E 20J-
at290 K°
E
RS
E
NI
4kTR
S
4kTR
F
DISABLE OPERATION
E =
O
E +(I R ) +4kTR
NI BN S S
2 2
NG +(I R ) +4kTR NG
BI F F
2 2
E =
N
E +(I R ) +4kTR
NI BN S S
+
2 2
4kTR
F
NG
I R
BI F
NG
2
+
OPA3695
www.ti.com ............................................................................................................................................... SBOS355A – APRIL 2008 – REVISED SEPTEMBER 2008
The OPA3695 offers an excellent balance between A current-feedback op amp such as the OPA3695
voltage and current noise terms to achieve a low provides exceptional bandwidth and slew rate, giving
output noise under a variety of operating conditions. fast pulse settling but only moderate dc accuracy.
The input noise voltage (1.8nV/ √ Hz) is very low for a The Electrical Characteristics show an input offset
unity-gain stable amplifier. This low input voltage voltage comparable to high-speed voltage-feedback
noise was achieved at the price of higher amplifiers. However, the two input bias currents are
noninverting input current noise (18pA/ √ Hz). As long somewhat higher and are unmatched. Whereas bias
as the ac source impedance looking out of the current cancellation techniques are very effective with
noninverting input is less than 100 Ω , this current most voltage-feedback op amps, they do not
noise does not contribute significantly to the total generally reduce the output dc offset for wideband
output noise. The op amp input voltage noise and the current-feedback op amps. Because the two input
two input current noise terms combine to give low bias currents are unrelated in both magnitude and
output noise using the OPA3695. Figure 43 shows polarity, matching the source impedance looking out
the op amp noise analysis model with all of the noise of each input to reduce the error contributions to the
terms included. In this model, all noise terms are output is ineffective. Evaluating the configuration of
taken to be noise voltage or current density terms in Figure 35 using a gain of +2V/V, using worst-case
either nV/ √ Hz or pA/ √ Hz. +25 ° C input offset voltage, and the two input bias
currents, gives a worst-case output offset range equal
to:
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.
Figure 43. Op Amp Noise Model
The total output spot noise voltage can be computed
The OPA3695 provides an optional disable feature
as the square root of the sum of all squared output
that can be used to reduce system power. If the V
DIS
noise voltage contributors. Equation 1 shows the
control pin is left unconnected, the OPA3695
general form for the output noise voltage using the
operates normally. This shutdown is intended only as
terms shown in Figure 43 .
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
(1)
limitation between the inverting node and the
Dividing this expression through by noise gain (NG =
noninverting node. Failure to properly account for this
1 + R
F
/R
G
) gives the equivalent input-referred spot
voltage may cause undesirable responses in the
noise voltage at the noninverting input, as shown in
output signal when multiplexed. Configuring the
Equation 2 .
amplifier for high gains helps minimize this impact,
but it is not ensured; proper analysis should be done
by the designer.
(2)
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