Datasheet

OPA2691
18
SBOS224D
www.ti.com
current noise terms to achieve low output noise. The inverting
current noise (15pA/
√Hz ) is significantly lower than earlier
solutions while the input voltage noise (1.7nV/
√Hz ) is lower
than most unity-gain stable, wideband, voltage-feedback op
amps. This low input voltage noise was achieved at the price
of higher noninverting input current noise (12pA/
√Hz ). As long
as the AC source impedance looking out of the noninverting
node is less than 100Ω, this current noise will 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 under a wide variety of operating conditions.
Figure 10 shows the op amp noise analysis model with all 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 .
DC ACCURACY AND OFFSET CONTROL
A current-feedback op amp like the OPA2691 provides
exceptional bandwidth in high gains, giving fast pulse settling
but only moderate DC accuracy. The Electrical Characteris-
tics 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 re-
duce the output DC offset for wideband current-feedback op
amps. Since the two input bias currents are unrelated in both
magnitude and polarity, matching the source impedance
looking out of each input to reduce their error contribution to
the output is ineffective. Evaluating the configuration of
Figure 1, using 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(MAX)
) + (I
BN
• R
S
/2 • NG) ± (I
BI
• R
F
)
where NG = noninverting signal gain
= ± (2 • 3.0mV) + (35µA • 25Ω • 2) ± (402Ω • 25µA)
= ±6mV + 1.75mV ± 10.05mV
= –14.3mV → +17.8mV
DISABLE OPERATION (SO-14 ONLY)
The OPA2691I-14D provides an optional disable feature that
may be used either to reduce system power or to implement
a simple channel multiplexing operation. If the
DIS control
pin is left unconnected, the OPA2691I-14D will operate
normally. To disable, the control pin must be asserted low.
Figure 11 shows a simplified internal circuit for the disable
control feature.
The total output spot noise voltage can be computed as the
square root of the sum of all squared output noise voltage
contributors. Equation 4 shows the general form for the
output noise voltage using the terms shown in Figure 10.
(4)
E E I R kTR NG I R kTR NG
O
NI
BN
SS
BI F F
=+
( )
+
( )
+
( )
+
22 22
44
Dividing this expression by the noise gain (NG = (1 + R
F
/R
G
))
will give the equivalent input referred spot noise voltage at
the noninverting input, as shown in Equation 5.
E E I R kTR
IR
NG
kTR
NG
N
NI
BN
SS
BI F F
=+
( )
++






+
22
2
4
4
(5)
Evaluating these two equations for the OPA2691 circuit and
component values presented in Figure 1 will give a total
output spot noise voltage of 8.08nV/
√Hz and a total equiva-
lent input spot noise voltage of 4.04nV/
√Hz . This total input
referred spot noise voltage is higher than the 1.7nV/
√Hz
specification for the op amp voltage noise alone. This reflects
the noise added to the output by the inverting current noise
times the feedback resistor. If the feedback resistor is re-
duced in high-gain configurations (as suggested previously),
the total input referred voltage noise given by Equation 5 will
approach just the 1.7nV/
√Hz of the op amp itself. For
example, going to a gain of +10 using R
F
= 180Ω will give a
total input referred noise of 2.1nV/
√Hz .
In normal operation, base current to Q1 is provided through
the 110kΩ resistor while the emitter current through the 15kΩ
resistor sets up a voltage drop that is inadequate to turn on
the two diodes in Q1’s emitter. As V
DIS
is pulled low, addi-
tional current is pulled through the 15kΩ resistor, eventually
turning on these two diodes (≈ 100µA). At this point, any
further current pulled out of V
DIS
goes through those diodes
FIGURE 10. Op Amp Noise Analysis Model.
4kT
R
G
R
G
R
F
R
S
1/2
OPA2691
I
BI
E
O
I
BN
4kT = 1.6E –20J
at 290°K
E
RS
E
NI
4kTR
S
√
4kTR
F
√
25kΩ 110kΩ
15kΩ
I
S
Control
–V
S
+V
S
V
DIS
Q1
FIGURE 11. Simplified Disable Control Circuit, Each Channel.