OPA3691 Datasheet, PDF (20/32 Pages) Burr-Brown (TI) – Triple Wideband, Current-Feedback OPERATIONAL AMPLIFIER With Disable

E_NI
1/3
R_S
I_BN
OPA3691
E_O
E_RS
√4kTR_S
4kT
R_G
R_F
√4kTR_F
R_G
I_BI
4kT = 1.6E – 20J
at 290°K
FIGURE 12. Op Amp Noise Analysis 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 4 shows the general form for the
output noise voltage using the terms shown in Figure 12.
(4)
( ) ^{( ) ( ) E}O ^{= ENI2 + I}BN ^RS ^{2 + 4kTR}S ^{NG2 + I}BI ^RF ^{2 + 4kTR}F^NG
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_N =
ENI ²
+
I_BN R_S
2
+ 4kTR_S
+

I_BI R_F
NG

2
+
4kTR_F
NG
(5)
Evaluating these two equations for the OPA3691 circuit and
component values (see Figure 1) will give a total output spot
noise voltage of 8.08nV/√Hz and a total equivalent 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 reduced 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 = 182Ω will give a total input referred
noise of 2.1nV/√Hz.
DC ACCURACY AND OFFSET CONTROL
A current-feedback op amp like the OPA3691 provides
exceptional bandwidth in high gains, giving fast pulse settling
but only moderate DC accuracy. The Electrical Characteris-
tics Table shows 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 effec-
tive with most voltage-feedback op amps, they do not gener-
ally reduce the output DC offset for wideband current-feed-
back op amps. Since the two input bias currents are unre-
lated 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 con-
figuration 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
The OPA3691 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 OPA3691 will operate normally.
To disable, the control pin must be asserted low. Figure 13
shows a simplified internal circuit for the disable control
feature.
+V_S
15kΩ
Q1
25kΩ
110kΩ
V_DIS
I_S
Control
–V_S
FIGURE 13. Simplified Disable Control Circuit.
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,
additional current is pulled through the 15kΩ resistor eventu-
ally turning on these two diodes (≈ 75µA). 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 shuts off the collector current out of Q1, turning the
amplifier off. The supply current in the disable mode is that
only required to operate the circuit of Figure 13. Additional
circuitry ensures that turn-on time occurs faster than turn-off
time (make-before-break).
When disabled, the output and input nodes go to a high
impedance state. If the OPA3691 is operating in a gain of +1,
this will show a very high impedance (4pF || 1MΩ) at the
output and exceptional signal isolation. If operating at a
gain greater than +1, the total feedback network resistance
(R_F + R_G) will appear as the impedance looking back into the
20
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