OPA2682 Datasheet, PDF (17/19 Pages) Burr-Brown (TI) – Dual, Wideband, Fixed Gain BUFFER AMPLIFIER With Disable

NOISE PERFORMANCE
The OPA2682 offers an excellent balance between voltage
and 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 (2.2nV√Hz) is
lower than most unity gain stable, wideband, voltage-feed-
back op amps. This low input voltage noise was achieved at
the price of higher non-inverting input current noise
(12pA/√Hz). As long as the AC source impedance looking out
of the non-inverting 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 6 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.
E_NI
OPA2682
E_O
R_S
I_BN
DC ACCURACY
The OPA2682 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. How-
ever, the two input bias currents are somewhat higher and
are unmatched. Bias current cancellation techniques will not
reduce the output DC offset for OPA2682. 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 = non-inverting signal gain
= ±(2 • 5.0mV) + (55µA • 25Ω • 2) ± (480Ω • 40µA)
= ±10mV + 2.8mV ± 19.2mV
= –26.4mV → +32.0mV
Minimizing the resistance seen by the non-inverting input
will give the best DC offset performance.
E_RS
√4kTR_S
4kT
R_G
R_F
√4kTR_F
R_G
I_BI
4kT = 1.6 • 10^–20
FIGURE 6. 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 6.
(1)
( ) ^{( ) ( ) E}O ^{= E}NI2 ^{+ I}BN^RS ^{2 + 4kTR}S ^{NG2 + I}BI^RF ^{2 + 4kTR}F^NG
DISABLE OPERATION (SO-14 Only)
The OPA2682N 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 OPA2682N will operate nor-
mally. To disable, the control pin must be asserted low.
Figure 7 shows a simplified internal circuit for the disable
control feature.
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 even-
tually turning on these two diodes (≈ 100µA). At this point,
any further current pulled out of V_DIS goes through those
diodes holding the emitter-base voltage of Q1 at approxi-
Dividing this expression by the noise gain (NG = (1+R_F/R_G))
will give the equivalent input-referred spot noise voltage at
the non-inverting input as shown in Equation 2.
(2)
( ) E_N =
E_NI2 +
I _BN R _S
2
+
4 kTR _S
+


I _BI R _F
NG


2
+
4 kTR _F
NG
+V_S
15kΩ
Q1
Evaluating these two equations for the OPA2682 circuit and
component values shown in Figure 1 will give a total output
spot noise voltage of 8.7nV/√Hz and a total equivalent input
25kΩ
110kΩ
spot noise voltage of 4.4nV/√Hz. This total input-referred
spot noise voltage is higher than the 2.2nV/√Hz specifica-
V_DIS
tion for the op amp voltage noise alone. This reflects the
I_S
Control
–V_S
noise added to the output by the inverting current noise times
the feedback resistor.
FIGURE 7. Simplified Disable Control Circuit.
®
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OPA2682