OPA1611
OPA1612
www.ti.com........................................................................................................................................................ SBOS450A –JULY 2009–REVISED AUGUST 2009
INPUT PROTECTION
current noise is negligible, and voltage noise
generally dominates. The low voltage noise of the
OPA161x series op amps makes them a good choice
for use in applications where the source impedance is
less than 1kΩ.
The input terminals of the OPA1611 and the
OPA1612 are protected from excessive differential
voltage with back-to-back diodes, as Figure 30
illustrates. In most circuit applications, the input
protection circuitry has no consequence. However, in
low-gain or G = +1 circuits, fast ramping input signals
can forward bias these diodes because the output of
the amplifier cannot respond rapidly enough to the
input ramp. This effect is illustrated in Figure 17 of
the Typical Characteristics. If the input signal is fast
enough to create this forward bias condition, the input
signal current must be limited to 10mA or less. If the
input signal current is not inherently limited, an input
series resistor (RI) and/or a feedback resistor (RF)
can be used to limit the signal input current. This
input series resistor degrades the low-noise
performance of the OPA1611 and is examined in the
following Noise Performance section. Figure 30
shows an example configuration when both
current-limiting input and feedback resistors are used.
The equation in Figure 31 shows the calculation of
the total circuit noise, with these parameters:
•
•
•
•
•
en = Voltage noise
In = Current noise
RS = Source impedance
k = Boltzmann’s constant = 1.38 × 10–23 J/K
T = Temperature in degrees Kelvin (K)
VOLTAGE NOISE SPECTRAL DENSITY
vs SOURCE RESISTANCE
10k
EO
Total Output
Voltage Noise
1k
100
10
RS
RF
Resistor
Noise
-
EO2 = en2 + (in RS)2 + 4kTRS
OPA1611
Output
1
100
1k
10k
100k
1M
RI
+
Source Resistance, RS (W)
Input
Figure 31. Noise Performance of the OPA1611 in
Unity-Gain Buffer Configuration
Figure 30. Pulsed Operation
BASIC NOISE CALCULATIONS
NOISE PERFORMANCE
Design of low-noise op amp circuits requires careful
Figure 31 shows the total circuit noise for varying
source impedances with the op amp in a unity-gain
configuration (no feedback resistor network, and
therefore no additional noise contributions).
consideration of
a
variety of possible noise
contributors: noise from the signal source, noise
generated in the op amp, and noise from the
feedback network resistors. The total noise of the
circuit is the root-sum-square combination of all noise
components.
The OPA1611 (GBW = 40MHz, G = +1) is shown
with total circuit noise calculated. The op amp itself
contributes both a voltage noise component and a
current noise component. The voltage noise is
commonly modeled as a time-varying component of
the offset voltage. The current noise is modeled as
the time-varying component of the input bias current
and reacts with the source resistance to create a
voltage component of noise. Therefore, the lowest
noise op amp for a given application depends on the
source impedance. For low source impedance,
The resistive portion of the source impedance
produces thermal noise proportional to the square
root of the resistance. Figure 31 plots this function.
The source impedance is usually fixed; consequently,
select the op amp and the feedback resistors to
minimize the respective contributions to the total
noise.
Copyright © 2009, Texas Instruments Incorporated
11
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