1
10
100
1k
10k
100 1k 10k 100k 1M
Resistor Noise
OPA166X
OPA165X
Source Resistance (Ω)
Voltage Noise (nV/ Hz )
Eo
2 = en
2 + (inRS)2 + 4KTRS
G003
OPA165x Output
RF
Input

+
RI
OPA1652
OPA1654
SBOS477 –DECEMBER 2011
www.ti.com
INPUT PROTECTION The equation in Figure 37 shows the calculation of
the total circuit noise, with these parameters:
The input terminals of the OPA1652 and OPA1654 •en= Voltage noise
are protected from excessive differential voltage with •in= Current noise
backtoback diodes, as Figure 36 illustrates. In most
circuit applications, the input protection circuitry has •RS= Source impedance
no consequence. However, in lowgain or G = +1 •k = Boltzmann’s constant = 1.38 ×10–23 J/K
circuits, fast ramping input signals can forward bias •T = Temperature in Kelvins (K)
these diodes because the output of the amplifier
cannot respond rapidly enough to the input ramp. If
the input signal is fast enough to create this forward
bias condition, the input signal current must be limited
to 10 mA 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 resistor degrades the
lownoise performance of the OPA165x and is
examined in the following Noise Performance section.
Figure 36 shows an example configuration when both
currentlimiting input and feeback resistors are used.
Figure 37. Noise Performance of the OPA165x in
UnityGain Buffer Configuration
BASIC NOISE CALCULATIONS
Design of lownoise op amp circuits requires careful
consideration of a variety of possible noise
contributors: noise from the signal source, noise
generated in the op amp, and noise from the
Figure 36. Pulsed Operation feedback network resistors. The total noise of the
circuit is the rootsumsquare combination of all noise
components.
NOISE PERFORMANCE The resistive portion of the source impedance
Figure 37 shows the total circuit noise for varying produces thermal noise proportional to the square
source impedances with the op amp in a unitygain root of the resistance. Figure 37 plots this equation.
configuration (no feedback resistor network, and The source impedance is usually fixed; consequently,
therefore no additional noise contributions). select the op amp and the feedback resistors to
The OPA165x (GBW = 18 MHz, G = +1) is shown minimize the respective contributions to the total
with total circuit noise calculated. The op amp itself noise.
contributes both a voltage noise component and a Figure 38 illustrates both inverting and noninverting
current noise component. The voltage noise is op amp circuit configurations with gain. In circuit
commonly modeled as a timevarying component of configurations with gain, the feedback network
the offset voltage. The current noise is modeled as resistors also contribute noise. The current noise of
the timevarying component of the input bias current the op amp reacts with the feedback resistors to
and reacts with the source resistance to create a create additional noise components. The feedback
voltage component of noise. Therefore, the lowest resistor values can generally be chosen to make
noise op amp for a given application depends on the these noise sources negligible. The equations for
source impedance. For low source impedance, total noise are shown for both configurations.
current noise is negligible, and voltage noise
generally dominates. The voltage noise of the
OPA165x series op amps makes them a better
choice for source impedances greater than or equal
to 1 kΩ.
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