Datasheet

1
10
100
1k
10k
100 1k 10k 100k 1M
Resistor Noise
OPA166x
OPA165x
Source Resistance ( )W
Voltage Noise (nV/
Hz)
E
o
2
= e
n
2
+ (i
n
R
S
)
2
+ 4KTR
S
G003
R
S
E
O
OPA166x
Output
R
F
Input
-
+
R
I
OPA1662
OPA1664
SBOS489 –DECEMBER 2011 www.ti.com
INPUT PROTECTION The equation in Figure 45 shows the calculation of
the total circuit noise, with these parameters:
The input terminals of the OPA1662 and OPA1664
• e
n
= Voltage noise
are protected from excessive differential voltage with
• i
n
= Current noise
back-to-back diodes, as Figure 44 illustrates. In most
circuit applications, the input protection circuitry has
• R
S
= Source impedance
no consequence. However, in low-gain 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 (R
I
) and/or
a feedback resistor (R
F
) can be used to limit the
signal input current. This resistor degrades the
low-noise performance of the OPA166x and is
examined in the following Noise Performance section.
Figure 44 shows an example configuration when both
current-limiting input and feeback resistors are used.
Figure 45. Noise Performance of the OPA166x in
Unity-Gain Buffer Configuration
BASIC NOISE CALCULATIONS
Design of low-noise op amp circuits requires careful
consideration of a variety of possible noise
contributors: noise from the signal source, noise
Figure 44. Pulsed Operation
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
NOISE PERFORMANCE
components.
Figure 45 shows the total circuit noise for varying
The resistive portion of the source impedance
source impedances with the op amp in a unity-gain
produces thermal noise proportional to the square
configuration (no feedback resistor network, and
root of the resistance. Figure 45 plots this equation.
therefore no additional noise contributions).
The source impedance is usually fixed; consequently,
The OPA166x (GBW = 22 MHz, G = +1) is shown
select the op amp and the feedback resistors to
with total circuit noise calculated. The op amp itself
minimize the respective contributions to the total
contributes both a voltage noise component and a
noise.
current noise component. The voltage noise is
Figure 46 illustrates both inverting and noninverting
commonly modeled as a time-varying component of
op amp circuit configurations with gain. In circuit
the offset voltage. The current noise is modeled as
configurations with gain, the feedback network
the time-varying component of the input bias current
resistors also contribute noise. The current noise of
and reacts with the source resistance to create a
the op amp reacts with the feedback resistors to
voltage component of noise. Therefore, the lowest
create additional noise components. The feedback
noise op amp for a given application depends on the
resistor values can generally be chosen to make
source impedance. For low source impedance,
these noise sources negligible. The equations for
current noise is negligible, and voltage noise
total noise are shown for both configurations.
generally dominates. The low voltage noise of the
OPA166x series op amps makes them a better
choice for low source impedances of less than 1 kΩ.
14 Copyright © 2011, Texas Instruments Incorporated
Product Folder Link(s): OPA1662 OPA1664