Datasheet

10k
1k
100
10
1
Voltage Noise Spectral Density, E
O
(nV/ )HzÖ
100
1k 10k 100k 1M
Source Resistance, R ( )W
S
OPA164x
OPA160x
E = e
O n S
+ (i R ) + 4kTR
n S
2 2 2
R
S
E
O
Resistor
Noise
OPA160x
Output
R
F
Input
-
+
R
I
OPA1602
OPA1604
www.ti.com SBOS474B –APRIL 2011–REVISED NOVEMBER 2011
INPUT PROTECTION The equation in Figure 33 shows the calculation of
the total circuit noise, with these parameters:
The input terminals of the OPA1602 and OPA1604
• e
n
= Voltage noise
are protected from excessive differential voltage with
• i
n
= Current noise
back-to-back diodes, as Figure 32 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 degrees Kelvin (K)
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 (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 OPA160x
and is examined in the following Noise Performance
section. Figure 32 shows an example configuration
when both current-limiting input and feeback resistors
are used.
Figure 33. Noise Performance of the OPA160x 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
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
Figure 32. Pulsed Operation
components.
The resistive portion of the source impedance
NOISE PERFORMANCE
produces thermal noise proportional to the square
Figure 33 shows the total circuit noise for varying
root of the resistance. Figure 33 plots this equation.
source impedances with the op amp in a unity-gain
The source impedance is usually fixed; consequently,
configuration (no feedback resistor network, and
select the op amp and the feedback resistors to
therefore no additional noise contributions).
minimize the respective contributions to the total
noise.
The OPA160x (GBW = 35MHz, G = +1) is shown with
total circuit noise calculated. The op amp itself
Figure 34 illustrates both inverting and noninverting
contributes both a voltage noise component and a
op amp circuit configurations with gain. In circuit
current noise component. The voltage noise is
configurations with gain, the feedback network
commonly modeled as a time-varying component of
resistors also contribute noise. The current noise of
the offset voltage. The current noise is modeled as
the op amp reacts with the feedback resistors to
the time-varying component of the input bias current
create additional noise components. The feedback
and reacts with the source resistance to create a
resistor values can generally be chosen to make
voltage component of noise. Therefore, the lowest
these noise sources negligible. The equations for
noise op amp for a given application depends on the
total noise are shown for both configurations.
source impedance. For low source impedance,
current noise is negligible, and voltage noise
generally dominates. The low voltage noise of the
OPA160x series op amps makes them a better
choice for low source impedances of less than 1kΩ.
Copyright © 2011, Texas Instruments Incorporated 11
Product Folder Link(s): OPA1602 OPA1604