OPA1602_1111 Datasheet, PDF (11/28 Pages) Texas Instruments – High-Performance, Bipolar-Input AUDIO OPERATIONAL AMPLIFIERS

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INPUT PROTECTION
The input terminals of the OPA1602 and OPA1604
are protected from excessive differential voltage with
back-to-back diodes, as Figure 32 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 (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.
R_F
-
Input
R_I
+
OPA160x
Output
Figure 32. Pulsed Operation
NOISE PERFORMANCE
Figure 33 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).
The OPA160x (GBW = 35MHz, 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,
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.
OPA1602
OPA1604
SBOS474B – APRIL 2011 – REVISED NOVEMBER 2011
The equation in Figure 33 shows the calculation of
the total circuit noise, with these parameters:
• e_n = Voltage noise
• i_n = Current noise
• R_S = Source impedance
• k = Boltzmann’s constant = 1.38 × 10^–23 J/K
• T = Temperature in degrees Kelvin (K)
10k
E_O
1k
R_S
OPA160x
100
OPA164x
10
1
100
Resistor
Noise
E_O2 = e_n2 + (i_n R_S)² + 4kTR_S
1k
10k
100k
1M
Source Resistance, R_S (W)
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
components.
The resistive portion of the source impedance
produces thermal noise proportional to the square
root of the resistance. Figure 33 plots this equation.
The source impedance is usually fixed; consequently,
select the op amp and the feedback resistors to
minimize the respective contributions to the total
noise.
Figure 34 illustrates both inverting and noninverting
op amp circuit configurations with gain. In circuit
configurations with gain, the feedback network
resistors also contribute noise. The current noise of
the op amp reacts with the feedback resistors to
create additional noise components. The feedback
resistor values can generally be chosen to make
these noise sources negligible. The equations for
total noise are shown for both configurations.
Copyright © 2011, Texas Instruments Incorporated
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