Datasheet

VOLTAGE NOISE SPECTRAL DENSITY
vs SOURCE RESISTANCE
10k
1k
100
10
1
100
1k 10k 100k 1M
Source Resistance, R ( )W
S
Resistor
Noise
E = e
O n S
+ (i R ) + 4kTR
n S
2 2 2
R
S
E
O
Total Output
Voltage Noise
Voltage Noise Spectral Density, E
O
(nV/ )Hz?
OPA1611
Output
R
F
Input
-
+
R
I
OPA1611
OPA1612
www.ti.com SBOS450B –JULY 2009–REVISED JULY 2011
INPUT PROTECTION current noise is negligible, and voltage noise
generally dominates. The low voltage noise of the
The input terminals of the OPA1611 and the
OPA161x series op amps makes them a good choice
OPA1612 are protected from excessive differential
for use in applications where the source impedance is
voltage with back-to-back diodes, as Figure 30
less than 1kΩ.
illustrates. In most circuit applications, the input
protection circuitry has no consequence. However, in The equation in Figure 31 shows the calculation of
low-gain or G = +1 circuits, fast ramping input signals the total circuit noise, with these parameters:
can forward bias these diodes because the output of
• e
n
= Voltage noise
the amplifier cannot respond rapidly enough to the
• I
n
= Current noise
input ramp. This effect is illustrated in Figure 17 of
• R
S
= Source impedance
the Typical Characteristics. If the input signal is fast
• k = Boltzmann’s constant = 1.38 × 10
–23
J/K
enough to create this forward bias condition, the input
signal current must be limited to 10mA or less. If the
• T = Temperature in degrees Kelvin (K)
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
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.
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
consideration of a variety of possible noise
source impedances with the op amp in a unity-gain
contributors: noise from the signal source, noise
configuration (no feedback resistor network, and
generated in the op amp, and noise from the
therefore no additional noise contributions).
feedback network resistors. The total noise of the
circuit is the root-sum-square combination of all noise
The OPA1611 (GBW = 40MHz, G = +1) is shown
components.
with total circuit noise calculated. The op amp itself
contributes both a voltage noise component and a
The resistive portion of the source impedance
current noise component. The voltage noise is
produces thermal noise proportional to the square
commonly modeled as a time-varying component of
root of the resistance. Figure 31 plots this function.
the offset voltage. The current noise is modeled as
The source impedance is usually fixed; consequently,
the time-varying component of the input bias current
select the op amp and the feedback resistors to
and reacts with the source resistance to create a
minimize the respective contributions to the total
voltage component of noise. Therefore, the lowest
noise.
noise op amp for a given application depends on the
source impedance. For low source impedance,
Copyright © 2009–2011, Texas Instruments Incorporated 11