Datasheet

100k 1M
SourceResistance,R (W)
S
100
1k 10k
10k
1k
100
10
1
VotlageNoiseSpectralDensity,E
O
R
S
E
O
E =e
O n n S S
+(i R ) +4kTR
2 2 2
ResistorNoise
OPA1611
OPA1641
OPA1641
OPA1642
OPA1644
SBOS484B –DECEMBER 2009–REVISED AUGUST 2010 www.ti.com
APPLICATION INFORMATION
The OPA1641, OPA1642, and OPA1644 are The equation in Figure 31 shows the calculation of
unity-gain stable, audio operational amplifiers with the total circuit noise, with these parameters:
very low noise, input bias current, and input offset
• e
n
= voltage noise
voltage. Applications with noisy or high-impedance
• I
n
= current noise
power supplies require decoupling capacitors placed
• R
S
= source impedance
close to the device pins. In most cases, 0.1mF
• k = Boltzmann's constant = 1.38 × 10
–23
J/K
capacitors are adequate. The front-page drawing
shows a simplified schematic of the OPA1641.
• T = temperature in degrees Kelvin (K)
For more details on calculating noise, see the next
OPERATING VOLTAGE
section on Basic Noise Calculations.
The OPA1641, OPA1642, and OPA1644 series of op
amps can be used with single or dual supplies from
an operating range of V
S
= +4.5V (±2.25V) and up to
V
S
= +36V (±18V). These devices do not require
symmetrical supplies; it only requires a minimum
supply voltage of +4.5V (±2.25V). For V
S
less than
±3.5V, the common-mode input range does not
include midsupply. Supply voltages higher than +40V
can permanently damage the device; see Absolute
Maximum Ratings table. Key parameters are
specified over the operating temperature range, T
A
=
–40°C to +85°C. Key parameters that vary over the
supply voltage or temperature range are shown in the
Typical Characteristics section of this data sheet.
NOISE PERFORMANCE
Figure 31 shows the total circuit noise for varying
Figure 31. Noise Performance of the OPA1611
source impedances with the operational amplifier in a
and OPA1641 in Unity-Gain Buffer Configuration
unity-gain configuration (with no feedback resistor
network and therefore no additional noise
contributions). The OPA1641, OPA1642, and BASIC NOISE CALCULATIONS
OPA1644 are shown with total circuit noise
Low-noise circuit design requires careful analysis of
calculated. The op amp itself contributes both a
all noise sources. External noise sources can
voltage noise component and a current noise
dominate in many cases; consider the effect of
component. The voltage noise is commonly modeled
source resistance on overall op amp noise
as a time-varying component of the offset voltage.
performance. Total noise of the circuit is the
The current noise is modeled as the time-varying
root-sum-square combination of all noise
component of the input bias current and reacts with
components.
the source resistance to create a voltage component
of noise. Therefore, the lowest noise op amp for a
The resistive portion of the source impedance
given application depends on the source impedance.
produces thermal noise proportional to the square
For low source impedance, current noise is negligible,
root of the resistance. This function is plotted in
and voltage noise generally dominates. The
Figure 31. The source impedance is usually fixed;
OPA1641, OPA1642, and OPA1644 family has both
consequently, select the op amp and the feedback
low voltage noise and extremely low current noise
resistors to minimize the respective contributions to
because of the FET input of the op amp. As a result,
the total noise.
the current noise contribution of the OPA164x series
is negligible for any practical source impedance,
which makes it the better choice for applications with
high source impedance.
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