OPA1641_15 Datasheet, PDF (10/29 Pages) Texas Instruments – High-Performance, JFET-Input AUDIO OPERATIONAL AMPLIFIERS

OPA1641
OPA1642
OPA1644
SBOS484B – DECEMBER 2009 – REVISED AUGUST 2010
APPLICATION INFORMATION
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The OPA1641, OPA1642, and OPA1644 are
unity-gain stable, audio operational amplifiers with
very low noise, input bias current, and input offset
voltage. Applications with noisy or high-impedance
power supplies require decoupling capacitors placed
close to the device pins. In most cases, 0.1mF
capacitors are adequate. The front-page drawing
shows a simplified schematic of the OPA1641.
OPERATING VOLTAGE
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
source impedances with the operational amplifier in a
unity-gain configuration (with no feedback resistor
network and therefore no additional noise
contributions). The OPA1641, OPA1642, and
OPA1644 are 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
OPA1641, OPA1642, and OPA1644 family has both
low voltage noise and extremely low current noise
because of the FET input of the op amp. As a result,
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.
The equation in Figure 31 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)
For more details on calculating noise, see the next
section on Basic Noise Calculations.
10k
E_O
1k
R_S
OPA1611
100
OPA1641
10
Resistor Noise
1
100
2
2
2
E_O = e_n + (i_n R_S) + 4kTR_S
1k
10k
100k
1M
Source Resistance, R_S (W)
Figure 31. Noise Performance of the OPA1611
and OPA1641 in Unity-Gain Buffer Configuration
BASIC NOISE CALCULATIONS
Low-noise circuit design requires careful analysis of
all noise sources. External noise sources can
dominate in many cases; consider the effect of
source resistance on overall op amp noise
performance. 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. This function is plotted in
Figure 31. The source impedance is usually fixed;
consequently, select the op amp and the feedback
resistors to minimize the respective contributions to
the total noise.
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