OPA1641 Datasheet, PDF (17/31 Pages) Texas Instruments – High-Performance, JFET-Input AUDIO OPERATIONAL AMPLIFIERS

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8 Application and Implementation
OPA1641, OPA1642, OPA1644
SBOS484C – DECEMBER 2009 – REVISED DECEMBER 2015
NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
8.1 Application Information
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, 0.1-μF capacitors are adequate. The
Simplified Internal Schematic shows a simplified schematic of the OPA1641.
Simplified Internal Schematic
V+
Pre-Output Driver
IN-
IN+
OUT
V-
8.1.1 Noise Performance
Figure 32 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 operational amplifier 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 operational amplifier 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 operational amplifier. 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 32 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 Basic Noise Calculations.
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Product Folder Links: OPA1641 OPA1642 OPA1644