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OPA1604AIDR Datasheet(PDF) 11 Page - Texas Instruments |
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OPA1604AIDR Datasheet(HTML) 11 Page - Texas Instruments |
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11 / 28 page  10k 1k 100 10 1 100 1k 10k 100k 1M Source Resistance, R ( ) W S OPA164x OPA160x E = e O n S + (i R ) + 4kTR n S 2 2 2 R S E O Resistor Noise OPA160x Output R F Input - + R I OPA1602 OPA1604 www.ti.com SBOS474B – APRIL 2011 – REVISED NOVEMBER 2011 INPUT PROTECTION The equation in Figure 33 shows the calculation of the total circuit noise, with these parameters: The input terminals of the OPA1602 and OPA1604 • en = Voltage noise are protected from excessive differential voltage with • in = Current noise back-to-back diodes, as Figure 32 illustrates. In most circuit applications, the input protection circuitry has • RS = Source impedance no consequence. However, in low-gain or G = +1 • k = Boltzmann ’s constant = 1.38 × 10–23 J/K circuits, fast ramping input signals can forward bias • T = Temperature in degrees Kelvin (K) 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 (RI) and/or a feedback resistor (RF) 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. 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 Figure 32. Pulsed Operation components. The resistive portion of the source impedance NOISE PERFORMANCE produces thermal noise proportional to the square Figure 33 shows the total circuit noise for varying root of the resistance. Figure 33 plots this equation. source impedances with the op amp in a unity-gain The source impedance is usually fixed; consequently, configuration (no feedback resistor network, and select the op amp and the feedback resistors to therefore no additional noise contributions). minimize the respective contributions to the total noise. The OPA160x (GBW = 35MHz, G = +1) is shown with total circuit noise calculated. The op amp itself Figure 34 illustrates both inverting and noninverting contributes both a voltage noise component and a op amp circuit configurations with gain. In circuit current noise component. The voltage noise is configurations with gain, the feedback network commonly modeled as a time-varying component of resistors also contribute noise. The current noise of the offset voltage. The current noise is modeled as the op amp reacts with the feedback resistors to the time-varying component of the input bias current create additional noise components. The feedback and reacts with the source resistance to create a resistor values can generally be chosen to make voltage component of noise. Therefore, the lowest these noise sources negligible. The equations for noise op amp for a given application depends on the total noise are shown for both configurations. 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 1k Ω. Copyright © 2011, Texas Instruments Incorporated 11 Product Folder Link(s): OPA1602 OPA1604 |
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