How to choose the best amplifier for low-noise design? Here are the tips
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When evaluating the performance of an amplifier for a low noise application, one of the considerations is noise. This article briefly explores the trade-offs involved in selecting the best amplifier for a low noise design.
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If you drive an op amp with a certain source resistance, the equivalent noise input is equal to the square root of the sum of the squares of: the amplifier's voltage noise; the voltage due to the source resistance; and the voltage due to the amplifier's current noise flowing through the source impedance.
If the source resistance is small, the noise generated by the source resistance and the current noise will have little effect on the total noise. In this case, the noise at the input is actually just the voltage noise of the op amp.
If the source resistance is large, the Johnson noise of the source resistance can be much higher than the voltage noise of the op amp and the voltage generated by the current noise. However, it should be noted that since Johnson noise only grows with the square root of the resistance, and the noise voltage affected by the current noise is proportional to the input impedance, the amplifier's current noise will dominate for sufficiently high input impedance values. When the amplifier's voltage and current noise are high enough, the Johnson noise will not dominate for any input resistance value.
If the noise contribution of an amplifier is negligible relative to the source resistance, then the op amp’s quality factor, Rs ,op, can be used to select it. This can be calculated from the amplifier’s noise specification:
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in: e n represents the voltage noise referred to the input end i n represents the current noise referred to the input end |
Figure 1 shows a comparison of voltage noise density versus R S, OP at 1 kHz for various Analog Devices high voltage (up to 44 V) op amps . The slanted lines show the Johnson noise associated with the resistor.
Figure 1. ADI's amplifier noise plot.
A similar graph can be made for a chosen frequency based on the data in the op amp data sheet . For example, the AD8599 has an input-referred voltage noise of about 1.07 nV/√Hz and an input-referred current noise of 2.3 pA/√Hz (1 kHz). Its Rs ,op value is about 465 S2 (1 kHz). In addition, note the following:
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The Johnson noise associated with this device is equivalent to a source resistance of approximately 69.6 Ω (see Figure 1);
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For source resistances greater than 465 Ω, the noise voltage due to the amplifier current noise exceeds the noise voltage due to the source resistance; the amplifier current noise becomes the dominant noise source.
To use this diagram (see Figure 2), follow steps 1 to 4.
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Usually, the source resistance is known (such as sensor impedance). If the resistance value is unknown, it is calculated based on the surrounding or front-end circuit components;
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Locate a given source resistance, such as 1 kΩ, on the Johnson noise line;
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Draw a horizontal line from the point determined in step 2 to the right side of the coordinate graph;
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Draw a straight line from the point determined in step 2 to the lower left. The slope is, for every 10-fold decrease in voltage noise there is a 10-fold decrease in resistance.
Figure 2. Selecting an op amp for a low-noise design.
The amplifiers to the lower right of the line are all good quality, low noise op , as shown in the shaded area of Figure 2.
When evaluating amplifier noise performance for a low noise design, all potential At the noise source.
The dominant noise contribution of an op amp depends on the source resistance as follows:
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Rs > > Rs, op; the current noise referred to the input end is dominant
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Rs = Rs, op; amplifier noise is negligible; resistor noise dominates
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Rs ＜＜Rs, op; the voltage noise referred to the input end is dominant
In general, interference signals can be reduced or eliminated by:
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Good layout techniques to reduce parasitic effects
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Good grounding techniques, such as separation of digital and analog grounds
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Good shielding
For resistive noise sources, follow these rules:
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Limit bandwidth based on application needs
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Keep the resistance value as low as possible
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Use low-noise resistors, such as those using large metal foil, wirewound, and metal film technologies.
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Minimize the number of resistive noise sources
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