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MAX3225EETP+ 其他数据使用手册 - Maxim Integrated（美信）
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Maxim Integrated（美信）
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接口芯片
封装:
TQFN-20
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MAXIM INTEGRATED PRODUCTS MAX3225EETP+ 芯片, RS232 收发器, 1MBPS, 5.5V, TQFN-20
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MAX3225EETP+数据手册
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Figure 1. Power-supply noise is added to the signal.
In application note 4992, "Reduce the Chances of Human Error: Part 1, Power and Ground," we
concentrated on power noise, ground, and layout. In this companion article we examine how to control
noise in the signal path. We discuss the characteristics of the signal and noise as we proceed through
the various circuit stages.
Is the major noise source harmonics of a switching power supply? If the power-supply noise is 50kHz
and higher and if our wanted signal is 1kHz, then filtering may be feasible. If we will be going into an
ADC next, the anti-aliasing filter may help.
Reducing Noise
What possibilities does an experienced circuit designer evaluate? The simplest fix is to amplify the signal
before the noise is added. Yes, straightforward enough, but unfortunately this solution most often cannot
be used because the incoming signal already has noise present. So now we get creative and consider
how to separate the signal from noise? Can we use highpass, lowpass, or bandpass filters? Can we
discriminate on the basis of amplitude, limiting, noise blanking, or coring? Can we use preemphasis
before the noise is introduced and deemphasis after to increase SNR? Can we discriminate on the basis
of time, i.e., sample at a minimum noise level or interference time? Can we time average, or if the signal
is repetitive, can we sum cycles or average in two, three, or more dimensions?
Turning from component functions, think about the system as a whole. What are the system goals? How
will the system be used? How will the human senses interact with the information provided by the
system? In short, consider all the information that we can muster.
The above set of questions reflects considerable design experience and knowledge. This is precisely why
smart companies hire experienced engineers and mentor the new engineers to build a solid design team.
We can use an example to illustrate the thinking and development process. The first observation finds
that the system has a high-gain operational amplifier at its input. The op amp feeds an analog-to-digital
converter (ADC). Occasionally a large noise pulse appears on the signal and this causes the op amp to
saturate. The op amp recovery time might be as long as milliseconds or seconds. How will we approach
the issue? Since every case will be different, we will just ask questions and point out possible solutions.
First, gather data to try to understand what is happening. Can we fix the source? No. So we decide that
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