实用模拟电路设计技术Section1

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Over the last several years, single-supply operation has become an increasingly
important requirement as systems get smaller, cheaper, and more portable.
Portable systems rely on batteries, and total circuit power consumption is an
important and often dominant design issue, and in some instances, more important
than cost. This makes low-voltage/low supply current operation critical; at the same
time, however, accuracy and precision requirements have forced IC manufacturers
to meet the challenge of “doing more with less” in their amplifier designs.

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Modern system design increasingly makes use of high speed ICs as circuit building
blocks. With bandwidths going up and up, demands are placed on the designer for
faster and more power efficient circuits.

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The trend in ADCs and DACs is toward higher speeds and higher resolutions at
reduced power levels. Modern data converters generally operate on ±5V (dual
supply) or +5V (single supply). There are now a few converters which operate on a
single +3V supply. This trend has created a number of design and applications
problems which were much less important in earlier data converters, where ±15V
supplies were the standard.

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An exciting new application for wideband, low distortion ADCs is called
undersampling, harmonic sampling, bandpass sampling, or Super-Nyquist
Sampling. To understand these applications, it is necessary to review the basics of
the sampling process.

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There are many applications for data acquisition systems in measurement and
process control. All data acquisition applications involve digitizing analog signals for
analysis using ADCs. In a measurement application, the ADC is followed by a digital
processor which performs the required data analysis. In a process control
application, the process controller generates feedback signals which typically must
be converted back into analog form using a DAC.

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One of the most commonly asked applications questions is: “What happens if
external voltages are applied to an analog integrated circuit with the supplies
turned off?” This question describes situations that can take on many different
forms: from lightning strikes on cables which propagate very high transient voltages
into signal conditioning circuits, to walking across a carpet and then touching a
printed circuit board full of sensitive precision circuits. Regardless of the situation,
the general issue is the effect of overvoltage stress (and, in some cases, abuse) on
analog integrated circuits. The discussion which follows will be limited in general to
operational amplifiers, because it is these devices that most often interface to the
outside world. The principles developed here can and should be applied to all analog
integrated circuits which are required to condition or digitize analog waveforms.
These devices include (but are not limited to) instrumentation amplifiers, analog
comparators, sample-and-hold amplifiers, analog switches and multiplexers, and
analog-to-digital converters.

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Dynamic range of an op amp may be defined in several ways. The most common
ways are to specify Harmonic Distortion, Total Harmonic Distortion (THD), or Total
Harmonic Distortion Plus Noise (THD + N).
Harmonic distortion is measured by applying a spectrally pure sinewave to an op
amp in a defined circuit configuration and observing the output spectrum. The
amount of distortion present in the output is usually a function of several
parameters: the small- and large-signal nonlinearity of the amplifier being tested,
the amplitude and frequency of the input signal, the load applied to the output of the
amplifier, the amplifier's power supply voltage, printed circuit board layout,
grounding, power supply decoupling, etc. Therefore, any distortion specification is
relatively meaningless unless the exact test conditions are specified.

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While there is no doubt that computer analysis is one of the most valuable tools that
the analog designer has acquired in the last decade or so, there is equally no doubt
that analog circuit models are not perfect and must be verified with hardware. If the
initial test circuit or "breadboard" is not correctly constructed, it may suffer from
malfunctions which are not the fault of the design but of the physical structure of
the breadboard itself. This section considers the art of successful breadboarding of
high performance analog circuits.

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