ADI高速ADC测试与评估方法详解
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更新于2024-07-19
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高速ADC测试与评估是现代电子设计中的关键环节，特别是在模拟器件制造商Analog Devices, Inc.的高速转换器组中。本文档，AN-835，作为应用笔记，详细阐述了该公司在高精度ADC（模拟到数字转换器）的特性分析和生产测试过程中采用的方法。它并非产品数据手册的替代品，而是为工程师提供了一种理解和优化高性能ADC性能的参考指南。
文档首先定义了测试的范围，强调了其作为参考材料的作用，但用户在实际操作时仍需参照产品数据表以获取完整的技术参数。动态测试部分是文档的核心，其中涉及了关键的硬件设置。测试过程通常包括：
1. 信号发生器：用于生成模拟信号，模拟真实世界的应用场景，以便对ADC进行全面的性能验证。
2. 带通滤波器：用来过滤掉信号中的噪声，确保测试结果的准确性，仅保留需要转换的信号频率范围。
3. 测试适配器或框架：为信号源和ADC之间的连接提供物理支持，并可能包含环境控制，如温度和电源管理。
4. 低噪声电源：确保测试环境中的电源稳定且噪声水平足够低，以减少干扰。
5. 编码源：通常集成在评估板上，负责信号的编码和解码，以供ADC处理。
6. 数据采集模块：用于捕获和记录ADC输出的数据，这是衡量性能的关键步骤，包括采样率、信噪比(SNR)、失真度（如SINAD和最差杂散抑制）等指标。
7. 数据分析软件：用于解析和评估采集的数据，通过复杂的算法计算各项性能指标，如通过计算噪声密度来评估信噪比(SNR)。
Analog Devices提供了专用的应用硬件和软件工具，旨在帮助用户在实验室环境中高效地进行ADC的性能评估。这些工具不仅简化了测试流程，还能够确保结果的可重复性和一致性。
AN-835提供了一个全面的框架，帮助工程师理解高速ADC测试的各个方面，从理论原理到实际操作方法，这对于确保这些高精度器件在实际应用中的性能至关重要。在实际操作中，结合产品数据表和其他相关技术文档，用户可以更准确地评估ADC的性能，从而做出更好的设计决策。
AN-835 Application Note
Rev. B | Page 6 of 28
ANALOG SIGNAL SOURCE
Usually, dynamic testing employs a Rohde & Schwarz SMA/
SMHU/SMG/SMGU, an Agilent 8644 signal generator, a Wenzel
crystal oscillators or a Valpey Fisher crystal oscillator. These
sources have proven to provide exceptional performance (low
phase noise, flat frequency response, and reasonable harmonic
performance) for frequencies of a few kilohertz to those of a few
gigahertz. Harmonic performance of these generators is typically
not as good as the intrinsic linearity of a given ADC, mandating
the need for additional filtering between the signal generator
and the analog input to the ADC.
ANALOG SIGNAL FILTER
Both fixed frequency and tunable frequency band-pass filters
are used for device testing. The fixed frequency filters are typically
smaller than tunable filters and often provide slightly better
performance. Tunable filters allow testing across a wide range
of frequencies using one filter. Several filter manufacturers, including
K&L Microwave, TTE, and Allen Avionics, Inc., provide excellent
filters for ADC testing.
There are two types of filters that are often used for ADC testing:
low-pass filters and band-pass filters. These can be used
individually or combined to yield the level of performance
required for an application.
Low-pass filters are a good choice when a wide range of analog
frequencies must be applied to the ADC. However, they allow
noise to pass from the signal generator to the ADC. This noise
may reduce the level of performance measured for the ADC. A
typical low-pass filter is the J97 available from TTE. Usually, low-
pass filters have a transition band that defines where the pass
band ends and the stop band begins. Along with this specification,
a guaranteed stop-band rejection is specified. In the case of the
J97, the transition band is defined to be between 1.0 times and
1.2 times the 3 dB frequency, and the guaranteed stop-band
rejection is 80 dB. Energy beyond 1.2 times the 3 dB frequency
is reduced by a minimum of 80 dB.
Band-pass filters are used when analog frequencies are fixed
and will not be changed. Band-pass filters also eliminate much
of the wideband noise generated by signal sources and typically
provide the best performance for ADC testing. Filters such as
TTE’s Q70T series have a bandwidth defined as a percentage of
the center frequency. The more narrow the bandwidth, the less
noise that passes through the filter; however, the analog frequency
is more restricted, and there is a greater insertion loss. When a
center frequency is chosen, the bandwidth can be determined.
Ideally, a bandwidth of 5% to 6% should be selected, keeping in
mind that good noise performance is being traded for analog
frequency flexibility. As with low-pass filters, the band-pass filter
has a transition band that defines the shape between the 3 dB
frequency (above and below the center frequency) and the
frequency of the guaranteed stop-band performance. In the
case of TTE’s Q70T, the stop-band rejection is 60 dB.
0
–10
–20
–30
–40
–50
–60
40 1201008060
ATTENUATION (dB)
FREQUENCY (MHz)
05941-005
IL 0.3dB @ 10MHz
Figure 5. Typical Performance of TTE’s J97
0
–10
–20
–30
–40
–50
–60
85 11511095 100 10590
ATTENUATION (dB)
FREQUENCY (MHz)
08941-006
IL 3dB @ 100MHz
Figure 6. Typical Band-Pass Performance of TTE’s Q70T
As noted previously, band-pass filters may only have a stop-band
rejection of 60 dB, meaning that signals that fall into the stop
band will be rejected by 60 dB. If, for example, a signal source
has a harmonic that is 25 dB below the fundamental, the effective
level of the harmonic is −85 dBc after a Q70T filter. For many
high performance ADCs, this is not sufficient. When performance
of −100 dBc or better is required, it is common to cascade a
band-pass filter with a low-pass filter. When selecting a low-pass
filter to follow a band-pass filter, the low-pass frequency should be
selected such that stop-band performance of the low-pass filter
optimally filters any harmonics that pass through the band-pass
filter. With a J97 low-pass filter, stop-band rejection is reached at
1.2 times the 3 dB frequency. If the second harmonic of the band-
pass filter is set equal to 1.4 times the low-pass 3 dB frequency, it
ensures that all harmonics passing through the band-pass filter are
filtered and that the additional insertion loss of the low-pass filter
does not significantly reduce the level of the desired pass band. In
this case, the low-pass frequency should equal 1.4 times the band-
pass frequency, and theoretically, the cascaded rejection should
be about 140 dB. Although this is difficult to achieve in practice
due to coupling and radiation effects, this technique is a useful
one and can achieve well beyond −100 dBc harmonic rejection.
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