High-Speed A/D performance metrics

在高速情况下，主要考虑信号参数的频率范围、失真和噪声。During system definition，setting time 、 acquisition time、static precision-related（integral nonlinearity INL、differential nonlinearity DNL）tend not to be primary parametes.

Analog Bandwidth

 a measure of the analog signal integrity versus increasing frequency.

In the second or third Nyquist zones the input signal frequency is near or above the sampling frequency.

Signal-to-Noise Ratio

SNR:dBc(dB relative to carrier:the effective noise floor of the converter device) or dBFS(dB relative to the A/D full-scale: amaximunm noise-limited dynamic range)

Spuious-Free Dynamic Range

The ratio of the power of the fundamental to the highest other 

spectral component (either spur or harmonic).

Third-Order Intermodulation Distortion (IMD3)

IMD3 is the ratio of the power of the fundamental (at frequencies f and f ) to the power of the worst spectral component at either frequency 2f – f or 2f – f .

 

 

Amplifier Performance Metrics

General factors:Gain. Dc-coupled sigal source. Conversion of a single-ended path to a differerntial signal path. Lever shift.

Small-Signal Bandwidth 类似于对输入信号的最小响应时间

where f–3dB is the –3dB small-signal bandwidth.

a measure of the ability of a device to act as a linear gain element over a defined frequency span. It is implicitly assumed that the signal level is small enough that its presence does not change the operating point of the amplifier; this metric also serves as the noise bandwidth of the amplifier.

Slew Rate/Large-Signal Bandwidth

the measurement range for ΔVOUT is the voltage deflection that covers 10% to 90% of the rising edge of the output waveform (or 90% to 10% of the falling edge).

在开环情况下得到上述公式，输入信号太快，输出无法响应会造成失真。

As a first-order approximation, the highest frequency in the desired signal should be five times less than

the large-signal BW of the amplifier if distortion is to be minimized.？？输入信号最大频率<<放大器信号频率的1/5.

Noise/SNR

关于模型可以参考笔记 SLVA043、SBOA066

在低频时flicker noise or 1/f noise     1M 噪声小于0.06

高频时 flat-band or thermal noise（频域上是常数）10KHz以上为主要噪声

Selecting an Amplifier

在放大器前做滤波最好了。For optimal system performance, it is generally best to place antialiasing filters before the driver amplifier.The interface filter is generally a low-pass filter (LF) or bandpass filter (BF) to limit the out-of-band noise.Recall that flat-band noise on the amplifier output has a wide bandwidth, and if the out-of-band noise is not attenuated, it can alias into the passband, thus increasing the noise floor.

Noise

LF 66MHz，The equivalent noise bandwidth of a single-pole RC filter is (p/2)f–3dB. This value can be interpreted as the cutoff frequency of an ideal brick-wall filter. If passband flatness is critical, then the –3-dB corner of a single-pole RC filter must be 6.5 times greater than the highest signal frequency. 滤波带宽6.5倍于输入 SLWA053，SBAA108

Many high-speed applications require an operating bandwidth that is much greater than the op amp 1/f noise bandwidth（对低频影响大）。

噪声相互独立，故

The 66-MHz, single-pole, low-pass filter has an equivalent noise bandwidth of (p/2) × f–3dB = 1.57 × 66 MHz = 104 MHz (see SLVA043), which allows the calculation of an equivalent filter output noise voltage over the passband.

 

 

Distortion SFDR

SLYT133；amplifier distortion products and ADC distortion products are correlated with one another。The result of this condition is that the analysis is restricted to a worst-case assumption that individual amplifier distortion components and individual ADC distortion components sum as voltages rather than as powers。

ADC distortion：the spurious-free dynamic range (SFDR), the worst-case spur relative to the full-scale voltage, which may not be harmonically related to the desired signal.

Amplifier distortion：HD2, HD3, and/or IMD2/3, which are all harmonically related to the desired signal。HD=harmonic distrion

 

If the voltages are referenced to the ADC full-scale voltage and the signal is a simple sinusoid (carrier),

the expression becomes:

Where no distinction has yet been made between HD2 or HD3, and the quantities in the second row of the equation are linear and have no units. Note that HD in this case refers to a single harmonic spur—such as HD2 or HD3—and not the composite total harmonic distortion (THD).

many board-related factors can specifically affect HD2 and have little impact on HD3.

These 

factors include power supply bypassing as well as mechanical and electrical symmetry of the layout. If a 

differential signal path is unsymmetrical in terms of component values and/or signal path routing, then its 

ability of the differential signal path to provide the natural attenuation of HD2 can be compromised. For 

additional information on best practices for high-speed PC board layouts, see application report SBAA113.

the first Nyquist zone？？