"高精度ADC噪声分析与设计技巧"

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This e-book, "Fundamentals of Precision ADC Noise Analysis," published by Texas Instruments in September 2020, provides a detailed analysis of ADC noise in high-precision laboratory settings. The book covers the different types of ADC noise and offers design tips and tricks to reduce noise using delta-sigma ADCs. The authors delve into the fundamentals of ADC noise analysis, exploring the various types of noise that can affect the accuracy and precision of ADC measurements. They provide insights into the sources of noise, such as thermal noise, flicker noise, and quantization noise, and discuss the impact of noise on the overall performance of ADCs. One of the key areas of focus is the analysis of noise in Texas Instruments' high-precision ADC series, offering a comprehensive understanding of the noise characteristics specific to these devices. Through this analysis, the authors aim to provide engineers and researchers with the knowledge and tools necessary to optimize the performance of precision ADCs in their applications. In addition to understanding the different types of noise, the book also presents practical design tips and tricks to minimize noise in delta-sigma ADCs. This includes recommendations for the layout and grounding of the ADC circuit, as well as considerations for analog and digital signal processing to mitigate noise effects. Overall, the e-book serves as a valuable resource for anyone working with precision ADCs, offering in-depth insights into ADC noise analysis and practical guidance for noise reduction. With its comprehensive coverage of the topic and practical recommendations, it is a must-read for engineers, researchers, and anyone looking to optimize the performance of precision ADCs in their designs. This e-book from Texas Instruments is a valuable contribution to the field of ADC noise analysis, providing engineers with the knowledge and tools to address noise challenges and achieve high-precision measurements in their applications.

Fundamentals of Precision ADC Noise Analysis 12 September 2020 I Texas Instruments

Chapter 1: Introduction to ADC noise

Absolute vs. relative noise parameters

An important characteristic of all of the equations in Table 2

is that they involve some ratio of values. These are “relative

parameters.” As the name implies, these parameters provide

a noise performance metric relative to some absolute value,

usually the input signal (decibels relative to carrier [dBc]) or

the FSR (decibels relative to full scale [dBFS]).

Figure 12 shows an output spectrum of the ADS127L01

using an input signal at –0.5 dBFS, where full scale is 2.5V.

If you choose a system input signal that is not referenced to

the same full-scale voltage, or if the input signal amplitude

varies from the value deﬁned in the data sheet, you should

not necessarily expect to achieve data-sheet performance,

even if all of your other input conditions are identical.

Similarly, for DC noise parameters, you can see from Table 2

that effective resolution is relative to the ADC’s input-referred

noise performance at the given operating conditions, as

well as at the ADC’s FSR. Since FSR depends on the ADC’s

reference voltage, using a reference voltage other than

what’s listed in the data sheet has an effect on your ADC’s

performance metrics.

For high-resolution ADCs, increasing the reference voltage

increases the maximum input dynamic range, while input-

referred noise stays the same. That is because high-

resolution ADC noise performance is largely independent of

the reference voltage. For low-resolution ADCs, where noise

is dominated by LSB size, increasing the reference voltage

actually increases input-referred noise, while the maximum

input dynamic range remains approximately the same.

Table3 on the following page summarizes these effects.

Noise parameter Definition Noise test Equation (units)

Input-referred noise Resolution or internal noise of the ADC (plus programmable gain amplifier [PGA]

noise for integrated devices) specified as a noise voltage source applied to the

ADC’s input pins (before gain).

n/a

Measured (V ,V )

RMS PP

SNR Ratio of the output signal amplitude to the output noise level, not including

harmonics or DC.

Input sine wave (AC)

10log (dBc)

()

Signal(RMS)

Noise(RMS)

THD Indication of a circuit’s linearity in terms of its effect on the harmonic content of a

signal, given as the ratio of the summed harmonics to the RMS signal amplitude.

Input sine wave (AC)

10log

(dBc)

()

Harmonics



()V

Signal(RMS)

SINAD Ratio of the RMS value of the output signal to the RMS value of all other spectral

components, not including DC.

Input sine wave (AC)

10log

(dBc)

()

Harmonics



Noise(RMS)

+ V

()V

Signal(RMS)

Effective resolution Dynamic range figure of merit using the ratio of full-scale range (FSR) to RMS

noise voltage to define the noise performance of an ADC.

DC-input (DC)

log

(bits)

FSR

(

)

N,RMS

Noise-free resolution Dynamic range figure of merit using the ratio of FSR to peak-to-peak noise

voltage to define the maximum number of bits unaffected by peak-to-peak noise.

DC-input (DC)

log

(bits)

FSR

(

)

N,PP

Noise-free counts Figure of merit representing the number of noise-free codes (or counts) that you

can realize with noise.

DC-input (DC)

()Noise-free resolution

(counts)

ENOB Figure of merit relating the SINAD performance to that of an ideal ADC’s

resolution with a certain number of bits (given by the ENOB).

Input sine wave (AC)

(bits)

SINAD( )–1.76dBc dB

6.02

Table 2. Typical ADC noise parameters with deﬁnitions and equations.

Frequency (kHz)

Amplitude (dB)

04080 120 160 200 240

-180

-160

-140

-120

-100

-80

-60

-40

-20

=4kHz, V

=–0.5 dBFS, HR mode, WB2, 512 kSPS,

32768 samples

ADS127L01

Figure 12. ADS127L01 FFT, with the input voltage (V

) referenced to

fullscale.

Table 2 summarizes AC and DC noise parameters, their deﬁnitions, the applicable noise test and equations.

Fundamentals of Precision ADC Noise Analysis 13 September 2020 I Texas Instruments

Chapter 1: Introduction to ADC noise

Therefore, to characterize an ADC’s maximum dynamic

range, most ADC manufacturers specify effective resolution

and noise-free resolution, using the assumption that the FSR

is maximized. Or, in other words, if your system does not

use the maximum FSR (or whatever FSR the manufacturer

used to characterize the ADC), you should not expect to

achieve the effective or noise-free resolution values speciﬁed

in the data sheet.

Let’s illustrate this point by using a 1-V reference voltage

with an ADC whose data-sheet noise is characterized with a

reference voltage of 2.5 V. Continuing with the ADS127L01

as an example, Table 4 shows that using a 2.5-V reference

voltage and a 2-kSPS data rate in very low power mode

results in 1.34 µV

RMS

of input-referred noise and an effective

resolution of 21.83 bits.

However, using a 1-V reference voltage reduces the FSR

to 2 V. You can use this 2-V value to calculate the new

expected effective resolution (dynamic range), given by

Equation 5:

(5)

Changing the reference voltage reduces the ADC’s FSR,

which in turn reduces its effective resolution (dynamic range)

compared to the data-sheet value by more than 1.3 bits.

Equation 6 generalizes this loss of resolution:

(6)

where % utilization is simply the ratio of the actual FSR to

the FSR at which the ADC’s noise is characterized.

While this apparent resolution loss may seem like a

drawback to using high-resolution delta-sigma ADCs, recall

that while the FSR is decreasing, the input-referred noise

is not. Therefore, I suggest performing ADC noise analysis

using an absolute noise parameter, or one that is measured

directly. Using an absolute noise parameter eliminates the

dependence on the input signal and reference voltage

characteristic of relative noise parameters. Additionally,

absolute parameters simplify the relationship between ADC

noise and system noise.

For ADC noise analysis, I recommend using input-

referred noise. I’ve bolded this phrase because it’s not

common practice to use input-referred noise to deﬁne

ADC performance. In fact, a majority of engineers speak

exclusively in terms of relative parameters such as effective

and noise-free resolution, and are deeply concerned when

they cannot maximize those values. After all, if you need to

use a 24-bit ADC to achieve 16-bit effective resolution, it

feels like you’re paying for performance that the ADC cannot

actually provide.

Reference voltage Parameter

Low-resolution

ADCs

High-resolution

ADCs

Increases

Dynamic range Stays the same Increases

Input-referred noise Increases Stays the same

Decreases

Dynamic range Stays the same Decreases

Input-referred noise Decreases Stays the same

Table 3. Effect of changing reference voltage on ADC noise parameters.

Mode

Data rate

(SPS)

RMS_noise

(µV

RMS

) ENOB

High resolution (HR)

512,000 7.40 19.37

128,000 5.12 19.90

32,000 2.74 20.80

8,000 1.41 21.76

Low power (LP)

256,000 7.22 19.40

64,000 4.97 19.94

16,000 2.65 20.85

4,000 1.37 21.80

Very-low power

(VLP)

128,000 6.97 19.45

32,000 4.80 19.99

8,000 2.57 20.89

2,000 1.34 21.83

Table 4. ADS127L01 noise performance: low-latency ﬁlter, AVDD = 3V,

DVDD = 1.8 V and V

REF

= 2.5 V.

log

= log

–6

= 20.51 bits

FSR 2 V

((

))

Noise,RMS

V 1.34×10 V

= –1.32

bits

= log (% ) = logutilization

2 V

(

)

resolution (dynamic

range) loss

Fundamentals of Precision ADC Noise Analysis 14 September 2020 I Texas Instruments

Chapter 1: Introduction to ADC noise

However, an effective resolution of 16 bits doesn’t

necessarily tell you anything about how much of the FSR

you use. You may only need 16 bits of effective resolution,

but if the minimum input signal is 50 nV, you will never be

able to resolve that with a 16-bit ADC. Therefore, the true

beneﬁt of a high-resolution delta-sigma ADC is the low

levels of input-referred noise it offers. It does not mean that

effective resolution is unimportant—just that it is not the best

way to parameterize a system.

Ultimately, if the ADC cannot resolve both the minimum

and maximum input signals, maximizing SNR or effective

resolution is irrelevant. And unlike effective resolution, you

can typically derive the ADC’s required input-referred noise

directly and easily from the system speciﬁcations. This

characteristic makes input-referred noise analysis more

ﬂexible to system changes. Additionally, it enables easy

comparison between different ADCs in order to choose a

speciﬁc ADC for any application.

Key takeaways

Here is a summary of important points to better

understand ADC noise measurement methods

andparameters:

Different measurements quantify

different types ofnoise:

• To measure AC noise performance, use

the AC-signal applied test.

• To measure DC noise performance, use

the input-short test.

• ADC end applications typically determine

the noise measurement type.

Noise is introduced via all signal-

chain components.

• In general, assume that the input signal is

equal to the FSR.

There are two types of

noise parameters:

• Relative—calculated using a ratio of

measuredvalues.

• Absolute—measured directly.

Input-referred noise is the absolute

measure of an ADC’s resolution (the

smallest measurable signal). Noise-

free bits and effective resolution are

relative parameters that describe the

dynamic range of theADC.

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