ADC AC Measurement

Measure AC performance metrics of ADC output

• Library:

• Mixed-Signal Blockset / ADC / Measurements & Testbenches

• 

Description

The ADC AC Measurement block measures ADC AC performance metrics such as signal to noise ratio (SNR), signal to noise and distortion radio (SINAD), spurious free dynamic range (SFDR), effective number of bits (ENOB), noise floor, and conversion delay. You can use ADC AC Measurement block to validate the ADC architectural models provided in Mixed-Signal Blockset™, or you can use an ADC of your own implementation

Ports

Input

digital — Converted digital signal from ADC

scalar

Converted digital signal from an ADC, specified as a scalar.

Data Types: fixed point | single | double | int8 | int16 | int32 | uint8 | uint16 | uint32

ready — Indicates whether analog to digital conversion is complete

scalar

Indicates whether the analog to digital conversion is complete, specified as a scalar.

Data Types: double

start — External conversion start clock

scalar

External conversion start clock, specified as a scalar. The signal at the start port indicates when the analog to digital conversion process starts.

Data Types: double

Parameters

Distortion measurement type — Type of distortion to measure

Harmonic (default) | Intermodulation

Type of distortion the ADC AC Measurement block is set to measure, specified as Harmonic or Intermodulation.

Programmatic Use

Block parameter: DistortionMeasurement

Type: character vector

Values: Harmonic | Intermodulation

Default: Harmonic

Analog stimulus frequency (Hz) — Frequency of the analog input signal to ADC

positive real scalar | positive real valued vector

Frequency of the analog input signal to an ADC block, specified as a positive real scalar in hertz. Analog stimulus frequency must match the input frequency to the ADC device under test.

• When the ADC AC Measurement is set to measure the Harmonic distortion, the default value of Analog stimulus frequency is 10000.

• When the ADC AC Measurement is set to measure the Intermodulation distortion, the default value of Analog stimulus frequency is [9000,11000].

Analog stimulus frequency needs to satisfy two requirements:

• All the output codes of the ADC must be activated.

• The Analog stimulus frequency must not share any common multiples other than 1 with the Start conversion frequency.

To satisfy both the conditions, use the equation $$f_{\text{analog}}=\frac{J}{M}{f}_{\text{start}}$$ [2],

where:

f_analog is the analog signal frequency,

f_start is the start conversion frequency,

$$M>2^{Nbits}·\pi$$, where Nbits is the number of bits of the ADC,

and J is an integer with no common factors with M.

Programmatic Use

Block parameter: InputFrequency

Type: character vector

Values: positive real scalar | positive real valued vector

Default: 10000

Resolution bandwidth (Hz) — Resolution bandwidth

positive real scalar

Resolution bandwidth, specified as a positive real scalar in hertz. This parameter defines the smallest positive frequency that can be resolved. By default, this parameter is calculated automatically. You can deselect Set automatically to customize the value.

• When the ADC AC Measurement is set to measure the Harmonic distortion, the default value of Resolution bandwidth (Hz) is 1000.

• When the ADC AC Measurement is set to measure the Intermodulation distortion, the default value of Resolution bandwidth (Hz) is 900.

Programmatic Use

Block parameter: RBW

Type: character vector

Values: positive real scalar

Default: 1000

Number of bits — Number of physical bits in ADC

5 (default) | positive real integer

Number of physical bits in ADC, specified as a unitless positive real integer. Number of bits must match the resolution specified in the ADC block.

Programmatic Use

Block parameter: NBits

Type: character vector

Values: positive real integer

Default: 5

Start conversion frequency (Hz) — Frequency of the start conversion clock of the ADC

10e6 (default) | positive real scalar

Frequency of the start conversion clock of the ADC, specified as a positive real scalar in hertz. Start conversion frequency must match the frequency of the start conversion clock of the ADC block.

Programmatic Use

Block parameter: Frequency

Type: character vector

Values: positive real scalar

Default: 10e6

Hold off time (s) — Delays measurement analysis to avoid corruption by transients

0 (default) | nonnegative real scalar

Delays measurement analysis to avoid corruption by transients, specified as a nonnegative real scalar in seconds.

Programmatic Use

Block parameter: HoldOffTime

Type: character vector

Values: nonnegative real scalar

Default: 0

Recommended min. simulation stop time (s) — Minimum time simulation must run for meaningful result

0.009 (default) | positive real scalar

Minimum time the simulation must run to obtain meaningful results, specified as a positive real scalar in seconds.

For AC measurement, the simulation must run so that the ADC can generate 6 spectral updates of the ADC output. The time to generate one spectral output based on Welch's method [1] is given by:

$$t=\frac{1.5·SamplingFrequency}{RBW}$$

where SamplingFrequency and RBW are the sampling frequency and resolution bandwidth of the spectrum estimator inside the ADC Testbench block.

This parameter is only reported by the testbench and is not editable.

Data Types: double

Set as model stop time — Automatically set recommended min. simulation stop time as model stop time

button

Click to automatically set the Recommended min. simulation stop time (s) as the stop time of the Simulink^® model.

Output result to base workspace — Store detailed test results to base workspace

off (default) | on

Store detailed test results in the base workspace for further processing at the end of simulation. By default, this option is not selected.

Workspace variable name — Name of the variable that stores detailed test results

adc_ac_out (default) | character string

Name of the variable that stores detailed test results, specified as a character string.

Dependencies

This parameter is only available when Output result to base workspace is selected.

Programmatic Use

Block parameter: VariableName

Type: character vector

Values: character string

Default: adc_ac_out

Show spectrum analyzer during simulation — Displays spectrum analyzer during simulation

off (default) | on

Displays spectrum analyzer during simulation. By default, this option is not selected.

More About

SNR

Signal to noise ratio or SNR is the ratio of the RMS signal amplitude to the mean value of the root-sum-squares of all other spectral components, excluding the DC and first five harmonics.

SINAD

Signal to noise and distortion ratio, or SINAD is the ratio of the RMS signal amplitude to the mean value of the root-sum-squares of all other spectral components and harmonics, excluding DC.

SFDR

Spurious free dynamic range or SFDR is the ratio of the RMS signal amplitude to the RMS value of the peak spurious content, measured over the entire first Nyquist zone (DC to half of sampling frequency).

ENOB

Effective number of bits or ENOB represents the actual resolution of an ADC after considering internal noise and errors. It is given by $$\text{ENOB}=\frac{\text{SINAD}-1.76}{6.02}$$.

References

[1] Spectrum Analyzer, DSP System Toolbox, MathWorks Documentation.

[2] IEEE Std 1241-2010. "IEEE Standard for Terminology and Test Methods for Analog-to-Digital Converters," pp. 29-30, 14 January 2011.