SAR ADC

N-bit successive approximation register (SAR) based ADC

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Libraries:
Mixed-Signal Blockset / ADC / Architectures

Description

Successive Approximation Register (SAR) based ADC consists of a sample and hold circuit (SHA), a comparator, an internal digital to analog converter (DAC), and a successive approximation register.

When the ADC receives the start command, SHA is placed in hold mode. The most significant bit (MSB) of the SAR is set to logic 1, and all other bits are set to logic 0.

The output of the SAR is fed back to a DAC, whose output is compared with the incoming input signal. If the DAC output is greater than the analog input, MSB is reset, otherwise it is left set. The next MSB is now set to 1, and the process is repeated until every bit the SAR is compared. The final value of the SAR at the end of this process corresponds to the analog input value. The end of the conversion process is indicated by the ready signal.

Ports

Input

analog — Analog input signal

scalar

Analog input signal, specified as a scalar.

Data Types: double

start — External clock to start conversion

scalar

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

Data Types: double

Output

digital — Converted digital output signal

scalar

Converted digital output signal, returned as a scalar.

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

ready — Determines whether analog to digital conversion is complete

scalar

Determines whether the analog to digital conversion is complete, returned as a scalar.

Data Types: double

Parameters

Configuration

Number of bits — Number of physical output bits

8 (default) | positive real integer in the range [1, 26]

Number of physical output bits, specified as a unitless positive real integer in the range [1, 26]. Number of bits determines the resolution of the ADC.

Programmatic Use

• Use get_param(gcb,'NBits') to view the current Number of bits.

• Use set_param(gcb,'NBits',value) to set Number of bits to a specific value.

Data Types: double

Input range (V) — ADC dynamic range

[-1 1] (default) | 2-element row vector

ADC dynamic range, specified as a 2-element row vector in volts.

Programmatic Use

Block parameter: InputRange

Type: character vector

Values: 2-element row vector

Default: [-1 1]

Data Types: double

Use external start clock — Connect to external start conversion clock

on (default) | off

Select to connect to an external start conversion clock. By default, this option is selected. If you deselect this option, a Sampling Clock Source block inside the SAR ADC is used to generate the start conversion clock.

Conversion start frequency (Hz) — Frequency of internal start conversion clock

10e3 (default) | positive real scalar

Frequency of internal start conversion clock, specified as a positive real scalar in Hz. Conversion start frequency determines the rate of the ADC.

Dependencies

This parameter is only available when Use external start clock is not selected.

Programmatic Use

• Use get_param(gcb,'StartFreq') to view the current value of Conversion start frequency (Hz).

• Use set_param(gcb,'StartFreq',value) to set Conversion start frequency (Hz) to a specific value.

Data Types: double

RMS aperture jitter (s) — RMS aperture jitter added to the start conversion clock

0 (default) | real nonnegative scalar

RMS aperture jitter added as an impairment to the start conversion clock, specified as a real nonnegative scalar in s. Set RMS aperture jitter value to zero if you want a clean clock signal.

Dependencies

This parameter is only available when Use external start clock is not selected.

Programmatic Use

• Use get_param(gcb,'StartClkJitter') to view the current value of RMS aperture jitter (s).

• Use set_param(gcb,'StartClkJitter',value) to set RMS aperture jitter (s) to a specific value.

Data Types: double

SAR Frequency (Hz) — Frequency of SAR clock

2e7 (default) | real scalar

Frequency of the SAR clock, specified as a real scalar in Hz. SAR Frequency (Hz) must be high enough to allow the ADC to perform Nbits comparison, where Nbits is the Number of bits of the ADC. The block has one cycle overhead due to algebraic loop removal. So, the clock must run for one additional cycle before the output is ready. So, the SAR Frequency (Hz) (f_SAR) is given by the equation $$f_{\text{SAR}}\ge (Nbits+1)f_{\text{start}}$$, where f_start is the Conversion start frequency.

Programmatic Use

• Use get_param(gcb,'SARFreq') to view the current value of SAR Frequency (Hz).

• Use set_param(gcb,'SARFreq',value) to set SAR Frequency (Hz) to a specific value.

Match input scale — Inherit output polarity and data type from input

off (default) | on

Inherit the output polarity and data type from the analog input signal to the ADC. If you select this option, the ADC ignores the Output polarity and Output data type parameters. The ADC output has the same polarity as the input and the data type is double.

Output polarity — Defines ADC output polarity

Auto (default) | Bipolar | Unipolar

Defines the ADC output data polarity.

If Output polarity is set to Auto, the minimum and maximum values of the output are determined by the polarity of the Input range.

If Output polarity is set to Bipolar, the outputs are between -2Nbits-1 and 2Nbits-1-1.

If Output polarity is set to Unipolar, the outputs are between 0 and 2Nbits-1.

Dependencies

To enable this parameter, deselect the Match input scale option.

Programmatic Use

Block parameter: OutputPolarity

Type: character vector

Values: Auto | Bipolar | Unipolar

Default: Auto

Output data type — Defines ADC output data type

fixdt(1,Nbits) (default) | fixdt(0,Nbits) | double | single | int8 | int16 | int32 | uint8 | uint16 | uint32 | Inherit: Inherit via back propagation

Defines ADC output data type.

Unsigned integers and fixed-point types (fixdt(0,Nbits)) are not available when the Output polarity is set to Bipolar or Auto.

Signed integers and fixed-point types (fixdt(1,Nbits)) are not available when the Output polarity is set to Unipolar.

Dependencies

To enable this parameter, deselect the Match input scale option.

Programmatic Use

Block parameter: OutDataType

Type: character vector

Values: double | single | int8 | int16 | int32 | uint8 | uint16 | uint32 | fixdt(0,Nbits) | fixdt(1,Nbits) | Inherit: Inherit via back propagation

Default: fixdt(1,Nbits)

Impairments

Enable Linearity Impairments — Enable linearity impairments in ADC simulation

off (default) | on

Select to enable linearity impairments such as offset error and gain error in ADC simulation. By default, this option is deselected.

Offset error — Shifts quantization steps by specific value

1 LSB (default) | real scalar

Shifts quantization steps by specific value, specified as a scalar in least significant bit (LSB) or %.

Note

The full scale range of the converter is defined as the difference between the last and first code on the +0.5 LSB compensated transfer curve. In a +0.5 LSB compensated transfer curve, first code is 0.5 LSB wide while the last code is 1.5 LSB wide. The input values must be considered within the full scale range of the converter.

Note

LSB is calculated by the equation $$\text{LSB = }\frac{\text{Full scale range of converter}}{2^{NBits}}$$.

Dependencies

This parameter is only available when Enable Linearity Impairments is selected.

Programmatic Use

• Use get_param(gcb,'OffsetError') to view the current value of Offset error (LSB).

• Use set_param(gcb,'OffsetError',value) to set Offset error (LSB) to a specific value.

Data Types: double

Gain error — Error on slope of ADC transfer curve

2 LSB (default) | real scalar

Error on the slope of the straight line interpolating ADC transfer curve, specified as a real scalar in %FS, FS, or LSB.

Note

The full scale range of the converter is defined as the difference between the last and first code on the +0.5 LSB compensated transfer curve. In a +0.5 LSB compensated transfer curve, first code is 0.5 LSB wide while the last code is 1.5 LSB wide. The input values must be considered within the full scale range of the converter.

Note

LSB is calculated by the equation $$\text{LSB = }\frac{\text{Full scale range of converter}}{2^{NBits}}$$.

Dependencies

To enable this parameter, select Enable Linearity Impairments in the Impairments tab.

Programmatic Use

Block parameter: GainError

Type: character vector

Values: real scalar

Default: 2 LSB

Data Types: double

Version History

Introduced in R2019a