SAR ADC Symbol
Definition: The SAR ADC symbol represents a Successive Approximation Register analog-to-digital converter that converts an analog input voltage to an n-bit digital code by iteratively comparing the input to a binary-weighted DAC output over n clock cycles, depicted as a rectangular block with analog input pin AIN, reference voltage pin REF, digital output pin DOUT, and chip-select pin CS, with common implementations including 12-bit devices such as the MCP3208.
Also known as: successive approximation ADC, SAR converter, successive approximation register ADC, SAR analog-to-digital converter, MCP3208.
What the SAR ADC symbol means
The SAR ADC symbol in a circuit diagram represents the most widely used architecture for medium-speed, high-resolution analog-to-digital conversion. The SAR ADC accepts an analog voltage at its AIN pin, compares it to a precision reference voltage at REF, and produces an n-bit digital result at DOUT over n conversion clock cycles — one bit resolved per cycle using a binary search algorithm.
In data-acquisition schematics, the SAR ADC symbol indicates where continuous analog signals (from sensors, transducers, or signal-conditioning stages) are converted to digital values for processing by a microcontroller, FPGA, or DSP. The CS pin controls when the device begins conversion or drives its output onto a shared SPI bus, allowing multiple ADCs to share a single microcontroller interface.
How to identify the SAR ADC symbol
The SAR ADC symbol is drawn as a rectangle labelled 'SAR ADC' or 'ADC' with AIN (analog input) and REF (reference voltage) pins on the left edge, and DOUT (digital serial data output) and CS (chip select, active low) pins on the right edge. A clock pin (CLK or SCLK) is often shown for the SPI interface. The symbol resembles a generic IC block and is distinguished by the AIN/REF/DOUT/CS pin labelling. More detailed symbols may show an MISO/MOSI/SCLK/CS SPI interface group.
Function in a circuit
A SAR ADC converts an analog input in n clock cycles using a successive approximation algorithm. In cycle 1, the MSB (bit n−1) is tested: a DAC within the ADC generates Vref/2 and the comparator checks whether AIN > Vref/2. If yes, the MSB is set to 1 and retained; if no, it is cleared to 0. In cycle 2, the next MSB is tested by adding or subtracting Vref/4 to the previous result. This binary search continues for n cycles until all bits are resolved. The final n-bit code is clocked out serially on DOUT (SPI-compatible) or presented in parallel, with CS controlling bus access.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617 does not define a dedicated symbol for SAR ADC circuits. IEC practice uses a functional block rectangle labelled 'A/D' or 'ADC' with labelled signal pins per IEC 60617-12. |
|---|---|
| ANSI/IEEE 315 | ANSI/IEEE 315 specifies a general analog-to-digital converter symbol as a rectangular block with an 'A/D' label and appropriate pin annotations. Specific SAR architecture is not called out — the architecture is identified by device part number or annotation. |
| Key difference | No IEC vs ANSI difference in SAR ADC symbol representation. Both use a generic A/D rectangular block. Architecture identification (SAR vs sigma-delta vs pipeline) is conveyed by device label, not symbol shape. |
Terminals / pins
| Pin | Name |
|---|---|
| ain | AIN |
| ref | REF |
| dout | DOUT |
| cs | CS |
Typical values
Resolution: 8 to 18 bits (common SAR range); 12-bit is the most common general-purpose choice. Sample rate: 100 kSPS to 5 MSPS (architecture and power budget dependent). Reference voltage (REF): typically VDD or an external precision reference (e.g., 4.096 V). Supply voltage: 2.7–5.5 V (device dependent). SPI clock: up to 2 MHz (MCP3208). INL/DNL: ±0.5–1 LSB typical for 12-bit devices. Input range: 0 to VREF (single-ended) or ±VREF/2 (differential).
Where the SAR ADC symbol is used
- Microcontroller-based sensor data-acquisition — converting temperature, pressure, and current sensor outputs to 12-bit digital values via SPI
- Industrial process control — 12–16 bit SAR ADCs on PLC analog input modules for 4–20 mA loop digitisation
- Medical instrumentation — 16–18 bit SAR ADCs for ECG, blood pressure, and EEG signal digitisation requiring high resolution and low noise
- Battery management systems — measuring cell voltages and currents with 12–16 bit ADCs for charge and discharge control
- Audio codec front ends — 16-bit SAR ADCs for voice-band (8 kHz) and audio-band (48 kHz) sampling in telephony equipment
- Motor control current sensing — 12-bit SAR ADC sampling phase current every PWM switching period for field-oriented control algorithms
Example
In a Raspberry Pi data-acquisition project, the SAR ADC symbol shows AIN connected to a 0–4.096 V analog sensor output, REF connected to a 4.096 V precision voltage reference, DOUT connected to the Raspberry Pi MISO pin, CS to GPIO 8 (CE0), and CLK to SCLK; the host reads the 12-bit result by asserting CS LOW, clocking 16 bits on the SPI interface at 1 MHz, and extracting the 12-bit value from the MCP3208 response packet.
Key facts
- SAR ADCs resolve one bit per clock cycle, requiring exactly n cycles for an n-bit conversion; a 12-bit SAR ADC requires 12 clock cycles plus overhead for a total conversion time of typically 1–10 µs.
- The MCP3208 (Microchip) is an 8-channel, 12-bit, 100 kSPS SAR ADC with SPI interface commonly used with Arduino and Raspberry Pi; it operates from 2.7–5.5 V with a maximum SPI clock of 2 MHz.
- SAR ADCs require a sample-and-hold (S/H) circuit at the input, which is typically integrated on-chip, to freeze the input signal during the multi-cycle conversion period and prevent aperture error.
- Total Harmonic Distortion (THD) and Signal-to-Noise-and-Distortion ratio (SINAD) are the key dynamic accuracy metrics for SAR ADCs; a 12-bit ADC has a theoretical SINAD of 74 dB (6.02 × N + 1.76).
- The SAR architecture achieves the best balance of resolution, speed, and power consumption for medium-speed applications (1 kSPS–5 MSPS); sigma-delta ADCs exceed SAR in resolution at low speeds; pipeline ADCs exceed SAR in speed at lower resolution.
- A precision external voltage reference connected to the REF pin (e.g., LM4040, REF3040) is essential for achieving the ADC's specified accuracy; using VDD as the reference couples power-supply noise into the measurement.
- Reference designator for ADCs in schematics is A or U followed by a number (e.g., U1, A1); no single IEC 60617 designator is universally standardised for ADCs.
Frequently asked questions
What does the SAR ADC symbol mean in a circuit diagram?
The SAR ADC symbol represents a successive approximation register analog-to-digital converter that converts an analog voltage at AIN to an n-bit digital code at DOUT over n clock cycles. It is the standard block for adding analog measurement capability to a microcontroller or digital system via SPI or parallel interface.
What does a SAR ADC symbol look like?
The SAR ADC symbol is a rectangle labelled 'SAR ADC' or 'ADC' with AIN (analog input) and REF (reference voltage) pins on the left, and DOUT (serial data output) and CS (chip select) pins on the right. A CLK pin for the SPI clock is typically also shown. The block resembles a generic IC but is identified by its AIN/REF/DOUT/CS labelling.
How does a successive approximation ADC work?
A SAR ADC uses a binary search: in the first clock cycle it tests if the input exceeds VREF/2 and sets the MSB accordingly. Each subsequent cycle tests the next bit by adding or subtracting VREF/2^n to narrow the range. After n cycles, all n bits are resolved and the result is output on DOUT. An n-bit SAR ADC requires exactly n conversion clock cycles.
What is the difference between a SAR ADC and a sigma-delta ADC?
A SAR ADC resolves one bit per cycle with a Nyquist sampling rate, achieving medium-to-high resolution (8–18 bit) at moderate speeds (1 kSPS–5 MSPS). A sigma-delta ADC uses oversampling and noise shaping to achieve very high resolution (16–24 bit) at low speeds (1 SPS–250 kSPS). SAR ADCs are preferred for general-purpose and multi-channel applications; sigma-delta for precision instrumentation.
What is the reference pin (REF) on an SAR ADC?
The REF pin sets the full-scale input voltage range for the ADC. An input equal to VREF produces the maximum digital code (2^n − 1); an input of 0 V produces code 0. A precision external reference (e.g., 4.096 V) improves accuracy; using VDD as VREF couples power-supply noise into the conversion result.
What resolution SAR ADC should I use?
8-bit resolution (256 steps) suffices for coarse measurements such as light levels or rough temperature readings. 12-bit (4096 steps) is the standard choice for sensor data-acquisition, motor control, and audio. 16-bit is used for precision instruments and medical devices where the smallest detectable change must be below 1 mV on a 5 V range.
What standard defines the SAR ADC symbol?
No single IEC 60617 or ANSI/IEEE 315 standard defines a dedicated SAR ADC symbol. IEEE 315 specifies a generic A/D converter rectangular block. The SAR architecture is identified by device label or annotation, not by a distinct symbol shape. IC manufacturer datasheets (Microchip, TI, Analog Devices) establish the de-facto pin-label conventions.
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