Microchip MCP4921-E/MS 12-Bit DAC: Datasheet, Application Circuit, and Programming Guide
The Microchip MCP4921-E/MS is a single-channel 12-bit Digital-to-Analog Converter (DAC) offering a compact and cost-effective solution for generating analog signals from a digital source. Housed in an 8-pin MSOP package, it is designed for space-constrained applications requiring high precision and low power consumption. This article provides an overview of its key specifications, a typical application circuit, and a fundamental programming guide.
Datasheet Overview and Key Features
The MCP4921 operates from a single 2.7V to 5.5V supply, making it suitable for both 3.3V and 5V microcontroller systems. Its 12-bit resolution provides 4096 (2^12) possible output levels, enabling fine control over the analog output. The device incorporates an external voltage reference (VREF) input, which sets the full-scale output range. This allows for flexibility; for example, using a 3.3V reference yields a 0V to 3.3V output swing.
Key parameters from the datasheet include:
Interface: SPI-compatible serial interface (supports modes 0,0 and 1,1) with a clock speed of up to 20 MHz.
Settling Time: 4.5 µs (typical) to within ±1/2 LSB of the final value, allowing for relatively fast signal generation.
Integrated Output Amplifier: Provides a low-impedance output buffer, capable of driving loads directly.
Low Power Consumption: Typical operating current of 215 µA during normal operation, which is crucial for battery-powered devices.
LDAC Pin: Allows for simultaneous update of multiple DACs when used in a system.
Typical Application Circuit
A basic application circuit for the MCP4921-E/MS is straightforward. The core components include:
1. Power Supply Decoupling: A 0.1 µF ceramic capacitor placed as close as possible between the VDD pin and ground is essential to filter high-frequency noise.
2. Voltage Reference (VREF): A stable, low-noise reference voltage must be applied to the VREF pin. This can be supplied from a dedicated reference IC or a filtered supply rail, depending on the required output accuracy.
3. Analog Output (VOUT): The output pin drives the load. For dynamic signals, proper transmission line techniques may be needed.
4. SPI Connections: The microcontroller (MCU) connects to the DAC's CS (Chip Select), SCK (Serial Clock), and SDI (Serial Data Input) pins.
5. LDAC and SHDN: The LDAC pin can be tied low for immediate updates, while the SHDN (Shutdown) pin is typically pulled high to VDD to keep the device active.
Programming Guide (SPI Protocol)
Controlling the MCP4921 involves sending a 16-bit data packet over the SPI bus. This packet contains both configuration bits and the 12-bit data value.
The 16-bit word is structured as follows:
Bit 15 (A/B): Selects the DAC. For the single-channel MCP4921, this bit is a "don't care" but is often set to 0.
Bit 14 (BUF): Controls the input buffer for the reference input. Set to 1 to buffer VREF, or 0 for unbuffered.
Bit 13 (GA): Selects the output gain. Set to 1 for 1x (VOUT = VREF D/4096) or 0 for 2x gain (VOUT = 2 VREF D/4096). Note: The output cannot exceed VDD.
Bit 12 (SHDN): Output shutdown control. Set to 1 for active operation, or 0 to shut down the output buffer (high-impedance output).
Bits 11-0 (D11-D0): The 12-bit data value (D) to be converted.
A simple code snippet for an Arduino (using its SPI library) to set an output voltage would look like this:
```cpp
include
const int CS_pin = 10; // Define Chip Select pin
void setup() {
pinMode(CS_pin, OUTPUT);
digitalWrite(CS_pin, HIGH);
SPI.begin();
}
void setDAC(int value) {
// Ensure value is within 0-4095 range
value = constrain(value, 0, 4095);
// Build the 16-bit command:
// Bit 15: 0 (ignored), Bit 14: 0 (unbuffered), Bit 13: 1 (1x gain), Bit 12: 1 (Active mode)
byte highByte = (0b00110000) | (value >> 8); // Combine config bits with top 4 bits of data
byte lowByte = value & 0xFF; // Low 8 bits of data
digitalWrite(CS_pin, LOW); // Select the DAC
SPI.transfer(highByte);
SPI.transfer(lowByte);
digitalWrite(CS_pin, HIGH); // Deselect the DAC (latches the data)
}
void loop() {
// Example: Generate a ~2.5V output with a 3.3V reference
// (2500mV / 3300mV) 4095 ≈ 3102
setDAC(3102);
delay(1000);
}
```
ICGOOODFIND
The MCP4921-E/MS is an excellent choice for designers seeking a simple, reliable, and precise single-channel DAC. Its small form factor, SPI interface, and flexible reference input make it incredibly versatile for applications like programmable voltage sources, automated test equipment, digital potentiometer replacement, and audio waveform generation. Understanding its datasheet parameters, application circuit requirements, and the structure of its control word is key to a successful implementation.
Keywords:
1. SPI Interface
2. 12-Bit Resolution
3. Voltage Reference
4. Output Buffer
5. Serial Data Packet
