AD7840JN

The AD7840JN is a complete 14-bit voltage-output digital-to-analog converter manufactured by Analog Devices using proprietary LC²MOS (Linear Compatible CMOS) technology — an advanced mixed-signal process that integrates precision bipolar analog circuitry with low-power CMOS digital logic on a single die. The AD7840 provides a fully self-contained DAC solution: it includes a 14-bit R-2R ladder DAC, a 3V buried zener reference voltage source, a precision output buffer amplifier, and a complete digital control interface with double-buffered latch architecture. The double-buffered structure — consisting of a 14-bit input latch and a 14-bit DAC latch — allows new data to be loaded into the input register while the DAC maintains its current output, then simultaneously updates the DAC output under control of the LDAC (Load DAC) pin. This is essential for systems where multiple DACs must update simultaneously. The device supports both parallel and serial data loading: in parallel mode, 14 bits of data are written directly via the data bus with fast 21ns setup time, enabling direct connection to 16-bit microprocessors and DSPs; in serial mode, data is clocked in at rates up to 6 MHz with four different data format options (MSB-first/LSB-first, left-justified/right-justified). The analog output provides a true bipolar range of ±3V from a 3V reference, generated using internal bipolar offset circuitry. The AD7840 is fully characterized for dynamic performance parameters including signal-to-noise ratio (80dB typ) and total harmonic distortion, making it suitable for audio and waveform generation applications as well as traditional DC control functions. With only 70mW typical power consumption from ±5V supplies and a compact 24-pin DIP footprint, the AD7840JN remains a versatile choice for industrial control, ATE, and instrumentation applications that require a complete, standalone 14-bit DAC solution.

The AD7840 is a 14-bit voltage-mode DAC based on an R-2R (resistor ladder) architecture. The DAC section consists of a network of precision thin-film resistors in an R-2R ladder configuration, with NMOS single-pole double-throw switches that connect each ladder rung to either the reference voltage or ground depending on the corresponding bit value. The three MSBs use a segmented architecture with seven separate switch groups driven by a 3-bit decoder, while the remaining 11 LSBs use a conventional R-2R ladder structure. The output of the resistor network is a current that is proportional to the digital input code and the reference voltage; this current is converted to a voltage by the internal output amplifier. The on-chip 3V buried zener reference provides a stable, low-drift voltage reference for the DAC — buried zener references achieve excellent long-term stability and low temperature coefficient compared to bandgap references. The bipolar output range of ±3V is achieved by offsetting the unipolar DAC output with a precision reference-derived bias voltage, so that a mid-scale digital input (code 2000H for 14 bits) produces 0V output, full-scale produces +3V, and zero-scale produces -3V. The double-buffered latch structure consists of two cascaded registers: the input latch receives new data from the parallel or serial interface when WR is asserted, and the DAC latch transfers the input latch data to the DAC when LDAC is asserted. This two-stage architecture allows pre-loading of new data while the current output is maintained, and enables synchronous updates across multiple DACs when their LDAC pins are tied together. The overall device operates from dual ±5V supplies, with the analog and digital sections separated to minimize noise coupling.

The AD7840JN is packaged in a 24-pin plastic DIP (PDIP-24). Key pin assignments:

Note: DB5 through DB13 serve as the 14-bit parallel data bus; some pins serve dual functions depending on PAR/SER mode. Please consult the full datasheet for complete pin descriptions and timing specifications.

• Digital Signal Processing (DSP) Systems: Used as the analog output front-end in DSP-based systems for audio processing, filtering, and waveform generation, leveraging the fast parallel interface that directly connects to ADSP-2100 and TMS320 series DSPs
• Industrial Process Control: Provides precise analog control voltages for PID controllers, valve positioners, temperature controllers, and other industrial automation equipment requiring 14-bit resolution setpoint generation
• Automatic Test Equipment (ATE): Used in semiconductor test systems, board-level testers, and calibration equipment to generate programmable reference voltages and bias signals with high accuracy
• Programmable Voltage Sources: Forms the core of digitally controlled power supplies, voltage calibrators, and laboratory bench instruments where precise, software-programmable analog outputs are required
• Servo and Motor Control: Generates analog command signals for servo amplifiers, motor drives, and positioning systems, providing smooth 14-bit resolution control with low glitch
• Waveform and Function Generation: Used in arbitrary waveform generators and function generators where the 80dB SNR and 20kHz full-power bandwidth support high-quality analog waveform synthesis
• Precision Instrumentation: Found in oscilloscopes, multimeters, and data loggers for offset adjustment, gain control, and trigger level generation
• Data Acquisition Output Stages: Serves as the analog output channel in bidirectional data acquisition systems, providing the DAC counterpart to ADC inputs in closed-loop control and monitoring systems