Analog Devices Inc. AD7651AST

Overview

AD7651AST from Analog Devices is a 16-bit, 100 kSPS PulSAR® charge redistribution SAR analog-to-digital converter operating from a single 5 V supply. It integrates an internal 2.5 V reference (±7 ppm/°C drift), supports unipolar 0 V to 2.5 V analog input, and delivers no pipeline delay with parallel/serial (SPI/QSPI/MICROWIRE/DSP-compatible) interface options. It is used in precision data acquisition systems requiring high DC accuracy and low power at medium speed.

For engineers reviewing the AD7651AST datasheet, AD7651AST pinout, AD7651AST application, or AD7651AST equivalent, this page provides verified technical context, real-world interface timing constraints, confirmed package mapping to 48-lead LQFP, and validated alternative selections for instrumentation-grade ADC replacement scenarios.

Technical Context

The AD7651AST implements a charge redistribution SAR architecture with on-chip error correction circuitry, enabling 16-bit resolution without missing codes over –40°C to +85°C. Its internal reference provides 2.48 V to 2.52 V output at 25°C and ±7 ppm/°C temperature drift, eliminating external reference dependency in space-constrained designs.

It supports dual-mode digital interfacing: parallel 16-bit bus (with BYTESWAP and OB/2C configuration) and flexible serial modes (master/slave, internal/external clock, active-high/low SYNC). Conversion is initiated by CNVST edge control, with BUSY signaling completion and no latency between sample hold and result availability.

Key Specifications

Pinout & Package

AD7651AST is packaged in a 48-lead LQFP (ST-48) with exposed pad, rated for –40°C to +85°C operation. Pin assignments are identical to AD7651 in LQFP format per Analog Devices' official pinout documentation.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar 16-bit SAR ADC applications.

Compared with AD7651AST, AD7652ASTZ trades integrated reference for higher speed and pseudo-differential input, while ADS8860IPWR reduces size and supply voltage at the cost of interface flexibility and reference integration - making AD7651AST optimal for 5 V, reference-free, medium-speed instrumentation where pin compatibility and ease of use are prioritized.

Availability

AD7651AST is available at Aetrix Electronics and suitable for portable data loggers, medical patient monitors, and industrial process controllers requiring stable component supply with guaranteed long-term manufacturability.

Supply support for AD7651AST includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

Analog Devices is a global leader in high-performance analog, mixed-signal, and digital signal processing semiconductors, founded in 1965 and headquartered in Wilmington, MA.

The AD7651AST belongs to Analog Devices' PulSAR® family of precision SAR ADCs, designed specifically for instrumentation, medical, and industrial applications demanding high DC accuracy, low power, and flexible digital interfacing without pipeline latency.

FAQ

What is the maximum recommended load capacitance for AD7651AST's SDOUT pin in serial interface mode?

The AD7651AST SDOUT pin is specified for a maximum load capacitance of 10 pF in serial interface modes (master or slave), as defined in the Absolute Maximum Ratings and Timing Specifications sections. Exceeding 10 pF may violate setup/hold timing margins - particularly t22 (SDOUT valid setup time) and t23 (SDOUT valid hold time) - leading to data corruption. For reliable operation, keep PCB trace length short and avoid stubs or connectors on SDOUT.

Does AD7651AST support true differential input, and how is it configured?

No, AD7651AST does not support true differential input. It is a unipolar, pseudo-differential SAR ADC with a single-ended analog input (IN) referenced to INGND. The functional block diagram and Pin Configuration section confirm IN and INGND as the only analog input terminals. True differential variants (e.g., AD7675, AD7676) are listed separately in Table 1 of the datasheet and are not pin-compatible or functionally interchangeable with AD7651AST.

Can AD7651AST operate with OVDD = 3.3 V while AVDD/DVDD = 5 V?

Yes, AD7651AST supports mixed-supply operation: OVDD may be set to 3.3 V (within 2.7 V to 5.25 V range) while AVDD and DVDD remain at 5 V. This allows direct interfacing with 3.3 V logic families (e.g., FPGA I/O banks) without level shifters. However, the OVDD supply must be decoupled independently, and its ground (OGND) must be tied to DGND - not AGND - to prevent digital noise coupling into analog sections.

What is the purpose of the BYTESWAP pin on AD7651AST, and how does it affect data alignment?

The BYTESWAP pin on AD7651AST controls byte ordering in parallel interface mode: when LOW, D[7:0] carries the LSB byte and D[15:8] carries the MSB byte; when HIGH, D[7:0] carries the MSB byte and D[15:8] carries the LSB byte. This allows seamless alignment with big-endian or little-endian host processors without firmware bit manipulation - critical for deterministic real-time data transfer in industrial controllers using AD7651AST.

How does the PDREF pin affect reference selection and power consumption in AD7651AST?

When PDREF is driven LOW, the AD7651AST internal 2.5 V reference is enabled and drives the REF pin; when HIGH, the internal reference is disabled and an external reference must be applied to REF. Power consumption drops by ~3 mA (AVDD current) when PDREF = HIGH and no external reference is connected - but full functionality requires either internal reference (PDREF = LOW + PDBUF = LOW) or valid external reference (PDREF = HIGH + 2.3 V to AVDD–1.85 V applied).