Analog Devices Inc. AD7781CRUZ

Overview

AD7781CRUZ from Analog Devices is a 20-bit, low-power sigma-delta ADC optimized for bridge sensor front ends, featuring pin-programmable gain (1 or 128), dual update rates (10 Hz / 16.7 Hz), simultaneous 50 Hz/60 Hz rejection, and integrated bridge power-down switch. It delivers 44 nV RMS noise (C grade, gain=128, 10 Hz) and operates from 2.7 V to 5.25 V across −40°C to +105°C - enabling high-precision weigh scale and pressure measurement systems with ultra-low quiescent current (330 μA typical).

For engineers reviewing the AD7781CRUZ datasheet, AD7781CRUZ pinout, AD7781CRUZ application, or AD7781CRUZ equivalent, this page provides verified technical context, exact pin functions, real-world resolution/noise trade-offs at both filter settings, confirmed bridge-switch R_ON (9 Ω), and validated alternatives for load-cell signal chain design.

Technical Context

The AD7781CRUZ implements a fully differential 20-bit sigma-delta modulator with on-chip programmable gain amplifier (PGA), internal 64 kHz oscillator, and dual-mode digital filter - all controlled via dedicated pins (GAIN, FILTER, PDRST). Its analog input channel accepts ±39 mV (gain=128) or ±5 V (gain=1) differential signals referenced to an external REFIN pair, with common-mode voltage ≥0.5 V and absolute input limits tied to AVDD/GND.

It uses offset binary coding and outputs a 32-bit word per conversion: 20-bit result + 4 zero bits + 8 status bits. The 2-wire serial interface (SCLK/DOUT/RDY) is SPI-compatible with Schmitt-triggered SCLK, and supports independent DVDD (2.7–5.25 V) for logic-level flexibility. Power-down mode reduces current to 10 μA while opening the BPDSW switch to disconnect bridge sensors.

Key Specifications

Pinout & Package

AD7781CRUZ is supplied in a 16-lead TSSOP package (body width 4.4 mm, JEDEC MO-153). Pin functions are fixed and non-multiplexed; all digital control pins (GAIN, FILTER, PDRST) are CMOS-compatible with DVDD-referenced thresholds.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar precision sigma-delta ADC applications.

Compared with AD7781CRUZ, AD7190BRUZ trades ultra-low power for higher resolution and bandwidth, while ADS1232IPWR reduces cost and size but shifts bridge control and noise rejection to external components - making AD7781CRUZ uniquely balanced for portable, battery-operated bridge sensor systems demanding integrated power management and robust EMI immunity.

Availability

AD7781CRUZ is available at Aetrix Electronics and suitable for weigh scales, pressure transducers, and portable instrumentation requiring stable component supply, long-term industrial temperature support, and guaranteed lifecycle continuity.

Supply support for AD7781CRUZ 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 AD7781CRUZ belongs to Analog Devices' precision sigma-delta ADC product line, engineered specifically for low-power, high-accuracy bridge sensor interfaces in industrial and portable measurement systems.

FAQ

What is the maximum allowable bridge excitation current for AD7781CRUZ's BPDSW?

The AD7781CRUZ's bridge power-down switch supports up to 30 mA continuous current with ≤9 Ω on-resistance. This accommodates standard 350 Ω load cells excited at 5 V (14.3 mA) or 3.3 V (9.4 mA) without voltage drop-induced gain error. Exceeding 30 mA risks thermal stress and degraded switch performance - always verify total bridge current including sensor and conditioning circuitry before final layout.

Does AD7781CRUZ require an external crystal or can it operate with its internal oscillator?

AD7781CRUZ operates autonomously using its internal 64 kHz ±3% oscillator and requires no external crystal or clock source. This simplifies BOM and layout for cost-sensitive applications. The internal clock directly drives the sigma-delta modulator and digital filter - eliminating timing jitter concerns associated with external sources while maintaining specified noise and linearity performance across −40°C to +105°C.

How does the AD7781CRUZ handle 50 Hz and 60 Hz interference simultaneously?

AD7781CRUZ achieves simultaneous 50 Hz and 60 Hz rejection through its fixed digital filter architecture - delivering 63–90 dB attenuation at both frequencies regardless of selected update rate (10 Hz or 16.7 Hz). This is implemented in hardware, not firmware, so no configuration or recalibration is needed. The rejection stems from precise notch placement relative to f_ADC, enabling robust operation in globally deployed equipment without region-specific tuning.

What is the absolute input voltage limit for AIN(+) and AIN(−) on AD7781CRUZ at gain=128?

At gain=128, the AD7781CRUZ specifies absolute AIN voltage limits of GND + 450 mV to AVDD − 1.1 V (for FILTER = 0) or GND + 1.1 V to AVDD − 1.1 V (for FILTER = 1, AVDD ≤ 3.6 V). Violating these limits degrades linearity and increases noise. For example, with AVDD = 3.3 V and FILTER = 0, AIN(+) must stay within 0.45 V–2.2 V - requiring careful biasing of bridge midpoints to maintain common-mode ≥0.5 V.

Can AD7781CRUZ interface directly with a 3.3 V microcontroller SPI port?

Yes - AD7781CRUZ supports independent DVDD (2.7–5.25 V), allowing direct connection to 3.3 V microcontrollers. SCLK and DOUT/RDY logic levels track DVDD, and the Schmitt-triggered SCLK accepts 100 mV hysteresis at 3 V. No level-shifting is required. However, ensure DVDD and AVDD share a clean ground plane and that decoupling capacitors (0.1 μF ceramic) are placed near each supply pin to prevent digital noise coupling into the analog section.