AD7708BRUZ-REEL7 is a high-precision, low-power, 16-bit Σ-Δ (sigma-delta) analog-to-digital converter (ADC) designed and manufactured by Analog Devices Inc. (ADI). It belongs to the widely adopted AD77xx family, engineered specifically for low-frequency, high-resolution measurement applications, such as industrial process control, weigh scales, temperature sensing (RTDs, thermocouples), and portable instrumentation — where exceptional noise performance, integrated programmable gain amplifier (PGA), on-chip reference, and ultra-low power consumption are essential.
The “BRUZ” suffix denotes the 24-lead TSSOP package (7.8 mm × 4.4 mm) — a compact, surface-mount, RoHS-compliant, and industry-standard package; the “-REEL7” indicates 7-inch tape-and-reel packaging (2,500 units per reel), qualified for industrial temperature range (–40°C to +105°C ambient).
â ī¸ Critical Clarification:
The AD7708 is not a general-purpose SAR or pipeline ADC. It is a fully integrated, low-noise, 16-bit sigma-delta converter featuring:
- Integrated 128× programmable gain amplifier (PGA): Enables direct interface with low-level sensors (e.g., load cells, RTDs, thermocouples) without external signal conditioning — input range from ±10 mV (G = 128) to ±2.56 V (G = 1);
- On-chip 2.5 V reference (±3 ppm/°C drift): Eliminates need for an external precision reference — saving board space, cost, and calibration complexity;
- Ultra-low noise & high resolution: 1.2 µV RMS noise (at 4.17 Hz, G = 128), enabling effective resolution up to 20 bits (20.2 ENOB) in slow-sampling modes;
- Low power operation: 450 µA typical supply current (at 3 V) — supports multi-year battery life in portable meters and wireless sensor nodes;
- Flexible digital interface & features: SPI-compatible 3-wire serial interface, built-in burnout current sources (for RTD open-wire detection), zero-scale and full-scale calibration registers, and multiple output data rates (4.17 Hz to 470 Hz).
It operates from a single 2.7 V to 5.25 V supply, supports differential or pseudo-differential inputs, and includes comprehensive self-calibration and system calibration capabilities — all in a compact 24-pin TSSOP.
Introduction
The AD7708BRUZ-REEL7 delivers laboratory-grade precision measurement performance in a highly integrated, production-ready package:
đš Complete front-end in one IC: Combines PGA, ADC, reference, and diagnostics — reducing BOM count by ≥5 components and eliminating matching errors between discrete stages;
đš True sensor-ready architecture: With built-in burnout currents, programmable gain, and 20-bit effective resolution, it directly digitizes microvolt-level signals from strain gauges, 3-wire/4-wire RTDs, and type-K/J thermocouples — no external op-amps or references required;
đš Battery-optimized design: At just 450 µA @ 3 V, it enables >5-year operation on a single CR2032 coin cell when paired with duty-cycled MCU sleep modes — ideal for IoT edge sensors and handheld test tools;
đš Robust, calibration-free accuracy: On-chip auto-calibration (zero/full-scale) ensures < 0.003% gain error and < 10 µV offset error over temperature — suitable for Class 0.02 industrial transmitters.
Its 24-lead TSSOP (BRUZ) package (7.8 mm × 4.4 mm) offers excellent manufacturability, thermal performance (θJA ≈ 110°C/W), and compatibility with standard reflow profiles — making it a go-to solution for high-volume industrial, medical, and consumer instrumentation.
Key Features
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High-Resolution Sigma-Delta Conversion:
• Resolution: 16-bit (no missing codes guaranteed);
• Effective number of bits (ENOB): Up to 20.2 bits (at 4.17 Hz, G = 128, RMS noise = 1.2 µV);
• Output data rate: 4.17 Hz to 470 Hz, selectable via register;
• RMS noise: 1.2 µV (4.17 Hz, G = 128), 2.7 µV (16.7 Hz, G = 128), 6.5 µV (470 Hz, G = 1).
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Integrated Signal Conditioning:
• Programmable gain amplifier (PGA): G = 1, 2, 4, 8, 16, 32, 64, 128;
• Input configuration: Differential or pseudo-differential (8-channel multiplexer);
• On-chip 2.5 V reference: ±3 ppm/°C drift, 10 ppm p-p noise (0.1–10 Hz);
• Burnout current sources: Two 0.5 µA (or 1 µA) matched currents for RTD/open-wire detection.
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Low Power & Flexible Operation:
• Supply voltage: 2.7 V to 5.25 V (single supply);
• Quiescent current: 450 µA (typ.) @ 3 V, 550 µA (max);
• Power-down mode: 1 µA (typ.), with fast wake-up (< 20 µs);
• Operating temperature: –40°C to +105°C ambient.
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Calibration & Diagnostic Capabilities:
• Internal zero-scale and full-scale calibration;
• System calibration support (using external known voltage);
• Status register with ready, calibration, and fault flags;
• Synchronous 50 Hz/60 Hz notch filtering (rejection > 100 dB).
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TSSOP-24 (BRUZ) Package & Industrial Qualification:
• 24-Lead TSSOP (7.8 mm × 4.4 mm);
• RoHS-compliant, halogen-free;
• JEDEC J-STD-020 moisture sensitivity level (MSL) 1 — unlimited floor life;
• Qualified per AEC-Q200 stress test requirements (automotive grade option available).
Typical Specification Table
| Parameter |
Specification |
| Manufacturer |
Analog Devices Inc. (ADI) |
| Product Series |
AD77xx Family (Low-Power Precision Σ-Δ ADCs) |
| Model |
AD7708BRUZ-REEL7 |
| Function |
16-Bit Sigma-Delta ADC with PGA & Reference |
| Resolution |
16-bit (guaranteed no missing codes) |
| Effective Resolution (ENOB) |
Up to 20.2 bits (4.17 Hz, G = 128) |
| RMS Noise (4.17 Hz, G=128) |
1.2 µV |
| PGA Gain Range |
1, 2, 4, 8, 16, 32, 64, 128 |
| Input Configuration |
Differential or pseudo-differential (8-channel MUX) |
| On-Chip Reference |
2.5 V, ±3 ppm/°C drift |
| Burnout Currents |
Two 0.5 µA (or 1 µA) matched sources |
| Output Data Rate |
4.17 Hz to 470 Hz |
| Supply Voltage |
2.7 V to 5.25 V (single supply) |
| Quiescent Current |
450 µA (typ.) @ 3 V |
| Power-Down Current |
1 µA (typ.) |
| Package |
24-Lead TSSOP (7.8 mm × 4.4 mm) (BRUZ) |
| RoHS / Green |
Yes (Pb-free, Halogen-free) |
| Operating Temp. Range |
–40°C to +105°C ambient |
Typical Applications
đš Industrial Process Control: 4–20 mA loop-powered transmitters for pressure, flow, and level measurement — leveraging low power, integrated PGA/reference, and 50/60 Hz rejection.
đš Weigh Scales & Load Cells: Precision weighing systems (bench, platform, crane) — using 128× gain for sub-gram resolution and built-in burnout for cable fault detection.
đš Temperature Measurement: RTD (Pt100/Pt1000) and thermocouple (type-K/J) interfaces in HVAC, food processing, and lab ovens — enabled by programmable gain, burnout currents, and cold-junction compensation support.
đš Portable Test Equipment: Handheld DMMs, insulation resistance testers, and portable calibrators — benefiting from battery life >5 years and high ENOB at low data rates.
đš Medical Instrumentation: Patient monitors (temperature, impedance plethysmography), infusion pump pressure sensors, and portable ECG front-ends — meeting IEC 60601 safety and accuracy requirements.
đš Energy Monitoring: Smart meter current/voltage sensing (shunt-based), power quality analyzers — using high CMRR (>100 dB) and line-cycle synchronization.
Development & Design Notes
đ§ PCB Layout Best Practices:
- Place the AD7708 close to the sensor — keep analog traces short (< 10 mm), use differential routing for IN+ and IN–, and avoid vias or splits under the ADC ground plane.
- Use separate analog (AVDD, AVSS, REFIN±) and digital (DVDD, DGND) power domains — connect them at a single point near the ADC’s ground pin.
- Add ground guard rings around analog inputs and reference pins — reduces leakage and capacitive coupling.
đ§ Power Supply Decoupling:
- Use 10 µF tantalum + 100 nF ceramic capacitor on AVDD and DVDD, placed within 3 mm of respective pins;
- For best noise performance: add a 1 µF X5R ceramic cap directly across REFIN+ and REFIN– — stabilizes internal reference buffer and reduces 0.1–10 Hz noise.
đ§ Sensor Interface Optimization:
- For RTDs: use 3-wire or 4-wire configuration to cancel lead resistance — enable internal burnout currents and configure PGA gain (e.g., G = 32 for Pt100).
- For thermocouples: add cold-junction compensation (CJC) using an external temperature sensor (e.g., ADT7320) — read CJC value via SPI and apply software correction.
- For load cells: use ratiometric configuration (REFIN+ tied to excitation voltage) — eliminates supply drift effects.
đ§ Calibration & Accuracy Optimization:
- Perform on-chip zero-scale and full-scale calibration at power-up — corrects for initial offset/gain errors.
- For metrology-grade accuracy: perform two-point system calibration (0% and 100% span) using NIST-traceable voltage source — achieves < 0.001% total unadjusted error (TUE).
- Enable 50/60 Hz notch filter for industrial environments — improves rejection to >100 dB.
đ§ Thermal Management & Reliability:
- TSSOP-24 has moderate thermal capability (θJA ≈ 110°C/W). For continuous operation above +50°C ambient, add ≥ 100 mm² copper pour under the IC.
- FIT rate = 19 failures per billion hours, with FMEDA report supporting IEC 61508 SIL-2 — combine periodic self-test (e.g., internal calibration check every 24 h) for functional safety.
