Low-Power Data Acquisition for High-Temperature Electronics

Low-Power Data Acquisition for High-Temperature Electronics

Downhole oil and gas tools, aerospace systems, and automotive electronics can operate far beyond standard industrial temperature ranges. In those environments, accuracy must be maintained while self-heating is minimized, because every additional milliwatt raises junction temperature.

The source presents a complete 16-bit, 600 kSPS SAR data-acquisition signal chain characterized from room temperature to 175°C. The design combines the ADC, driver, voltage reference, digital interface, high-temperature packaging, and PCB technology into a system-level reference.

Multi-sensor data-acquisition chain for a downhole instrument

1. System Architecture: A Low-Power SAR ADC at the Center

The core converter is AD7981, a 16-bit SAR ADC supporting up to 600 kSPS. Typical power at 600 kSPS is about 4.65 mW, and the converter automatically powers down between conversions, so average power scales approximately with sample rate.

That behavior is valuable in battery-powered and high-temperature systems because slow sensors do not force the converter to dissipate its full-rate power continuously.

AD7981 supply and digital-interface arrangement

2. ADC Driver: Control Kickback and Survive the Temperature

A SAR input is a dynamic capacitive load. At the beginning of acquisition, the internal CDAC injects charge back into the input node. A high source impedance degrades settling and THD, so the reference design uses AD8634 as a low-impedance driver.

AD8634 driving the AD7981 input through an RC network

AD8634 consumes roughly 1 mA per amplifier. Although its output is rail-to-rail, the input requires about 300 mV of headroom from the supplies, which is why the design includes a -2.5 V rail.

The RC network between driver and ADC both damps kickback and limits noise bandwidth. Too much filtering, however, increases settling error, so the network must be optimized rather than maximized.

3. RC Values Need Calculation and High-Temperature Bench Validation

Using AD7981 parameters including CIN = 30 pF and tCONV = 900 ns, the source estimates the front-end bandwidth for a 10 kHz input at 600 kSPS with CEXT = 2.7 nF.

VSTEP = 2π·fIN·VPEAK·tCONV·CIN / (CEXT + CIN)

The theoretical value corresponds to REXT of roughly 76.6 Ω. Bench optimization ultimately selects REXT = 85 Ω and CEXT = 2.7 nF, giving about 693.48 kHz -3 dB bandwidth and good performance across the extended temperature range.

4. Voltage Reference: Low Quiescent Current Is Not Enough

The design uses the 2.5 V ADR225 precision reference. Its low quiescent current suits a high-temperature low-power system, but the AD7981 REF input has dynamic impedance and draws transient current during conversion.

AD8634 is therefore also used as the reference buffer. A 49.9 Ω / 47 µF network provides a cutoff near 67 Hz, reducing reference noise while preserving a low dynamic source impedance.

ADR225 reference, AD8634 buffer, and local storage capacitor

Reference-pin current spikes can reach about 2.5 mA. Large low-ESR ceramic capacitors are common at room temperature, but suitable high-value high-temperature ceramics are limited. The reference design therefore uses a 47 µF low-ESR tantalum capacitor.

5. Total Power Remains Around 20 mW at 175°C

6. High-Temperature Reliability Is More Than the IC Rating

Packaging, bond-wire metallurgy, PCB material, surface finish, and solder all matter. A conventional gold-wire/aluminum-pad interface can form Au-Al intermetallic compounds at high temperature, leading to voids and brittle failure over time.

Voiding at a conventional Au-Al interface after high-temperature aging

The high-temperature devices in the source use an over-pad metallization process that creates a gold bond-pad surface, producing an all-gold bond system. This avoids Au-Al intermetallic growth and has been qualified in extended 195°C testing.

Gold-on-gold OPM bond structure after long high-temperature exposure

7. The PCB Must Also Be Designed for 175°C

Conventional FR-4 is not well suited to continuous 175°C operation because its glass-transition temperature is typically much lower. The reference board uses a four-layer polyimide PCB with a Tg above 240°C.

Analog and digital signals are routed on opposite sides of the ADC to control return currents. The REF storage capacitor is placed very close to REF and GND with a low-inductance connection. Nickel-gold surface finish is used, and SAC305 solder, with a melting point around 217°C, provides useful margin above the 175°C operating temperature.

High-temperature data-acquisition reference board

8. Measured Performance Remains Strong at High Temperature

The calculated room-temperature SNR is close to the measured value of about 86.2 dB. At 175°C, SNR drops only to roughly 84 dB, while THD remains better than about -100 dB.

SNR versus temperature

THD versus temperature

The result shows that high-temperature accuracy depends on the whole signal chain, not only on the ADC die.

9. Practical High-Temperature Design Checklist

1. Choose device ratings based on real junction temperature, not ambient temperature alone.

2. Prefer converters whose power scales with sample rate to minimize self-heating.

3. Treat both the SAR input and the reference pin as dynamic loads requiring low source impedance.

4. Validate RC values across temperature; do not rely only on room-temperature calculations.

5. Check package metallurgy, PCB Tg, surface finish, and solder melting point.

6. Separate analog and digital return paths and place the REF storage capacitor close to the ADC.

7. Complete SNR, THD, power, and long-duration reliability testing in a high-temperature chamber.

Conclusion

A 175°C data-acquisition system is not simply a room-temperature design populated with high-temperature parts. Reliable performance requires coordinated control of power, sampling kickback, reference transients, noise, package metallurgy, PCB materials, and assembly technology. The value of the source reference design is that all of these elements are validated as one signal chain.