Zero-drift, digitally-programmable instrumentation amplifier

The AD8231 features 80 percent lower input noise than competing devices while delivering rail-to-rail inputs and outputs at up to 1-MHz bandwidth—five times the throughput of the nearest competing zero-drift in-amp­. Unlike competing devices, the exceptionally low noise characteristics of the AD8231 allow designers of industrial sensors and controls to maintain dc precision over gain.

“Zero-drift instrumentation amplifiers can have high noise at high gains for a variety of reasons, including the presence of chopping noise,” said Steve Sockolov, product line director, Precision Signal Processing, Analog Devices. “However, the trade-off between offset drift and noise becomes untenable as the gain increases to the level demanded by today’s industrial equipment manufacturers. Analog Devices addressed this challenge using a proprietary auto-zero technique that dramatically lowers the in-amp’s noise profile while eliminating the need for external components, such as switched capacitors.”

Using a three-operational amplifier (op-amp) instrumentation architecture, the AD8231 enhances reliability by integrating auto-zero amplifiers to maintain a voltage offset of only 50 nV/°C across temperature and over years of operation. The AD8231 also features internal gain-setting resistors, which limit gain coefficient temperature drift to 10 ppm/°C. The device’s LFCSP (lead-frame chip-scale package) material was chosen for its reliability and robustness over temperature, which results in a part that is specified from –40°C to 125°C. The package comes with a centre paddle, which keeps the part securely soldered to the printed-circuit board in high-vibration environments.

The AD8231 features software programmable gains of 1, 2, 4, 8, 16, 32, 64, or 128, which are programmed through an easy-to-use three-pin interface. The gain can be set using digital logic, which allows users to repeatedly adjust gain once the in-amp has been designed into the system. A pin-strapping gain setting option is also included. The new device operates on a single supply from 3.3 V to 5 V and has rail-to-rail outputs, making it easy to interface to an analog-to-digital converter (ADC). The AD8231 has high impedance inputs of 10 gigaohms, and unlike competing parts, does not suffer performance degradation when using higher impedance sensors.

The AD8231 includes an additional on-chip op-amp, which further extends performance. The op-amp can be used to create a differential output to drive today’s high performance ADCs, or to add or customise gains. The integrated op-amp also provides a tool for additional error correction, level shifting, or filtering. Using the same low-noise auto-zero techniques as the in-amp, the on-chip op-amp also has rail-to-rail inputs and outputs. The ability of the AD8231 to eliminate noise-prone discrete components, such as switched capacitors, combined with the added on-chip signal conditioning of the integrated op-amp, cuts cost by 60 percent and reduces board space requirements by 50 percent compared to other zero-drift in-amps.

Manufactured using Analog Devices’ highly efficient iCMOS™ (industrial CMOS) process technology, the AD8231 achieves a noise level of 32 nV/rt-Hz, five times lower than competitive zero-drift in-amps. The AD8231 also features 122-dB CMR (common-mode rejection), allowing the in-amp to detect very low-amplitude signals in noisy environments.

Ideally suited for industrial controls and sensors requiring a single 3.3-V to 5-V power supply, ADI’s new single-supply in-amp easily interfaces to ADCs and microcontrollers by using the same power supply as digital components. The AD8231 is designed to work well with Analog Devices’ data converters (including the AD7942, AD7685 and AD7980 PulSAR® ADCs), the ADuC702x family of precision analog microcontrollers and ADI’s ADR121, ADR43x and ADR44x voltage reference.

The AD8231 zero-drift, digitally-programmable, gain in-amp is sampling now in a 16-lead LFCSP package with production quantities available in April 2007. The AD8231 is priced at $1.69 in 1,000-unit quantities.