Embedded control: Zero-drift, programmable instrumentation amplifier with diagnostics

National Semiconductor's LMP8358 simplifies pressure and thermocouple bridge measurements with easy detection of shorts, open circuits and degraded source connections.



National Semiconductor Corp. introduces what it's calling the industry's first zero-drift, programmable instrumentation amplifier with on-chip diagnostics. The LMP8358 simplifies pressure and thermocouple bridge measurements by allowing users to detect shorts, open circuits and degraded source connections in remote industrial applications. This diagnostic capability prevents circuit shutdowns in medical instruments, precision weigh scales, pressure sensors and motor control systems.

National Semiconductor instrumentation circuit
The LMP8358 uses patented techniques to measure and continuously correct its input offset voltage (5 uV typical), eliminating offset drift over time and temperature, as well as the effect of 1/f noise. This leads to a flat noise spectrum of 27 nV/sqrt Hz. The unit's programmable frequency compensation enables programmable bandwidths, and its offset compensation circuit does not introduce distortion and aliasing for signals lower than 12 kHz of bandwidth.

The unit is equipped with electromagnetic interference (EMI) suppression filters to reduce unwanted radio frequency (RF) interference from external sources, such as cell phones or nearby antennas. Additional filtering can be added with one external capacitor. It is built on National's CMOS7-5V process technology and provides an optimized signal path solution for precision systems when combined with the 16-bit ADC161S626 or 14-bit ADC141S626 analog-to-digital converters.

The LMP8358 provides glitch-free transitions between gain settings, which can be programmed to 10, 20, 50, 100, 200, 500 or 1000 through a serial peripheral interface (SPI) or parallel interface. All gain settings have less than 0.1 percent typical gain error. This enables manufacturers to perform system calibration by applying specific signal levels at various gains and temperatures. The unit's gain can be set to an arbitrary value using two external resistors.

Technical features
Supplied in 14-pin SOIC and 14-pin TSSOP packages, the LMP8358 operates from a single 2.7V to 5V supply and operates over a wide temperature range of -40 degrees C to 125 degrees C. The amplifier provides 10 uV maximum offset voltage and 50 nV/C maximum offset voltage drift. It features a gain bandwidth (GBW) of 8 MHz while consuming only 1.7 mA of supply current. A power-conserving shutdown mode reduces current consumption to less than 1 uA. The LMP8358's registers allow designers to program the chip's bandwidth, shutdown and fault detection circuitry. Designers also can "daisy chain" multiple LMP8358 ICs.

The LMP8358's ground-sensing CMOS input provides a high common-mode rejection ratio (CMRR) of 120 dB. It's capable of sensing differential input voltages in a common-mode range that extends from 100 mV below the negative supply to 1.4V below the positive supply. This makes the LMP8358 well-suited for interfacing with ground-referenced sensors, supply-referenced sensor bridges and any other application requiring precision and long-term stability.

To order samples and an evaluation board, visit www.national.com/pf/LM/LMP8358.html
- Edited by Renee Robbins, senior editor
Control Engineering News Desk
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