Programmable Amp Merges Precision And Flexibility For Sensor Correction

In any given batch of pressure or strain-bridge sensors, sensitivity (gain) and offset voltage may vary widely. These sensor-to-sensor variations, often as large as the sensor's full-scale output, represent errors that must be corrected before the sensor signals can be further processed. Each sensor may require a precision amplifier and several external components that can adjust gain and offset in the system. Other parts may be needed for fault protection, too.

The AD8555, a zero-drift, bridge-sensor signal amplifier from Analog Devices, provides the precision amplifier, gain and offset adjustment circuitry, and fault protection functions in a cost- and space-saving 4- by 4-mm leadframe chip-scale package (LFCSP). This integration sets the AD8555 apart from other amplifiers, yet its high levels of programming flexibility and dc accuracy distinguish it even more. These features are achieved by combining ADI's low-noise autozero and DigiTrim technologies.

Using DigiTrim, the gain and offset are digitally programmed. Trim information is input as a digital keyword sequence conveyed through a single-wire interface (see the figure).

The adjustment values can be temporarily programmed, evaluated, and readjusted for optimum accuracy before permanent adjustment. In the last step, the settings are stored by blowing polysilicon fuses on-chip. One advantage of using polysilicon fuses is their reliable performance over temperature. That capability is particularly relevant in automotive applications, where high temperatures may put other programming options such as EEPROM at risk.

After the trim is completed, the trim circuit is locked out to prevent any accidental retrimming by the end user. Previously, Analog Devices applied the DigiTrim technique solely to correct the internal offset of a CMOS amplifier by adjusting the internal balance of the amplifier's input stage. In the AD8555, the company reworked the technique so customers could adjust for system-level offsets and gain errors, or simply to adjust amplifier gain.

Programming range and resolution provide other measures of flexibility. Gain may be programmed from 70 to 1280 with better than 0.4% resolution. To achieve similar levels of adjustment, most existing amps would require manually selected external gain-setting resistors. Meanwhile, output offset voltage may be set with a resolution of better than 0.4% of the difference between the supply rails. In the AD8555, an internal 8-bit digital-to-analog converter generates the variable offset voltage on the amplifier's output.

While the DigiTrim technique lets users correct for the variations in gain and offsets associated with the external sensors, the AD8555's use of autozero techniques enable it to achieve low offset voltage, offset drift, and low noise internally. The maximum input offset voltage of the amplifier over temperature is 10 µV with a drift of just 50 nV/°C. ADI says these numbers are on par with the industry's most accurate amplifiers.

The amplifier specifies a typical voltage-noise-density of 25 nV/√Hz, measured at 1 kHz. Translated into a noise measurement over bandwidth, the noise density specification equals a noise voltage well below 1 µV p-p from dc to 10 Hz.

Fault detection in the amplifier protects against open, shorted, and floating inputs. Any of these conditions triggers the output voltage to be clamped to the negative supply rail (V_SS). Opens, shorts, and floating conditions are detected on either the positive or negative amplifier input. Shorts and floating conditions are also detected on the input to the amplifier's output voltage clamp.

The AD8555 implements a low-pass filter, which requires an external capacitor to set its cutoff frequency. The amplifier operates from single-supply voltages of 3 to 5.5 V. Other specifications include a signal bandwidth ranging from 40 to 460 kHz. The amplifier comes in either a 16-lead LFCSP or an eight-pin SOP. Pricing starts at $2.80 in 1000-unit lots.