The autoreferencing circuit in Figure 1 nulls out the error of a sensor, such as a pressure transducer, at its reference level – for example, at ambient pressure. The circuit is an analog-digital-feedback control system that uses a digitally programmable potentiometer to provide the variability. The circuit in Figure 1 is designed to accommodate a pressure transducer with a nominal 1 V±50-mV output at ambient pressure and provide a voltage of 1 V±1 mV. Amplifier IC1A is a summing/difference circuit whose inputs are the sensor's output voltage, VSENSE; a voltage shift, VSHIFT, of 100 mV; and a correction voltage, VCORR. IC1B functions as a comparator, comparing the output voltage of the summing amplifier with the ideal output voltage of the sensor, 1 V. The logic output of the comparator sets the direction for incrementing or decrementing the potentiometer's wiper, whose buffered wiper voltage provides the correction voltage, VCORR.
|Figure 1.||This autoreferencing circuit nulls out output errors at a sensor’s reference (ambient) condition.|
The potentiometer is a Catalyst 30-tap digitally programmable potentiometer with an increment/decrement interface. The correction voltage varies from 0 to 200 mV and subtracts from the shifted sensor voltage. Mathematically,
VOUT = (VSENSE + VSHIFT) – VCORR,
where 0 ≤ VCORR ≤ 200 mV, VSHIFT = 100 mV, and 0.95 V ≤ VSENSE ≤ 1.05 V. The 100-mV, 200-mV, and 1 V references for the circuit come from a 2.5 V reference, stepped down by a resistive divider and buffered by voltage followers. IC2A implements a square-wave oscillator whose frequency is approximately equal to 1/RC – in this case, 10 kHz. You program the autoreferencing circuit using the logic-input signals OSC and /CS. The circuit becomes disabled when OSC is low and /CS is high. When the circuit is disabled, VOUT is at its last corrected value.
The circuit becomes enabled and corrects the output voltage for a new sensor or different set of conditions when OSC is high and /CS is low. To store the current wiper setting of the digitally programmable potentiometer in nonvolatile memory, first make OSC low and then bring /CS from low to high. If power disappears and is later restored, the potentiometer goes to the corrected value stored in nonvolatile memory. The measured error in the system is less than 1 mV, but better performing amplifiers, a higher resolution potentiometer, and more accurate resistors can reduce the error to the low-microvolt region. This circuit uses three ICs and a handful of discrete parts and is an alternative, low-cost approach to more complex autoreferencing circuits using DACs, ADCs, and microprocessors.