Minimize additional offset voltage with the LTC2063 sensor

This low power precision gas sensor circuit operates from 0% to 30% oxygen and has a nominal 1V output in normal atmospheric oxygen concentration (20.9%) when the gas sensor is fully initialized. The total operating power consumption when powered from a single rail supply is less than 2.1μA.

Since the gas sensor produces 100μA in a normal oxygen environment and requires a 100Ω load resistor, the resulting input signal is typically about 10mV. The rail-to-rail input of the LTC2063 means that no additional DC level shifting is required until very low oxygen concentrations.

Because the LTC2063 has an extremely low input offset voltage (1μV typical and 5μV maximum), the gain of the mV stage input signal can be substantially increased without causing significant errors. In the configuration shown in Figure 9, when the non-inverting gain is 101V/V, the worst case input offset produces an offset of 0.5mV at the 1V output, which is an error of 0.05%.

Although a 100kΩ resistor in series with a gas sensor does not necessarily have the same accuracy requirements as the 10MΩ and 100kΩ resistors responsible for setting the gain, a similar resistor is required on both inputs, which is very important. This helps minimize the extra offset voltage caused by the thermocouple effect on the input to meet similar 0.1% accuracy requirements.

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