February 2, 1998

Signal conditioning precisely indicates humidity

William Whitehead, Lafayette, CO

The circuit in Figure 1 translates the level of humidity from 0 to 100% into a stable, respective dc signal of 0 to 100 mV. The heart of the circuit is IC₁, the IH3602-C relative-humidity sensor (Microswitch/HyCal Sensing Products, Freeport, IL). This device operates on 5V at 200 mA and provides a dc output of 0.8 to 4.0V over the 0 to 100% humidity range. The output impedance is 5 mA when sourcing, and on-chip circuitry preconditions the humidity-related charge of a thermoset polymer dielectric capacitor. The dc output contains a small amount of 1-kHz modulation, which is an artifact of typical switched-capacitor circuits. Device accuracy is ±2% relative humidity, and linearity is ±0.5%. The six-pin TO-5 package includes a precision thin-film, 1-kilohms RTD that you can use in applications that require temperature-corrected relative-humidity data.

The LT1236-5 provides a stable 5V supply for IC₁, which precludes any ratiometric changes in the sensor's output that would otherwise occur with less stable 5V supplies. Amplifier IC_2A's voltage follower buffers the sensor's high-impedance output. IC_2B, a 1-kHz, two-pole Butterworth filter, reduces the 1-kHz chopper modulation. The unique output stage comprising IC₃ is a precise, dc-accurate, ultralinear, noninverting summing/scaling amplifier. The LT1112 performs well for this function because it has high large-signal gain (A_VOL), low input bias current (I_B), and low input offset voltage (V_OS). The summing node of IC_3A is a handy spot to add a temperature-correcting term, as the figure indicates. C₁ and C₂ roll off any high-frequency gain/phase-related oscillations.

The 0% relative-humidity adjustment not only takes care of the nonzero output of IC₁, but also compensates for any residual offsets. You achieve a full-scale output of 100 mV using the 100% adjustment.

The circuit draws 2.56 mA, which, if battery-powered, results in a battery life of about 250 hours. You can drop the total consumption to approximately 625 mA by changing IC₂ and IC₃ to LT1078s and by powering IC₁ from the 5V output of the LT1121-5. This change lengthens battery life to more than 1000 hours. However, these changes also reduce the overall accuracy of the circuit and prevent IC₃ from driving anything but very light capacitive loads, such as high-impedance ADC inputs.

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