Barometer Signal Conditioner
The schematic diagram below show a complete barometric pressure signal conditioner that operates from a single 1.5V battery. We can get the best accuracy and stability if we use bonded strain gauge and capacitive-based transducer, but they are expensive. This design introduce a new type of semiconductor transducer that achieves 0.01″ Hg (inches of mercury) uncertainty over time and temperature. This would be suitable for portable application since it uses only a single cell battery.
The 6 k Ohm transducer T1 need exactly 1.5mA excitation current, so it needs a relatively high voltage source. Non-inverting input of A1 senses T1’s current by monitoring the voltage drop across the resistor string in T1’s return path. The inverting input of A1 is fixed by the 1.2V LT1004 reference.
The output of A1 biases the 1.5V powered LT1110 switcher. This switcher produces two outputs from L1. The rectified and filtered voltage from pin 4 is used to supply the power for A1 and T1. A1’s output, in turn, closes a feedback loop at the regulator. This loop control the step-upped voltage to give a constant current of 1.5mA through T1. This circuit design produce the required high excitation voltage while minimizing power consumption. This is done by making the switching regulator produces only enough voltage to satisfy T1’s current requirements.
Pin 1 and 2 of Li produce a fully floating, boosted voltage, which is rectified and filtered to power A2. Since A2 floats with respect to T1, it can look differentially across T1 outputs (pin 10 and 4). Practically, pin 10 acts as ground and pin 4’s output is measured by A2 using this reference point.
The gain of A2 is set to provide a convenient scale of 3.000 V = 30.00″ Hg. Calibrating this circuit is easy, just adjust R1 for 150mV across the 100 Ohm resistor in T1’s return path. This step set the T1 current to the calibration point as specified by the manufacturer. Next, adjust the scale to match the factor of 3.000V=30.00″Hg by adjusting R2. Even if the pressure standard is not available, we can calibrate the circuit using individual calibration data provided by the manufacturer. This circuit maintain 0.01″Hg accuracy over months over a wide range ambient pressure shift. This portable device is also give interesting altitude indication if we take it when driving over hills and freeway overpasses, since 0.01″Hg corresponds to about 10 feet increase from sea level. This circuit draws 14mA from the battery, permitting the circuit to operate for 250 hours if we use a single D cell battery. [Circuit schematic diagram source: Linear Technology Application Notes]