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Pressure Sensor Bridge Output

Calculate bridge output voltage for piezoresistive pressure sensors. Determine mV output from excitation voltage, sensitivity, and applied pressure.

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Formula

Vout=Vex×S×(P/PFS)V_out = V_ex × S × (P/P_FS)
S— Sensitivity (mV/V)
P_FS— Full-scale pressure (kPa)

How It Works

This calculator computes piezoresistive pressure sensor bridge output voltage, essential for process control engineers, HVAC system designers, and automotive sensor integrators. Piezoresistive pressure sensors contain a Wheatstone bridge of diffused or thin-film strain gauges on a silicon or steel diaphragm. Applied pressure deflects the diaphragm, changing resistance to unbalance the bridge. Output is Vout = Vex S (P/Pfs), where Vex is excitation voltage (5-10V typical), S is sensitivity in mV/V (typically 10-30 mV/V per OIML recommendations), P is applied pressure, and Pfs is full-scale pressure. A 20 mV/V sensor on 5V excitation produces 100 mV at full scale. Per IEC 61298, pressure sensor specifications include accuracy (+/-0.1 to +/-1% FS), nonlinearity (+/-0.1-0.5% FS), hysteresis (+/-0.05-0.2% FS), and thermal zero shift (typically +/-0.02% FS/C). Industrial sensors from Honeywell, Sensata, and Bosch achieve total error band (TEB) of +/-0.25% FS over -40 to +125C per AEC-Q100 automotive qualification.

Worked Example

Problem

Size signal conditioning for a Honeywell MLH500PSB01A pressure sensor (0-500 psi, 20 mV/V sensitivity) in a hydraulic system. Excitation is 10V, ADC is 12-bit with 5V reference.

Solution
  1. Full-scale output: Vfs = 10V * 20 mV/V = 200 mV
  2. Required amplifier gain: G = 4500 mV / 200 mV = 22.5 V/V (leave headroom for offset)
  3. Use INA128 with Rg = 50k/(G-1) = 50k/21.5 = 2.33 kOhm (use 2.32 kOhm 0.1%)
  4. Output at 350 psi: Vout = 200 mV (350/500) 22.5 = 3.15V
  5. ADC resolution: 5V/4096 = 1.22 mV/LSB
  6. Pressure resolution: 1.22 mV / 22.5 / 200 mV * 500 psi = 0.136 psi/LSB
  7. Sensor accuracy: +/-0.25% FS = +/-1.25 psi per datasheet TEB
  8. Common-mode voltage: Vex/2 = 5V (INA128 handles 0-5V Vcm on 5V supply)
Result: INA128 with G = 22.5 provides 0.14 psi resolution. Sensor TEB (+/-1.25 psi) dominates system accuracy.

Practical Tips

  • ✓Use ratiometric operation: connect both ADC reference and sensor excitation to the same regulated voltage; if supply fluctuates +/-5%, both scale proportionally and ratio Vout/Vex remains constant per Honeywell Technical Note HSC-AN-800
  • ✓For absolute accuracy, perform two-point calibration at zero pressure and known reference pressure (NIST-traceable calibrator) to correct for both offset and gain errors per ISO 17025 requirements
  • ✓Add 100 nF ceramic capacitors from each excitation line to ground, close to the sensor, to filter high-frequency noise from PWM switching that would appear as measurement noise

Common Mistakes

  • ✗Applying excitation exceeding sensor maximum: overvoltage causes bridge self-heating, shifting zero by 0.1-1% FS and degrading accuracy; verify maximum excitation (typically 5-12V) per manufacturer datasheet
  • ✗Installing sensor upside-down relative to calibrated orientation: many sensors include diaphragm dead-weight in zero calibration; orientation change causes offset shift equal to diaphragm pressure head (0.1-1% FS for liquid-filled sensors)
  • ✗Neglecting common-mode voltage at amplifier input: bridge output rides on Vex/2 common mode; choose INA with input range including Vex/2 on your supply rails per Texas Instruments INA128 datasheet

Frequently Asked Questions

Gauge sensors measure relative to atmospheric pressure (zero output at 1 atm); used for tire pressure, tank level, HVAC. Absolute sensors measure relative to vacuum (zero output at 0 Pa); used for altimeters, barometers, MAP sensors. Differential sensors measure pressure difference between two ports (positive for P1>P2, negative for P1<P2); used for flow measurement, filter monitoring, HVAC zone control. Honeywell HSC series and Bosch BMP390 offer all three types per respective product families.
Thermal zero offset is shift in bridge output at zero pressure caused by temperature-induced resistance imbalance. Specified as +/-0.02% FS/C typical per IEC 61298. Over 100C range, this is 2% FS error. Compensation methods: (1) analog using thermistor in bridge network, (2) digital using on-chip temperature sensor plus polynomial correction (integrated sensors like Honeywell HSC, Bosch BMP390). Digital sensors achieve +/-0.5% TEB over full temperature range versus +/-2% for uncompensated analog sensors.
Higher excitation gives larger output (better SNR) but increases self-heating. Self-heating error = (Vex^2/Rbridge) * thermal_resistance. For 350 Ohm bridge at 10V: P = 100/350 = 286 mW. With 0.1 C/mW thermal resistance, self-heating is 28.6 C, causing 0.6% FS error at 0.02%/C drift. Standard practice: use 5V for silicon bridges rated 5-10V maximum. For battery-powered applications, use minimum recommended voltage (often 3V) to reduce power to <26 mW per Honeywell HSC low-power modes.

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