SBOU024C august 2004 – july 2023 PGA309
A typical bridge pressure sensor is shown in Figure 5-1. For a given bridge excitation voltage (VEXC), the output voltage of the bridge (VP − VN) is a voltage proportional to the pressure applied to the sensor.
Span is the scale factor for VP − VN at full-scale pressure input relative to the bridge excitation (VBR+ − VBR−). Span is also called FSO (Full-Scale Output), FSS (Full-Scale Sensitivity), Sensitivity, or Gain. For example, with a bridge excitation voltage of 5V, a 2mV/V FSS implies that the bridge output will be 10mV at full-scale pressure.
Offset, also known as Zero, is the output of the bridge (VP − VN) with zero pressure applied. Often a bridge sensor’s Zero may be equal to or greater than its FSS for a given excitation voltage. Figure 5-2 graphically illustrates the definition of Span and Offset.
An ideal sensor would have span and offset curves over temperature, as shown in Figure 5-3. Real-world sensors have span and offset changes that change over temperature. Both span and offset have variations at +25°C, linear changes with temperature, and nonlinear changes with temperature. Figure 5-4 and Figure 5-5 illustrate span and offset changes over temperature for a bridge sensor with second-order nonlinearities. TC1 coefficients represent a linear change with temperature, and TC2 a second-order change with temperature.
Many bridge sensors have a nonlinear output with applied pressure. Figure 5-6 shows the non-ideal curves for both a positive and negative nonlinear bridge sensor output with applied pressure. The PGA309 provides calibration over temperature for both span and offset, and has dedicated linearization circuitry to linearize many types of bridge sensors whose outputs are not linear with applied pressure.