SBOU024C august 2004 – july 2023 PGA309
Some bridge sensor applications measure the temperature of the bridge sensor by the change in the bridge resistance. This is accomplished by adding a series resistor in either the top or the bottom of the bridge excitation connections. When this is done, the common-mode voltage range of the PGA309 inputs must be observed over the operating temperature range of the application.
Figure 2-20 shows a top-side series resistor (RT+) used to monitor the change in bridge resistance with temperature. For simplification of analysis, the effective bridge resistance is converted to one resistor (RBT), as shown. For a given temperature, RBT will be a fixed value; for this example, 1.8kΩ at 70°C. Since RT has a negligible change in temperature (50ppm/°C) compared with RBT (3500ppm/°C), RT is used to detect a change in RBT. For this application, the Temp PGA is configured for VEXC on the +input, and TEMPIN on the –input. The Temp ADC uses VEXC as its reference, VREFT. The Temp PGA is set to a gain of 8. Notice that two different values for VEXC will be analyzed to emulate the changing voltage on VEXC due to the linearization block adjusting VEXC to minimize error on the bridge sensor output with applied pressure. The squareboxed values show numerical results for VEXC = 2.9V and the oval-ringed values for VEXC = 2.4V. The final Temp ADC reading will be the same value regardless of what value VEXC is used by the linearization block.
Figure 2-21 shows a bottom-side series resistor (RT−) used to monitor the change in bridge resistance with temperature. Again, for simplification of analysis, the effective bridge resistance is converted to one resistor (RBT) as shown. For 70°C, RBT is 1.8kΩ for this example. RT is used to measure the change in RBT. The Temp PGA is configured for TEMPIN on the +input and GND on the –input. VEXC is selected as the Temp ADC reference, VREFT. The PGA gain is 8. The square-boxed values are results for VEXC = 2.9V and the oval-ringed values for VEXC = 2.4V. It is seen that the final Temp ADC reading will be the same regardless of the VEXC value.
If the linearization block is not used in the application, the bridge sensor top excitation connection is made to either VSA or VREF, instead of VEXC. In either of these cases, top-side (Figure 2-20) or bottom-side (Figure 2-21), external temperature sensing can be done by adding a series resistor, RT. The Temp ADC reference (VREFT) should be changed to the bridge excitation voltage (VSA or VREF) for the specific application. This yields a constant Temp ADC output at a given temperature independent of changes in the bridge excitation voltage.