7.3.19.2.1 TC and NL Coefficients
The PGA305 device implements third-order TC and NL compensation of the bridge offset, bridge span, and bridge nonlinearity. The equation has 16 coefficients, and hence requires at least 16 different measurement points to compute a unique set of 16 coefficients. Use Equation 7 to calculate the TC-compensated DAC output.
Equation 7. DATA_OUT = (h0 + h1T + h2T2 + h3T3) + (g0 + g1T + g2T2 + g3T3) × P + (n0 + n1T + n2T2 + n3T3) × P2 + (m0 + m1T + m2T2 + m3T3) × P3
The 16 different P ADC and T ADC measurements can be made, for example, at four temperatures and at four different pressures. Note that:
- P GAIN and T GAIN values must be set to a fixed value for all measurements.
- At each measurement point, the P ADC value and the T ADC value must be recorded in order to compute the 16 coefficients.
- Sometimes, it may be expensive to measure P ADC and T ADC at different temperatures and pressures. In this case, there are three approaches:
- Use a model of the bridge to estimate P ADC and T ADC measurements instead of actually measuring.
- Use batch modeling, in which a family of sense elements is characterized across temperature, and the TC coefficients of the compensation equation are determined prior to calibration. On a production line, measurements are made at a limited number of temperature and pressure set points, and coefficients are adjusted accordingly. Discuss with TI application engineers for details.
- Reduce the number of coefficients by reducing the order of TC compensation. Discuss the procedure to use fewer coefficients with TI application engineers.