5 Beta Error

Beta can be a factor of error when measuring temperature from a remote junction. Not all transistors have a beta issue. Beta is the current ratio between the collector and base. Beta represents the error present when the collector current leaks through the base. Low beta error tends to be with smaller technologies. As mentioned, the core design of TI remote temperature sensors forces either three or four currents on the D+ line to measure the delta Vbe across the remote junction. Different currents result in a different beta measurements. Equation 8 highlights the relationship between betas and the temperature result for a two-current architecture.

When the beta terms equal each other, the terms cancel out leading to no additional error due to Beta. Error comes from variations of beta from different current values. Adding radiation exposure to the remote temperature must not add additional beta errors to the BJT remote temperature result. The Gummel Plot highlights the most preferred beta characteristics for a BJT to match with the remote temperature sensors. When matching the transistors, TI recommends the BJT to follow the characteristics within the linear region shown in Figure 5-1. The blue line represents the Beta ratio following the right secondary y-axis. The red and gray lines follows the primary left y-axis.

When designing BJTs, the following plot can be useful when sizing the BJTs to match with our remote junctions. When sizing transistors to match with the remote sensors, TI recommends small delta beta measurements across the set D+ current values on the remote as highlighted in Figure 5-2.