Calculating R₂ and Input Range

R₂ can be calculated using the loop maximum current value as in Section 7.1.2, for 24mA loop current and 2.5V maximum voltage, 24.9Ω is the right value. Use 0.1% resistor or better with temperature drift coefficient lower than 10ppm/°C to get the XTR100 accuracy advantage.

Now calculate back the different nominal input voltages required for different current values as in Table 7-7.

The voltage levels are within the valid output range of AFE88101 DAC. See also the AFE88101 datasheet for the corresponding digital codes.

Selecting Q₁

Important parameters for selecting the pass transistor Q₁ include maximum collector emitter voltage V_CE, maximum power dissipation, low I_C leakage, and high current gain h_FE.

The best Q₁ selection depends on the system requirements, like available space and maximum loop voltage. Smaller packages, like SOT-23, which are beneficial regarding space saving, are typically limited to continuous power dissipation below 0.5W, so this package is an excellent choice for space-constrained application when loop voltage is clamped. SOT-23 transistors can also be used if the board has excellent heat sinking and loop voltage is low. SOT-23 transistors can offer higher current gains. When higher power handling is required, a BJT in larger package like SOT-89 or SOT-223 can be used.

As an example, select BC55PA NPN, which is a 60V transistor, with high current gain and has variants that can handle power dissipation from 0.4W up to 1.6W and comes in SOT1061 relatively small package.

Base and Emitter Transistor R_E and R_B

High transistor current gain requires emitter degeneration, which is the function of R_E. The resistor value must not be high as high value affects the voltage drop in case of high current. R_E = 10Ω results in a voltage drop less than 0.25V at the highest loop current.

Base resistor R_B is isolating the op amp from the capacitive load of the Q₁, which can affect the stability of the loop. Higher R_B with high base current (resulting from high loop current or lower current gain) leads to higher voltage drop and can exceed the op amp swing limits. At low values of h_FE = 60, and if the user wants to keep the R_B voltage drop less than 0.9V, the maximum value of R_B is 2.5kΩ.

Confirm that in all conditions, V_RE+V_BE+V_RB < V_CC−200mV.

Compensation Components R_C and C_C

Compensation R_C and C_C are determining the loop bandwidth. Optimal values of compensation capacitor C_C can be found using SPICE simulation by picking a value of R_C and running AC simulation for the loop while stepping C_C to get the best phase margin while keeping proper bandwidth. Simulation shows R_C = 20kΩ, C_C = 25pF are valid values for the example at hand.