9 I_DQ Monitoring

Monitoring the drain quiescent current (I_DQ) can detect if the PA is efficiently conducting the desired current. An external INA can be used to convert current measurements into voltage outputs for the AFE11612-SEP. The INA used in this example circuit is the INA240-SEP, which features a 20 V/V gain. With the following equation, the ideal voltage differential across the resistor is calculated, where V_DIFF is the maximum differential-input voltage across the INA’s inputs, V_OUT is the ADC range of 2.5 V or 5 V, and gain is 20 V/V. This calculation gives a maximum voltage differential of 125 mV for 2.5-V ADC range or 250 mV for 5-V ADC range across the resistor.

V_DIFF = V_OUT / GAIN

Next, use the following equation to calculate resistance. R_SENSE is the shunt resistor value; V_DIFF is the previously calculated maximum voltage drop of 125 mV or 250 mV; and I_{DQ_MAX} is the maximum current draw of the power amplifier.

R_SENSE = V_DIFF / I_{DQ_MAX}

Similar to the V_DRAIN monitoring circuit, an external RC filter is required for current charging.

Figure 9-1 I_DQ Monitor Circuit