SWRA679 January   2021 CC3200 , CC3220R , CC3220S , CC3220SF , CC3230S , CC3230SF , CC3235S , CC3235SF

 

  1.   Trademarks
  2. 1Introduction
    1. 1.1 Basics of the SAR ADC Architecture
    2. 1.2 Introduction to the CC32XX ADC
      1. 1.2.1 Main Features
      2. 1.2.2 ADC Sampling Operation
      3. 1.2.3 ADC Additional Information
  3. 2ADC Application Examples
    1. 2.1 Battery Voltage Measurements
      1. 2.1.1 Important Considerations
        1. 2.1.1.1 Extra Current Draw
        2. 2.1.1.2 Droop Correction
        3. 2.1.1.3 Offset Adjustment
        4. 2.1.1.4 Least Squares Fit
        5. 2.1.1.5 Choosing the Capacitor (for droop correction)
        6. 2.1.1.6 First Measurement
        7. 2.1.1.7 Time Between Measurements
  4. 3AC Measurements
  5. 4Useful References
    1. 4.1 Smart Thermostat
    2. 4.2 Measuring Air Quality With the Winsen MP503 Analog Sensor
    3. 4.3 Touch Position Detection With HMI Through Resistive Touchscreen
  6. 5References

2.1.1.1 Extra Current Draw

The equivalent resistance of R1 + R2 in series draws a current of Vcc/(R1 + R2) = 3.3V/370K ~= 9 μA. If the current draw is a concern, you may be tempted to simply increase the resistor values. However, this slows down the re-charging of the capacitor Cext after the pin is disconnected from the ADC.

GUID-20201021-CA0I-9MTK-ZLLQ-BLBLV19Z4PLK-low.gif Figure 2-2 Reducing Resistive Current Draw

An alternative is to disconnect R2 from the circuit by using an NMOS transistor controlled by the CC32xx device. This circuit is shown in Figure 2-2. In this circuit, an additional optional capacitor is shown (C1) that can be used to improve the overall frequency response and speed up the charging time. This capacitor, along with R1, R2, and Cext, work as a compensated attenuator. Note that this arrangement will not offer any noise filtering and should be used with caution.