We have a lot of queries on the ADC usage on the CC3200, mostly related to sensing the battery voltage and some sensors. The internal SAR ADC would draw a transient current while sampling the input and this can cause issues with measurement. So the input signal has to be buffered before feeding to the ADC. This can be done in two ways discussed below
1. DC measurement :
This is useful for battery voltage sensing, light detection etc..
a. Scale the measured voltage so that the full scale is less than 1.4V.
b. The scaling can be performed using a resistor divider with a capacitor. Capacitor connected between ADC input and ground.
c. The resistor value has to be chosen based on the bleeding current. Typical values range from 10K to 100K. If this current is a concern, then a MOSFET can be used to cut off the resistor from the supply.
d. The capacitor is chosen based on the settling time of the signal and the sampling input capacitance. In this case the internal sampling capacitor is 3.2pF. Assuming that the current needed to charge this cap is fully derived from the external capacitor, the value has to be at least 2^12 x 10 times larger. (for error to be less than tenth of an LSB)
e. The external cap should be > 0.13uF. Assume standard value of 0.22uF.
f. The capacitor can be increased to improve the accuracy, but the frequency response of the system would be limited due to the external RC combination. For e.g. using 10K + 10K resistor divider with 0.22uF cap , the 3dB bandwidth would be 144Hz. This should be good for most applications involving sensing close to DC frequencies.
2. For AC measurements
a. For all AC measurements that need higher bandwidth, the input of the ADC has to be driven using a dedicated OPAMP buffer.
An excellent application note is available here to arrive at the OPAMP selection.
http://www.ti.com/lit/ml/slyp166/slyp166.pdf
