Wideband Sense & Heater Control ADC muxing 74HC4052

 The dual 4 to 1 line mux U4 (74HC4052) allows two groups of four signals to be sensed by the microcontroller's two ADC input lines ADC0 and ADC1.

Wideband signals Vsx5, Ih, Vsx1, IpSense, DACV & Cal (VGND) voltages are sensed as well as H- via filtering provided by R112 and C103. also shown here is the H+ sense signal but it is not selected by the mux, but rather passed directly to the microcontroller via filter components R111 and C102.

Analogue user channel 1 USR1 (see Y5 connector input below) is also sensed by this mux.

ADC input protection BAT54S

Hi guys , do you know how do you protect your PIC adc inputs against reverse polarity ? Ì'm feeding battery volrtage to my PIC16F877A and would like to protect it against operator error.

The best protection scheme is a series resistor and a pair of schottky diodes (such as BAT54S) one to each power rail. The resistor should be sized to not burn out the diodes and no affect the sample accuracy of the A/D. The Microchip data sheets give info on the accuracy issue.

The schottky keeps the voltage drop below the processors internal protection diode forward drops and therfore prevents latchup and destruction.

We have used this approach for many years with complete success. Typically we use a 10K, 1206 resistor although sometimes go smaller depending on how we are using the A/D, which comes down to the sample rate. The same circuit is also effective on digital inputs that are exposed to the external world.