MC56F847xx 12bit ADC problem with voltages greater than 3.3V

[an1970]

We are using Triggered parallel mode for reading ADC channels in ADCA and ADCB.

We observed that if any one of the ADC channels in ADCA goes more than 3.3V, readings belonging to other channels of ADCA gets affected.  In our case, when ANA2 input was about 3.38Volts, ANA3 readings are always high ( 4095 ).  I reduced ANA2 input voltage to less than 3.25V and ANA3 readings are restored to correct value ( which is 2048 corresponding to 1.65 volts).

Do we have any problem in case ADC input goes in the range of 3.3V to 3.6V? Because even with diode protection we cannot prevent voltage going to these levels in hardware.

please reply

regards

anantha

[johnlwinters]

You may help this issue by changing the sampling sequence so that the out of bounds channel is not impacting the next sample.

For example, by sampling a signal twice in a row, the "memory" of the previous sample does not present a significant impact.

[xiangjun_rong]

Dear anantha saligram,

I think it is a known issue, when the ADC channel voltage exceeds 3.3V, it charges the capacitor of sample/hold with exceesive voltage, in scan mode, it can not discharge when the ADC samples the following channel, so it will take effect for the following channel in scan mode.

for example, ADCA sample CH0, CH3, CH5 in order in scan mode, if the CH3 voltage exceeds 3.3V, the sample value of CH5 may be incorrect.

It is said we will fix the bug in future.

Thank you for your interest in Freescale Semiconductor products and for the opportunity to serve you.

Best Regards,

XiangJun Rong