3 [Edit removed during grace period] edited Feb 23 '14 at 21:20 Spehro Pefhany 219k55 gold badges174174 silver badges452452 bronze badges 2 added 5 characters in body edited Feb 23 '14 at 19:57 Spehro Pefhany 219k55 gold badges174174 silver badges452452 bronze badges I'm not that familiar with the Atmel ADC converter, but I suggest that maybe the acquisition time for the S&H is not long enough. I think it should be 15usec or greater. The maximum time constant is 100K$$\\Omega\cdot 14\$$pF or 1.4usec, so 10 time constants is about 15usec. Alternately, do you have something like a 5.1V Zener diode on the input? That could easily cause errors in that range. I'm not that familiar with the Atmel ADC converter, but I suggest that maybe the acquisition time for the S&H is not long enough. I think it should be 15usec or greater. The maximum time constant is 100K$$\\Omega\cdot 14\$$pF or 1.4usec, so 10 time constants is about 15usec. Alternately, do you have something like a Zener diode on the input? That could easily cause errors in that range. I'm not that familiar with the Atmel ADC converter, but I suggest that maybe the acquisition time for the S&H is not long enough. I think it should be 15usec or greater. The maximum time constant is 100K$$\\Omega\cdot 14\$$pF or 1.4usec, so 10 time constants is about 15usec. Alternately, do you have something like a 5.1V Zener diode on the input? That could easily cause errors in that range. 1 answered Feb 23 '14 at 19:45 Spehro Pefhany 219k55 gold badges174174 silver badges452452 bronze badges I'm not that familiar with the Atmel ADC converter, but I suggest that maybe the acquisition time for the S&H is not long enough. I think it should be 15usec or greater. The maximum time constant is 100K$$\\Omega\cdot 14\$$pF or 1.4usec, so 10 time constants is about 15usec. Alternately, do you have something like a Zener diode on the input? That could easily cause errors in that range.