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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.

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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.