In a previous query, I asked what the effective input impedance was for the T100-MD-888+, and was told ( May 6 and 7, 2004 )
"there are already 2.2K resistor in series with all the AD1 to AD8 and AD1 to AD6 has 0.1uF capacitor in parallel to filter out high frequency noise on the analog inputs. AD7 (DA1) and AD8(DA2) do not have the 0.1uF capacitor."
As it turns out, the 0.1uf capacitance is presented directly to the DB15 pin, then the resistor is in series to the input to the CPU.
This effectively presents a capacitave loading to any source feeding it.
I have just run into this problem, with an op amp feeding the A/D inputs, and when connected to the T100MD-888+, the opamp( LTC1637) output showed an oscillation riding on a dc level.
I proceded to remove the CPU, and the problem persisted. Examining the T100MD-888+, and following traces, showed me that the 0.1uf caps were connected directly to the DB15S connector, then connected trough the 2.2 k resistor to the CPU. Shouldn't the resistor be presented first, then the cap for a RC filter?
The solution in my case was to install a 348 ohm resistor in series with the Op amp output.
So choose your op amps carefully, for use with the A/D inputs of the T100MD-888+. They require the ability to drive 0.1 uf capacitive load.
All comments are appreciated
