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ltc2448 input impedance problem

Hello all,

we're using the ltc2448 for our design (see schematic). We're able to control the ADC over SPI and we're also able to receive converted values. These values are somehow plausible.

But during conversion time we're seeing a voltage drop of the input signal.

For example with J3 open we have an analog input voltage of ca. 4.096 at ADC ch4 but during measurement this input voltage drops to ca. 3.90 V, we see this input voltage drop on the oscilloscope and in the ADC conversion values. This does not make much sense to us, can anybody shed some light on this behavior? Thank you very much.