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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.

 

BR

Matthias

Parents Reply
  • The ADC has a switched capacitor input. With the large RC at the input it does not have time to settle. I would suggest installing the optional buffers between MUXOUT and ADCIN shown on the schematic to solve this problem. The other alternative is to use a smaller filter that will have time to settle. The RC time constant should be less than 580ns to settle.

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