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LTC1855 input range.

I made a measurement circuit that must read with respect to the GND +/- 10V on the analog input pin, and I used your LTC1855.

I made the wiring diagram as per datasheet and I configured the channel in single ended mode.

But to my surprise I can only measure +/- 5V. If I apply + 5V I read the value 0x1FFF (positive full scale) if I increase the input voltage the reading becomes negative.

How should I configure to read +/- 10 Volt full scale, range indicated in the data sheet?

I attach the wiring diagram

Thanks in advance.


  • Hi,

    Your schematic shows the LTC1856 not the LTC1855. Please confirm which par you are using. If it is the LTC1855, the most logical assumption is that the part has been programmed for unipolar not bipolar operation. If bipolar operation was programmed, once FS was reached the output code would stop changing, the device would not go to negative values. Please provide an oscilloscope waveform showing CONVST, SCK, SDI and SDO to confirm this.

    Your schematic shows two of the inputs floating. CMOS inputs should never be left floating. This can lead to unpredictable behavior.

  • Hello,

    the part is LTC1855, because the LTC1856 was not available on mouser. I do the tests in bipolar, and connect the two free inputs to gnd. I will let you know the result of the test and send the photo of the logic analyzer. Thanks for the support Fausto

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