LTC1855 input range.

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

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

Hi

I did the tests,

I configured in differential mode, I selected channel 2/3 (2 input 3 connected to GND) I applied 5 volts in input and I read the value 0x1F6E,

then I raised the voltage to 6V and I read 0x259A, therefore a negative value.

While it had to stop at 0x1FFF but at 10 Volts.

Attached the screen of ogic analizer.

Any suggestions.

Thank you.

Test 5V

Test 6V

You are decoding the data improperly. For 5V I decode as 0FB7. For 6V I decode as 12CD.

FB7 calcilstes to 4.91V. 12CD calculates to 5.875V. Read off the positive edges of SCK.

I corrected the reading function on rising edge and now everything is ok.

Thanks very much for your help.

Fausto