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LTC1856 configuring and reading back differential channels

I'm testing the LTC1856 differential channels, Chan 6 & 7. I'm confused how to configure them and read them back.

Both channels work in single ended mode reading back 32767 0x7FFF with +10V input.

When I configure the channels as differential and input +10V on chan 6 and -10V on chan 7 I read back 32767 from both channels. I also get the same result when injecting +5V on chan 6 and -5V on chan 7?

I've configured both channels with the input word 00110000 which should configure the multiplexer for differential channels + on 6 and - on 7.

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  • Thanks for the quick response. I've taken care of the conversion result being shifted out next.

    It's the differential of + and - 10V I'm more concerned about. On my initial reading of the datasheet I thought the device could handle a differential input voltage of 20V, +10V on CH6 and -10V on CH7, and report it back as +10V read from CH6 and -10V read from CH7. But on further inspection it states on page 13 of the datasheet

    "The SAR contents (a 12-/14-/16-bit

    data word) which represents the difference of ADC+ and

    ADCare loaded into the 12-/14-/16-bit shift register."

    so it looks like the maximum reportable differential voltage is + and - 5V?  Everything greater than this is +full-scale on CH6 and -full-scale on CH7

    I could configure channels 6 and 7 as single ended referenced to COM which will give me +10 on CH6 and -10V on CH7 but won't remove the common mode noise.

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  • Thanks for the quick response. I've taken care of the conversion result being shifted out next.

    It's the differential of + and - 10V I'm more concerned about. On my initial reading of the datasheet I thought the device could handle a differential input voltage of 20V, +10V on CH6 and -10V on CH7, and report it back as +10V read from CH6 and -10V read from CH7. But on further inspection it states on page 13 of the datasheet

    "The SAR contents (a 12-/14-/16-bit

    data word) which represents the difference of ADC+ and

    ADCare loaded into the 12-/14-/16-bit shift register."

    so it looks like the maximum reportable differential voltage is + and - 5V?  Everything greater than this is +full-scale on CH6 and -full-scale on CH7

    I could configure channels 6 and 7 as single ended referenced to COM which will give me +10 on CH6 and -10V on CH7 but won't remove the common mode noise.

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