LTC2497 Conversion

Hi

I am using LTC2497. I am trying to get the ADC conversion reading, could anyone explain how does the conversion of 2's compliment to decimal works on this ADC? I tried many times and it does not seem to be working.

Thank you.

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  • 0
    •  Analog Employees 
    on Jun 8, 2020 3:14 PM

    The LTC2497 is 16 Bits + Sign. B22-B6 are in two's complement format. For example Code 1010 0000 0000 0000 0000 0000 

    B23 and B22 are 10. This indicates the output code is greater than or equal to 0 and less than full scale . Bits 22-6 are 0 1000 0000 0000 0000. This is equal to decimal +32768. 

    A negative code example would be 0111 1111 1111 1111 1111 1111. B23 and B22 are 01. This indicates the output code is less than 0 and reater than or equal to -FS. Bits 22-6 are 1 1111 1111 1111 1111. this is equal to decimal -1.

  • I did quite a few data sets and all point to having a amplified digital value of 2.78-2.80.

    Any possible error which might have caused this readings?

  • 0
    •  Analog Employees 
    on Jun 9, 2020 2:07 PM in reply to SLee144

    what is your reference voltage? Are you doing single-ended or differential measurement? Is the -input at ground or at vref/2? Decoding the input voltage of 0.507, assuming a 5V reference and the -input grounded, I come up with 0.5407V. That is in the ballpark. Remember +FS is only Vref/2. Can you provide a schematic or give more details? you might have a filter settling issue to account for the rest of the error.

  • 1. Vref = Vss = 5v

    2. Single ended

    3. -ve Input = GND

    I have attached the schematics below... Resistors: 0 ohms, I2C Res: 4.7k ohms, Caps = 0.1uF & 10uF

    Thank you for helping. 

  • 0
    •  Analog Employees 
    on Jun 10, 2020 1:53 PM in reply to SLee144

    My assumptions were correct. So the output code you are seeing is off by about 6.6%. Your schematic doesn't show what is driving the ADC inputs. COM is connected directly to ground. If the channel you are driving does not have a similar low impedance path to ground, the mismatch in impedance could be causing the error. Please include in your schematic how the inputs are being driven. Is your circuit built on a PCB with a solid ground plane? Are your bypass capacitors as close as possible to the pins being bypassed? What is the 5V_REF source?

  • Hi ghoover

    1. ADC inputs are driven by the molex connectors on the left.

    2. The ADC traces are on top layer and full plane on layer 2.

    3. I missed out the bypass caps.

    4. Vref is connected to my Vss. Will you recommend using a external Vref for this application?

    Thank you.

  • 0
    •  Analog Employees 
    on Jun 11, 2020 1:48 PM in reply to SLee144

    Telling me the ADC is driven by molex connectors still doesn't tell me what is driving the ADC. Is it an opamp, a sensor (what's the output impedance?), a battery? Is there a long trace between the molex connector and the ADC inputs? For Vss, what generates it? If it's a switching regulator, you may need something a little quieter.

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  • 0
    •  Analog Employees 
    on Jun 11, 2020 1:48 PM in reply to SLee144

    Telling me the ADC is driven by molex connectors still doesn't tell me what is driving the ADC. Is it an opamp, a sensor (what's the output impedance?), a battery? Is there a long trace between the molex connector and the ADC inputs? For Vss, what generates it? If it's a switching regulator, you may need something a little quieter.

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