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LMP90077 General Questions

Martin Kirstein1
Prodigy 80 points
Other Parts Discussed in Thread: LMP90077, LMP90100

Hi,

I have questions about the LMP90077.

1. The Resolution is 16 bit, which makes 0 - 65536 values. I want to use the device single ended. Now the datasheet Figure 71 shows that the MSB seems to be reserved for the sign + or minus... Also the ADC Calcuation in 10.1.3 gives me a maximum of 32768 for VINP = 5V; VINN = 0V. Now i wonder how to measure 16bit single ended ?

2. I did some testing with the WEBENCH Designer for this chip. If gives me values of 20bits ENOB and 17.5 bits NFR. How to interpret these values?

  • 0
    TI__Genius 16890 points

    Hi Martin,

    1.  As an example of how to measure single ended:  Set VA to 5V.  Set VIN0 as the VINN and VIN1 as VINP.  Define the channel as VIN1 - VIN0.  Set VREFN to ground and VREFP to a positive voltage like 5V.  Looking at the graph in Figure 71, when VIN0 and VIN1 = 0, the channel = 0V.  This will give you a value of 0x8000.  As VIN1 goes up in voltage the code will decrease to 0x0000 at 2.5V and then then count up to 0x7FFF when the channel = 5V.

    2.  The Webench designer uses the LMP90100 which is a 24 bit device, so that is why you are seeing values over 16 bits.


    Mike

  • 0 in reply to Michael Stout
    Prodigy 80 points

    Hi,

    thanks.

    So its basically like that at 5V vref: The lowest number is

    0V = 1000000000000000 (binary)

    76,29uV = 1000000000000001 (binary)

    152,59uV = 1000000000000010 (binary)

    .....

    2.5V = 0000000000000000 (binary)

    2,50007629V = 0000000000000001 (binary)

    2,50015259V = 0000000000000010 (binary)

    ...

    5V = 0111111111111111 (binary)

    Or did I miss something? Because u said " the code will decrease to 0x0000 " but maybe it actually increases?

  • 0 in reply to Martin Kirstein1
    TI__Genius 16890 points

    Hi Martin,

    The example you have is correct.  When the ADC is at the step just below 2.5V the ADC output will be 0xFFFF.  At 2.5V the output will be 0x0000.  At the next step above 2.5V it will be 0x0001 and then climb up to 0x7FFF.

    Mike