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Data output format and input waveform

Sandeep B
Prodigy 200 points
Other Parts Discussed in Thread: ADS42LB49, ADS42LB69

Dear Sir/Madam,

As per datasheet 8.3.5: Output Data Format

FFor a positive overdrive, the output code is 3FFFh for the ADS42LB49 and ADS42LB69 in offset binary output format; the output code is 1FFFh for the ADS42LB49 and ADS42LB69 in twos complement output format. For a negative input overdrive, the output code is 0000h in offset binary output format and 2000h for the ADS42LB49 and ADS42LB69 in twos complement output format.

Both ADS42LB69 and ADS42LB49 have the same overdrive code; if so, where do the 14 and 16 bit digital data come in?

As per section 9.2.2.1 Analog Input

Each input pin (INP, INM) must swing symmetrically between VCM + 0.5 V and VCM – 0.5 V, resulting in a 2-VPP differential input swing. 

Can you please explain how +0.5V and -0.5V with respect to VCM will give 2-VPP?

Please explain with waveform.

Regards,

B Sandeep.

  • 0
    TI__Expert 5660 points

    Hi Sandeep,

    It's differential so at a given time INP = 1V (0.5V - (-0.5V)) and INM = -1V (swing -0.5V - 0.5V) so the difference INP - INM = 1V - (-1V) = 2Vpp.

    Regards,

    Geoff

  • 0
    Prodigy 200 points

    Hi,

    Both ADS42LB69 and ADS42LB49 have the same overdrive code;

    For the identical digital code, why should one select ADS42LB69 over ADS42LB49?

    The ADS42LB69 is providing negative decimal values even if the 08 register is programmed with 0x10 for offset binary.

    The datasheet does not include a waveform for analogue input that would help one understand what the chip requires. Please provide me a waveform to better understand this.

    Regards,

    B Sandeep.

  • 0 in reply to Sandeep B
    TI__Expert 5065 points

    Hi Sandeep,

    Both the ADS42LB69 and ADS42LB49 output the same code when they are overdriven or underdriven when programmed with the same output format. So there is no difference between them regarding this aspect as you saw in the datasheet. This code is just to alert you of whether the analog inputs are being overdriven or underdriven.

    However, the resolution of the ADS42LB69 is 16 bits while inside the input voltage range, while ADS42LB49 has 12 bit resolution.

    Hopefully the picture below clears things.

    Best,

    Camilo

  • 0 in reply to Camilo Garcia Trujillo
    Prodigy 200 points

    Hi,

    Despite the offset binary output data format, the ADS42LB69 is producing negative decimal values.

    A distorted waveform is being produced as an output. if the waveform construction does not make use of signed decimal values.

    Regards,

    B Sandeep.

  • 0 in reply to Sandeep B
    TI__Expert 5065 points

    Hi Sandeep,

    Offset binary uses bits to represent negative or positive signed values. If you construct a waveform without using the negative values your waveform will not look the same. Refer to the table below as an example for an 8 bit scenario.

    Best,

    Camilo