AD7685

I am using AD7685 to read a CMOS linear camera sensors. The sensor has 512 pixels. It reads 512 consecutively at 125Khz, wait some time, then read it again. 

The first few AD readings, for some reasons, are always off a little bit. The first reading has the most error in it.  I wonder if it is issue with the AD7685, where there is sampling issues with the first few pixels due to the sudden signal level change. So checked the track time, it has more than 1.8us that the spec says. 

I looked the sensor output, I don't see the discrepancies for the first few pixels, they are all pretty smooth. 

I wonder if there is sleep low power mode before you start the first conversion.  Takes long time for the chip to get out of the sleep mode. 

-Andrew



Added a tag for internal tracking purposes.
[edited by: tschmitt at 7:11 PM (GMT -4) on 23 Sep 2020]
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  • 0
    •  Analog Employees 
    on Sep 23, 2020 7:10 PM

    Hi Andrew,

    Thanks for posting your query.

    My first guess is that you may be witnessing a reference voltage drive issue, meaning the voltage on the AD7685 REF pin is deviating across the first few samples (i.e. readings) before eventually settling out to its steady state value. The reason I think this is because the AD7685 REF input is a dynamic load, where it draws very little current while the AD7685 is idle (i.e. not converting) and then draws short bursts of peak current while it is converting. This behavior is common for all precision SAR ADCs, since the bit trials in the converter core require pulling charge from the REF input to generate an output code.

    Basically if the circuitry providing VREF to the REF input is not designed properly, i.e. with low output impedance/load regulation, this can cause the output voltage of the reference circuitry to ring for a few samples before it fixes itself. The response is similar to looking at the step response of an underdamped system.

    Can you share what components you are using in the circuit driving the AD7685 REF input? Did you include a REF decoupling capacitor placed very closely to the AD7685 REF input as recommended in the Voltage Reference Input section of the AD7685 data sheet?

    Thanks,

    Tyler

  • I am using ADR421BR as the 2.5V ref. And put several caps close to the ADC. 

    I am using the ADR435 to give me 2.5 vdc reference. 

    I was reading the AD4008 datasheet, it says on the first page: 

    "First conversion accurate"

    What does that infer? Is AD4008 series newer design versus AD7685? Please recommend a chip between 250K to 500Khz single ended 16bit ADC. 

  • 0
    •  Analog Employees 
    on Sep 24, 2020 3:44 PM in reply to honestgold

    Hi Andrew,

    Thanks for the info.

    "First conversion accurate" basically means that if the ADC is idle for a long period of time, and then you provide a CNV rising edge to kick off a conversion, the result for that conversion will be within specs in the data sheet. The AD4008 calls this out specifically in its data sheet but I believe the AD7685 is first conversion accurate as well.

    The AD4008 is indeed the newer design.

    For 250k - 500kSPS single ended 16-bit ADC recommendations, I would also recommend the AD4008 and AD7988-5. Both of these can run up to 500kSPS, just take note that their VDD supply constraints are different from the AD7685. (2.5V for AD7988-5 and 1.8V for AD4008.)

    Will give a response regarding the pixel measurement error topic in a reply to your newer comment.

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  • 0
    •  Analog Employees 
    on Sep 24, 2020 3:44 PM in reply to honestgold

    Hi Andrew,

    Thanks for the info.

    "First conversion accurate" basically means that if the ADC is idle for a long period of time, and then you provide a CNV rising edge to kick off a conversion, the result for that conversion will be within specs in the data sheet. The AD4008 calls this out specifically in its data sheet but I believe the AD7685 is first conversion accurate as well.

    The AD4008 is indeed the newer design.

    For 250k - 500kSPS single ended 16-bit ADC recommendations, I would also recommend the AD4008 and AD7988-5. Both of these can run up to 500kSPS, just take note that their VDD supply constraints are different from the AD7685. (2.5V for AD7988-5 and 1.8V for AD4008.)

    Will give a response regarding the pixel measurement error topic in a reply to your newer comment.

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