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ADS1602: Output won't change when input change

Cindy Chang
Intellectual 2475 points

Part Number: ADS1602
Other Parts Discussed in Thread: THS4551,

Hi,

We found that no matter how we change analog input, the digital output stuck at 24576 all the time.

We set /REFEN low to use internal reference, but we found that reference voltage values are not in the correct range.

VREFP= 1.223V, VREFN=1.216V, VMID= 1.339V when power ready and with input buffer made by THS4551.

All of them become 0V when input floating and still with all power rail.

Could you please help to figure out what may cause reference voltage be changed?

Thank you.

Best Regards,

Cindy

  • 0
    TI__Guru 51270 points

    Hello Cindy,

    Can you provide the schematic with the ADC and all capacitors, plus the input amplifier?

    Have you confirmed all power supplies are correct voltage levels?

    4.75V<AVDD<5.25V

    2.7V<DVDD<3.3V

    2.7V<IOVDD<5.25V

    Does the design include the capacitors shown in Figure 43 of the datasheet?

    Is the proper value of Rbias connected to pin 8 (37kOhm for 2.5MSPS)?

    Is /PD pin 17 pulled up to IOVDD voltage level?

    For proper operation, the /SYNC pin must be asserted after the power supplies have ramped to final value, per Figure 1.

    Is the CLK signal present on CLK pin 40 after power-up and before initialization /SYNC pulse?

    Regards,
    Keith Nicholas
    Precision ADC Applications

  • 0 in reply to Keith Nicholas
    Prodigy 130 points

    Hello Keith 

    Assist Cindy to supplement the information

    Can you provide the schematic with the ADC and all capacitors, plus the input amplifier? 

    => Please help review the attached circuit designDelta Sigma_ADC3.PDF

  • 0 in reply to Keith Nicholas
    Prodigy 130 points

    Hello  Keith 

    Continuing the problem, provide ADS1602 test report, please help confirm! 

    Thank you.

    Best Regards,

    Samson

    0317.ADS1602 TEST REPORT.pdf

  • 0 in reply to user5005898
    TI__Guru 51270 points

    Hello Samson,

    The Rbias resistor connected to pin 8 must be connected to analog ground (AGND).  It looks like you have a schematic error and connected to A+5V_2.

    Please correct and see if the part starts working correctly.

    Also, you can adjust Rbias to a higher value at lower clock frequency, but this also effects the accuracy of the internal reference.  It is suggested to only use Rbias=37kOhm when using the internal reference.

    In addition, the output capacitance connected to the VREFP, VMID, and VREFN pins will effect the stability of the internal reference buffer.  We have verified performance with the suggested values in Figure 43 of the datasheet.  I recommend removing the 3 additional 22uF caps, C338, C339, and C340.

    Regards,
    Keith

  • 0 in reply to Keith Nicholas
    Prodigy 130 points

    Dear  Keith

    Thank you for your assistance in confirming. I have followed your suggestion to deal with it, but found that the reference voltage level is incorrect. The test results are as follows:

    VREFP=3.26V , VREFN=1.81V , VMID=2.47V

    May I ask if the internal reference voltage level is wrong, is it caused by not designing RBias to 37K?

    In the current test situation, when the input analog voltage VP-VN range is greater than 3.2V, wrong results will be obtained. I think it is related to the internal reference voltage. Do you think so?

    Thank you.

    Best Regards,

    Samson

  • 0 in reply to user5005898
    TI__Guru 51270 points

    Hello Samson,

    Based on your measurements, the full scale input range will be +/-(VREFP-VREFN)=+/-(3.26-1.81)V=+/-1.45V.  Any input voltage (Vp-Vn) greater than +1.45V or less than -1.45V will cause a full scale reading.

    Please change Rbias to 37kOhm and measure reference voltages, VREFP (4V), VREFN(1V), and VMID(2.5V).  Yes, I think you need to change Rbias to 37kOhm to get proper reference voltage levels.

    Until the reference voltages are correct, you will not get correct conversion results.

    Regards,
    Keith