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Challenges & Requirements: Voltage Reference Design for Precision Successive-Approximation ADCs, Part 4

Troubleshooting reference issues
A poorly designed reference circuit can cause serious conversion errors. The most common manifestation of a reference issue is repeated or “stuck” codes from the ADC. This happens when noise on the reference input is large enough to cause the ADC to make an incorrect bit decision.

This may show up as the same code being repeated many times, even though the input is changing, or as a repeated string of ones or zeros in the less significant bits, as shown in Figure 9. The areas circled in red show where the ADC gets stuck, repeatedly returning the same code. The problem generally gets worse near full scale because the reference noise has a greater impact on the more significant bit decisions. Once an incorrect bit decision has been made, the remaining bits become filled with ones or zeros.

Figure 9

'Stuck' codes in ADC transfer function.

“Stuck” codes in ADC transfer function.

The most common reasons for these “stuck” bits are the size and placement of the reference capacitor, insufficient drive strength of the reference/reference buffer, or poor selection of the reference/reference buffer. These result in excess noise.

It is critical to place the reservoir capacitor close to the ADC’s reference input pin, using wide traces to connect it, as shown in Figure 10. The capacitor should have a low impedance path to ground using multiple vias to the ground plane. If the reference has a dedicated ground, the capacitor should be connected close to that pin using wide traces.

Because the capacitor acts as a charge reservoir, it needs to be large enough to limit droop and must have low ESR. Ceramic capacitors with X5R dielectric are a good choice. Typical values are in the 10 µF to 47 µF range, but smaller values can sometimes be tolerated depending on the current requirements of the ADC.

Figure 10

Typical reference capacitor layout.

Typical reference capacitor layout.

Insufficient drive strength is another issue, especially if low-power references or micropower reference buffers are used, since these typically have much higher output impedances that increase dramatically with frequency. This is particularly true when using higher throughput ADCs, since the current requirement is higher than at lower throughputs.

Excessive noise from either the reference or reference buffer, relative to the LSB size of the converter, can also result in stuck codes. Therefore the voltage noise of the reference circuit must remain a small fraction of the LSB voltage.

Conclusion
This article showed how to design a reference circuit for precision successive-approximation ADCs and highlighted how to identify some of their common problems. The calculations presented are a means to estimate the reference circuit drive strength and noise requirements so that a greater probability of success can be realized when testing the circuit in hardware.

References
AN-931 Application Note. Understanding PulSAR ADC Support Circuitry.

Kester, Walt. Data Conversion Handbook, Chapter 7. Data Converter Support Circuits.

Kester, Walt. “Which ADC Architecture Is Right for Your Application?” Analog Dialogue. Volume 39, Number 2, 2005.

Walsh, Alan. “Front-End Amplifier and RC Filter Design for a Precision SAR Analog-to-Digital Converter.” Analog Dialogue. Volume 46, Number 4, 2012.

8 comments on “Challenges & Requirements: Voltage Reference Design for Precision Successive-Approximation ADCs, Part 4

  1. sandrad.jerome
    May 22, 2014

    I researched lot of articles and the best I came across is yours. Behance Jeremy Renner

  2. amrutah
    May 24, 2014

    @sandrad:

    What is this Behance Jeremy Renner link for.  

    Is it your name or an adv.?

  3. amrutah
    May 24, 2014

    Alan,

       I agree that the reservior capacitor should be place very close with low ESR. But other important routes are

    1> from the capacitor to ADC

    2> ADC-capacitor array to ADC ground reference.

       The ESR can be a factor for the stability of the reference system, but the series resistance can cause significant drop and leads to wrong reference value and hence the ADC conversion accuracy.

  4. chirshadblog
    May 26, 2014

    @amrulah: Well yes indeed but it's a common issue to all the items isn't it? Nothing wrong with the manufacturer itself I guess

  5. AlanWalsh
    May 27, 2014

    Hi Amrutah,

    You are correct it is a bit of a delicate balance between insuring the driving circuit is stable but not too much series resistance or it will introduce reference droop which can be seen as a throughput dependent gainerror or worse (nonlinearity). The good news is that generally with such a large cap the driving buffer becomes seriously overdamped and almost can't oscillate but this has to be checked on a case by case basis.

    Best Rgds,

    Alan

  6. chirshadblog
    November 23, 2014

    @amrulah: Well any other alternative for this which can support many of the requirements or at least most of them ? 

  7. amrutah
    November 24, 2014

    Well any other alternative for this which can support many of the requirements or at least most of them ?

    @chirshadblog: It will be helpful, if you can be a little more specific with the highlighted “this” and “them”.  I am unable to correlate with the previous post.

    Thanks.

  8. Davidled
    December 2, 2014

    As long as the correct capacitor and resistor values meeting the current consumption of ADC is used, stuck issue could be reduced. Also, serial resistance could be considered, when designing the PCB board. It is assumed that PCB simulation might generate the predefined resistance.

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