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.
“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.
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.
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.
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.