How To Sequence Your SAR ADC Power Supply

How To Sequence Your SAR ADC Power Supply

Introduction

Sometimes we know that powering up integrated circuits simply applies the required voltage or current parameters to the circuit and we are good to go. Little do we are using a high-precision or high-performance system. It has so many different sections in the circuit that are properly cascaded and requires a sequence of applying power to the different sections of the circuit. Otherwise, it will not achieve its optimized performance. The worst case is it will damage the IC or ADC.

Power supply sequencing can prevent damage and extend the life of your power supply and any sensitive ICs and FPGAs in your design. In addition to controlling the startup timing to control the current draw, the power supply rails need to stay within the voltage tolerance requirements of FPGAs. These tolerances have become tighter with new process technologies, to the point where 3% total core voltage tolerance may be necessary. Sequencing the power supplies in a system can be accomplished in several ways:

  • Sequencing using a reset IC
  • Analog up/down sequencers
  • Simultaneous sequencing

Any design must include power sequencing, especially for intricate systems that require several power rails.

One of the mysteries or at least the cause of considerable confusion about successive approximation register (SAR) ADCs is calculating their exact power requirements at the system level. Datasheets can be confusing on this specification. SAR ADCs provide a low-power means to measure input signals. The power consumption often scales with the sample rate, making for a very efficient measurement system.

 

Analog and Digital Supplies

Digital control can also ease calibration parameters that may be programmed instead of tweaking a pot in an analog power supply. On the downside, digital control of a power supply involves many operating parameters and can be extremely complex. It requires a DSP (Digital Signal Processor) with sophisticated software, which can be difficult to troubleshoot.

On the other hand, analog control, having been used for 50+ years, is a tried and true method. Analog power supplies are stable and less prone to glitches, though they have limited flexibility. Parameters are defined during the design phase and cannot be changed later.

SAR ADC Power Supply Inputs

 SAR ADCs require a digital supply (DVDD), an analog supply (AVDD), and a digital input/output supply (OVDD). (AVDD) and (AVSS) are the power supply and ground for the analog portions of the circuitry. The reference voltage (Vref) is an analog reference voltage that allows the user to configure the upper range of the ADC. (Vref) is used instead of AVDD to provide the ADC reference. The ADC range will be between AGND and (Vref) allowing you to use the full dynamic range of the ADC even if the peak amplitude of your input signal is smaller than the supply voltage.

Unipolar and Bipolar SAR ADC

 Unipolar SAR

The simplest analog input type is the single-ended input. In this case, only one wire is required to get a signal from the source to the ADC. There will be a single input pin with no direct return or sense path back to the signal source. The conversion result will be generated with respect to the ground pin of the ADC. In this case, your supply will only swing from 0v or GND to AVDD.

 

 

 

Bipolar SAR

If the SAR ADC is a bipolar single-ended configuration, the allowable signal swing is between positive full scale and negative full scale relative to the ground. Again, full scale is typically set by the ADC reference input. In this case, your supply will swing from AVSS to AVDD.

 

Absolute Maximum Ratings

The Absolute Maximum Ratings section is available in all Analog Devices datasheets. It states what maximum voltages, currents, or temperatures can be applied to the pins or case of a device to avoid causing damage. From here, the absolute maximum rating can also determine the power sequence of a SAR ADC

Absolute Maximum Ratings for AD7380

Steps on How to Determine the Power Sequence of the SAR ADC:

  1. Using the Absolute Maximum Rating table, check the first-line parameter that is indicated on the table. For example, table 5 of the AD7380 Absolute Maximum Ratings.

            As we can see here, this table indicates that the Vcc to GND is the first parameter that should be supplied with the specified voltage.

  1. Next in line is the Vlogic to GND. We can see here that the first and second parameter is not yet correlated with each other. So, either of the two can be supplied first.

        3. Next in line is the Analog Input Voltage to GND. We can see here that the parameter rating is -0.3 V to Vref +0.3 V, Vcc +0.3V, 4V therefore we conclude that before supplying analog input voltage to ground, the Reference voltage should be present first.

Conclusion

Example Part: AD7381-4

To compare the other SAR ADCs that include the power supply sequencing rules we have here an example SAR ADC which is the AD7381-4 datasheet that includes the rules for applying the power to the ADC. Below is the screenshot in the datasheet of the P.S. Sequence of the AD7381-4. From here you don’t need to have to use the Absolute Maximum Ratings to determine the power supply sequence. 

 

References:

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