When you use an 8- to 14- bit analog-to-digital converter (ADC) in your system , it is important to understand the converter's voltage reference path. Figure 1 shows a circuit that can accommodate the dynamics of the ADC reference input. In the figure, the voltage reference chip provides the voltage-foundation for the conversion process and capacitor C L1 is designed to absorb the ADC 's internal reference circuit [REF 1] current peaks and filter reference noise. In this circuit, it is not only important to reduce the voltage reference noise, but also to balance the stability of the internal voltage reference amplifier.
 
 
Figure 1. A low-pass filter is installed between the reference and the ADC.
Series Voltage Reference Circuit for 8- to 14- Bit Analog-to-Digital Converters
 
When using this circuit to solve the noise problem, the ADC transfer function (Equation 1 ) shows the effect of voltage reference noise.
              ( Equation 1)
 
In the formula, V IN is the input voltage of the ADC , N is the number of ADC bits, and V REF is the reference voltage of the ADC . The V REF variable includes all errors associated with the reference chip, such as accuracy, temperature variation, noise, etc. In any case, the reference error will become part of the ADC system gain error.
 
You can calibrate out most of these errors with your system processor or controller. If you are measuring a few points from the ADC negative full scale to the positive full scale, you will see gain errors among these errors, which are related to the input voltage of the converter. The error you cannot calibrate out with a processor or controller is noise. In Figure 2 , you can see that the reference noise at the output of the converter increases with the analog input voltage.
 
 
Figure 2 Effects of voltage reference and ADC noise on converter output code
 
Most voltage reference data sheets have an output voltage noise specification for the frequency range of 0.1 to 10 Hz . Some vendors provide a voltage reference output noise density specification. This specification is generally for wideband regional noise, for example: 10 kHz noise density, etc. Regardless of how the vendor specifies its reference noise, adding a low-pass filter can reduce the overall noise of the reference output. You can design this filter using a capacitor and the capacitor's equivalent series resistance (ESR) (see Figure 1 ). You can also ensure the stability of the design by using the same techniques suggested in Reference 2 .
 
Figure 1 shows the complete circuit diagram of our hypothetical reference system configured with an 8- to 14- bit converter. The accuracy of the voltage reference in this system is important; however, you can use any hardware or software to calibrate out the initial error. On the other hand, absorbing the current peaks on the ADC reference pin while eliminating or reducing the reference noise requires a certain degree of characterization and hardware filtering skills.
 
Next time, we will investigate and design a voltage reference circuit suitable for 16- bit converters and above.
 
References:
1.      "Boosting the Mixing Signal Voltage Reference" by Baker, Bonnie
2.      "Just Use a 100Ω Resistor" by Baker, Bonnie
3.      Oljaca, Baker, “ Part 2 : How the Voltage Reference Affects ADC Performance,” TI Analog Applications Magazine, Q3 2009 .