I. Key ADC Parameters Explained

1. Resolution

Resolution refers to the smallest change in the analog signal that can be detected by the ADC. It is defined as the full-scale voltage divided by $2^n$, where n is the number of bits. Higher resolution means finer signal granularity.

2. Conversion Rate

The conversion rate is the reciprocal of the time it takes to complete one analog-to-digital conversion.

• Integrating ADCs: milliseconds (low-speed)

• SAR ADCs: microseconds (medium-speed)

• Flash/Hybrid ADCs: nanoseconds (high-speed)

3. Sampling Time and Sample Rate

Sampling time is the interval between two conversions. To ensure accurate results, the sample rate must be less than or equal to the conversion rate. Common units are ksps (kilo samples per second) and Msps (million samples per second).

4. Quantizing Error

Caused by the ADC’s limited resolution. It’s the maximum deviation between the actual analog input and the ideal linear output. Usually expressed as ±0.5 LSB or ±1 LSB.

5. Offset Error

This occurs when the output is not zero while the input is zero. It can often be adjusted externally using a potentiometer.

6. Full Scale Error

The difference between the actual and ideal input signal when the ADC output is at full scale.

7. Differential Nonlinearity (DNL)

The maximum deviation between the steps of adjacent digital output values.

8. Integral Nonlinearity (INL)

The largest deviation of the ADC’s actual transfer function from the ideal straight line across all codes.

9. Total Harmonic Distortion (THD)

THD measures the harmonic distortion present in the output due to ADC non-linearity. It is the sum of all harmonic components and is frequency dependent.

10. Signal-to-Noise Ratio (SNR)

SNR indicates the ratio between signal and noise levels. It is typically expressed in decibels (dB) and calculated as:

• Power ratio: 10log(Ps/Pn)

• Voltage ratio: 20log(Vs/Vn)
  SNR is affected by resolution, linearity, jitter, and other factors. ENOB (Effective Number of Bits) is commonly used to represent ADC SNR performance.

11. Power Supply Rejection Ratio (PSRR)

PSRR measures the ADC’s ability to suppress changes in the output caused by fluctuations in the power supply.

• AC PSRR: Measured with ripple voltage at 100Hz

• DC PSRR: Measured by changing supply voltage ±5%

12. Common Mode Rejection Ratio (CMRR)

CMRR is the ratio of differential gain to common-mode gain. It evaluates the ADC’s ability to reject common-mode noise.

13. Effective Resolution

Due to internal noise, an ADC’s effective resolution is often lower than its nominal resolution. For example, a 12-bit ADC may deliver only 11-bit effective resolution in practice.

14. Input Impedance

Input impedance affects ADC accuracy. Low source impedance or input buffering (often built into Sigma-Delta ADCs) is recommended to ensure minimal loading and accurate conversion.

II. Types of ADCs

III. How to Select the Right ADC

Choosing the right ADC involves evaluating several criteria:

1. Resolution

Choose a resolution that meets or exceeds your signal accuracy needs. A good rule is to select one bit higher than your minimum requirement.

2. Sampling Rate

Ensure the conversion rate exceeds twice the maximum frequency of the input signal to avoid aliasing.

3. Number of Channels

Some ADCs integrate multiple input channels with internal multiplexers. Useful for multi-sensor systems.

4. Input Type

Determine if your signal requires single-ended or differential input, and check voltage range compatibility.

5. Interface Type

Select from SPI, I²C, parallel, or UART depending on your system needs.

6. Input Signal Characteristics

Check whether the signal is bipolar or unipolar, and whether the ADC can handle the voltage levels.

7. Power Supply

Some ADCs require dual supply (±15V), while others are optimized for single 3.3V or 5V operation.

8. Reference Voltage

Use a precise external reference voltage for better accuracy if supported.

9. Power Consumption

Low-power ADCs are preferred in battery-operated systems. CMOS technology often offers better efficiency.

10. Package Type

Available in DIP, SOIC, QFN, BGA, etc. Choose based on assembly and space constraints.

11. Track-and-Hold (T/H)

For high-speed or AC signals, T/H circuits are essential to capture accurate snapshots of the input.

12. Rail-to-Rail Output

Some ADCs (and DACs) can operate rail-to-rail, meaning their output can swing from 0V to the full supply voltage—useful for full-scale signal usage.

Conclusion

Understanding ADC parameters such as resolution, SNR, THD, INL, and DNL is critical when selecting the right ADC for your application. Whether you're building a battery-powered sensor or a high-speed data acquisition system, this guide offers the technical foundation you need to make an informed decision.