Activity: Analog to Digital Conversion

Objective

The purpose of this lab activity is to explore the concepts of analog to digital conversion by building explanatory examples.

Analog-to-Digital converters (ADC) translate analog signals, real world signals like temperature, pressure, voltage, current, distance, or light intensity, into a digital representation of that signal. This digital representation can then be processed, manipulated, computed, transmitted or stored.

Figure 1. Analog to Digital conversion

An ADC samples an analog waveform at uniform time intervals and assigns a digital value to each sample. The digital value appears on the converter’s output in a binary coded format. The value is obtained by dividing the sampled analog input voltage by the reference voltage and them multiplying by the number of digital codes. The resolution of converter is set by the number of binary bits in the output code.

Figure 2. Digital output code

An ADC carries out two processes, sampling and quantization. The ADC represents an analog signal, which has infinite resolution, as a digital code that has finite resolution. The ADC produces 2N digital values where N represents the number of binary output bits. The analog input signal will fall between the quantization levels because the converter has finite resolution resulting in an inherent uncertainty or quantization error. That error determines the maximum dynamic range of the converter.

Figure 3. Quantization Process

The sampling process represents a continuous time domain signal with values measured at discrete and uniform time intervals. This process determines the maximum bandwidth of the sampled signal in accordance with the Nyquist Theory. This theory states that the signal frequency must be less than or equal to one half the sampling frequency to prevent aliasing. Aliasing is a condition in which frequency signals outside the desired signal band will, through the sampling process, appear within the bandwidth of interest. However, this aliasing process can be exploited in communications systems design to down-convert a high frequency signal to a lower frequency. This technique is known as under-sampling. A criterion for under-sampling is that the ADC has sufficient input bandwidth and dynamic range to acquire the highest frequency signal of interest.

Figure 4. Sampling Process

Sampling and quantization are important concepts because they establish the performance limits of an ideal ADC. In an ideal ADC, the code transitions are exactly 1 least significant bit (LSB) apart. So, for an N-bit ADC, there are 2N codes and 1 LSB = FS/2N, where FS is the full-scale analog input voltage. However, ADC operation in the real world is also affected by non-ideal effects, which produce errors beyond those dictated by converter resolution and sample rate. These errors are reflected in a number of AC and DC performance specifications associated with ADCs.

Figure 5. Transfer Function for an Ideal ADC

Any analog input in this range gives the same digital output code.

Materials

ADALM2000 Active Learning Module
Solder-less breadboard, and jumper wire kit
1 OP482 operational amplifiers
2 AD654 Voltage-to-Frequency Converter
3 1kΩ resistor
5 10kΩ resistor
1 nF capacitor

Flash ADC

Background

Flash analog-to-digital converters, also known as parallel ADCs, are one of the fastest way to convert an analog signal to a digital signal. Flash ADCs are ideal for applications requiring very large bandwidth, but they consume more power than other ADC architectures and are generally limited to 8-bit resolution. Typical examples include data acquisition, satellite communication, radar processing, sampling oscilloscopes, and high-density disk drives.

Flash ADCs are made by cascading high-speed comparators. For an N-bit converter, the circuit employs 2N-1 comparators. A resistive-divider with 2N resistors provides the reference voltage. Each comparator produces a 1 when its analog input voltage is higher than the reference voltage applied to it. Otherwise, the comparator output is 0. The point where the code changes from ones to zeros is the point at which the input signal becomes smaller than the respective comparator reference-voltage levels.

Consider the circuit presented in Figure 6.

Figure 6. Flash ADC - analog side circuit

The circuit represents the analog side of a 2-bit Flash ADC with the architecture known as thermometer code (unary code) encoding. For this type of circuit additional logical circuitry is used to decode the unary code to appropriate digital output code.

As mentioned before, flash ADCs are made using high-speed comparators, but for convenience, we are going to use the OP482 quad op-amp to illustrate the principle. Alternatively, you can build the circuit using four AD8561 comparators.

Hardware Setup

Build the following breadboard circuit for 2-bit Flash ADC.

Figure 7. Flash ADC - analog side breadboard circuit

Procedure

Supply the circuit to +/-5V from the power supply. Configure AWG1 of the Signal Generator to Rising Ramp Sawtooth with 5V amplitude, 2.5 offset and 100Hz frequency. Use AWG2 as 5V constant reference voltage for the ADC.

Configure the Logic Analyzer so that the digital channels DIO0, DIO1, DIO2, DIO3 form a channel group decoded for Unary code.

A plot with the output signals are presented in Figure 8.

Figure 8. Flash ADC - output code

The plot represents the output thermometer code for the 2-bit Flash ADC with all possible output values, by varying the input analog voltage through the entire available range (0-5V).