voltage converter

Compact A-D Converter Schematic Circuit Diagram

Building an Analog-to-Digital (A-D) Converter with Discrete Components

Despite the ready availability of good and cost-effective integrated analog-to-digital (A-D) converters, constructing one from individual components can provide valuable insights. The featured converter relies on a Type TLC274 IC, housing four comparators, and the output of these stages also serves as the converter’s output.

The primary challenge in A-D converter design lies in generating a precisely tracked reference voltage. Each bit necessitates a small digital-to-analog (D-A) converter to create the required reference. Typically, this hurdle is overcome by assigning the Most Significant Bit (MSB) comparator half the supply voltage as a reference, generated by the potential dividers R1-R2. Subsequently, for each successive bit, the preceding bits are added to the reference voltage, involving a few resistors. For bit B, this involves using bits C and D; for bit A, bits B and D are utilized. The translation of these bit levels is accomplished through IC2 and R1-R14.

Compact A-D converter Schematic diagram

Constructing a Regulated 5-V Analog-to-Digital (A-D) Converter Supply

The regulated 5-V supply voltage serves as the foundation for the reference voltage in this setup. Ideally, the output levels of the buffers should be 5 V (logic 1) or 0 V (logic 0). To ensure good linearity, the resistors in the potential divider should be of close tolerance. The accuracy of the resistors becomes increasingly critical as the resolution (number of bits) of the A-D converter rises. It’s important to note that the output level of the buffers in IC2 deviates more from the ideal as the provided current increases. In simpler terms, the resistor values need to be relatively high. This challenge is mitigated by connecting two non-inverting gates in parallel to enhance fan-out. In the prototype, with the specified values, the voltage drop at the MSB output buffer (which provides the highest current) was a mere 6 mV, negligible compared to the LSB value of 312.5 mV.

The conversion speed is contingent on the propagation times of the comparators and buffers. Although the TLC274 performs adequately, for optimal results, true comparators are recommended. The input sensitivity of the circuit is adjustable using P1. Diodes D5 and D6 act as safeguards for the opamp inputs against excessive potentials. The current draw with all LEDs off is approximately 4 mA, while with all four LEDs on, it rises to about 20 mA.


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