1 / 18

Analog to Digital Converters Electronics Unit – Lecture 7

Analog to Digital Converters Electronics Unit – Lecture 7. Representing a continuously varying physical quantity by a sequence of discrete numerical values. 03 07 10 14 09 02 00 04. Conversion Methods (selected types, there are others). Ladder Comparison Successive Approximation

deo
Download Presentation

Analog to Digital Converters Electronics Unit – Lecture 7

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Analog to Digital ConvertersElectronics Unit – Lecture 7 Representing a continuously varying physical quantity by a sequence of discrete numerical values. 03 07 10 14 09 02 00 04 Electronics 7

  2. Conversion Methods(selected types, there are others) Ladder Comparison Successive Approximation Slope Integration Flash Comparison Electronics 7

  3. Ladder Comparison Electronics 7

  4. - + Single slope integration • Charge a capacitor at constant current • Count clock ticks • Stop when the capacitor voltage matches the input • Cannot achieve high resolution • Capacitor and/or comparator Vin Counting time StartConversion StartConversion Enable S Q R N-bit Output Counter C IN Clk Oscillator Electronics 7

  5. Successive Approximation Electronics 7

  6. Flash Comparison If N is the number of bits in the output word…. Then 2N comparators will be required. With modern microelectronics this is quite possible, but will be expensive. Electronics 7

  7. Pro and Cons Slope Integration & Ladder Approximation Cheap but Slow Electronics 7

  8. Pro and Cons Flash Comparison Fast but Expensive Slope Integration & Ladder Approximation Cheap but Slow Electronics 7

  9. Pro and Cons Successive Approximation The Happy Medium ?? Slope Integration & Ladder Approximation Cheap but Slow Flash Comparison Fast but Expensive Electronics 7

  10. Resolution Suppose a binary number with N bits is to represent an analog value ranging from 0 to A There are 2N possible numbers Resolution = A / 2N Electronics 7

  11. Resolution Example Temperature range of 0 K to 300 K to be linearly converted to a voltage signal of 0 to 2.5 V, then digitized with an 8-bit A/D converter 2.5 / 28 = 0.0098 V, or about 10 mV per step 300 K / 28 = 1.2 K per step Electronics 7

  12. Resolution Example Temperature range of 0 K to 300 K to be linearly converted to a voltage signal of 0 to 2.5 V, then digitized with a 10-bit A/D converter 2.5 / 210 = 0.00244V, or about 2.4 mV per step 300 K / 210 = 0.29 K per step Is the noise present in the system well below 2.4 mV ? Electronics 7

  13. Quantization Noise Each conversion has an average uncertainty of one-half of the step size ½(A / 2N) This quantization error places an upper limit on the signal to noise ratio that can be realized. Maximum (ideal) SNR ≈ 6 N + 1.8 decibels(N = # bits) e.g. 8 bit → 49.8 db, 10 bit → 61.8 db Electronics 7

  14. Signal to Noise RatioRecovering a signal masked by noise Some audio examples In each successive example the noise power is reduced by a factor of two (3 db reduction), thus increasing the signal to noise ratio by 3 db each time. Example 1 Example 2 Example 3 Example 4 Electronics 7

  15. Conversion Time Time required to acquire a sample of the analog signal and determine the numerical representation. Sets the upper limit on the sampling frequency. For the A/D on the BalloonSat board, TC ≈ 32 μs, So the sampling rate cannot exceed about 30,000 samples per second (neglecting program overhead) Electronics 7

  16. Data Collection – Sampling Rate The Nyquist Rate Asignal must be sampled at a rate at least twice that of the highest frequency component that must be reproduced. Example – Hi-Fi sound (20-20,000 Hz) is generally sampled at about 44 kHz. External temperature during flight need only be sampled every few seconds at most. Electronics 7

  17. Activity E7a Do the HuSAC® a party game for techies... Human Successive Approximation Converter Electronics 7

  18. Activity E7b Data Acquisition Using BalloonSat Electronics 7

More Related