Bipolar DAC & Frequency-Based Converters - PDF

Bipolar DAC  Some DACs are designed to output a voltage that ranges from plus to minus some maximum when the input binary ranges over the counting states.  Although computers frequently use 2s complement to represent negative numbers, this is not common with DACs.  Instead, a simple offset...

Bipolar DAC  Some DACs are designed to output a voltage that ranges from plus to minus some maximum when the input binary ranges over the counting states.  Although computers frequently use 2s complement to represent negative numbers, this is not common with DACs.  Instead, a simple offset-binary is frequently used, wherein the output is simply biased by half the reference voltage.  The bipolar DAC relationship is then given by: chapter 3 part 2 1 chapter 3 part 2 2 Conversion Resolution  The conversion resolution is a function of the reference voltage and the number of bits in the word.  The more bits, the smaller the change in analog output for a 1- bit change in binary word, and hence the better the resolution.  The smallest possible change is simply given by chapter 3 part 2 3 chapter 3 part 2 4 1. Digital input: Typically, digital input is a parallel binary word composed of a number of bits specified by the device specification sheet. 2. Power supply: The power supply is bipolar at a level of ± 12 to ± 18 V as required for internal amplifiers. Some DACs operate from a single supply. chapter 3 part 2 5 3. Reference supply: A reference supply is required to establish the range of output voltage and resolution of the converter. This must be a stable, low-ripple source. In some units, an internal reference is provided. 4. Output: The output is a voltage representing the digital input. 5. Data latch: Many DACs have a data latch built into their inputs. When a logic command is given to latch data, whatever data are on the input bus will be latched into the DAC, and the analog output will be updated for that input data 6. Conversion time: A DAC performs the conversion of digital input to analog output virtually instantaneously. chapter 3 part 2 6 chapter 3 part 2 7 Frequency-Based Converters  Which an analog sensor signal can be converted into a digital signal. This is based upon converting the sensor signal into a variable frequency and then using this frequency as input to a counter for a fixed interval of time.  The output of the counter is then a measure of the frequency and thus the sensor signal. chapter 3 part 2 8 General diagram of a frequency-based analog-to-digital converter chapter 3 part 2 9 1. An as yet not identified device converts the sensor signal into a proportional frequency Fs. 2. This frequency signal is typically a square wave, as suggested in the figure. 3. The square wave is fed to an n-bit counter, which counts every rising (or falling) edge of the wave and hence every cycle. 4. The counter often has a latch on the output that allows the counter to be accumulating a new count of input frequency while still maintaining the previous output. chapter 3 part 2 10  A conversion cycle starts with a start-convert (SC) signal from the computer.  This clears the counter and triggers a one-shot convert multivibrator (MV), which controls the operation.  The MV is a simple digital IC that, when triggered, outputs a single pulse of some desired time duration.  This pulse acts as a start/stop signal to the counter.  The e falling edge of also signals the computer that a conversion is complete (EOC).  The computer can then read the count by enabling the tri- state output of the counter latch with the RD signal taken low. chapter 3 part 2 11  typical design starts from the range of frequency of the converted sensor signal, f(min) to F (max)  For maximum resolution, we then make the count time, , such that if the sensor signal produces the maximum frequency, the count will also be at its maximum. chapter 3 part 2 12  Sensor-to-Frequency Conversion  The LM331 is a common voltage-to- frequency converter useful in frequency-based ADCs chapter 3 part 2 13 The 555 timer is useful for generation of a frequency that depends upon resistance or capacity chapter 3 part 2 14