ADC / DAC Questions

Discussion in 'Homework Help' started by James4553, Jun 7, 2008.

  1. James4553

    Thread Starter Active Member

    Jun 7, 2008
    35
    0
    Hello all

    I need some help with the following questions.
    Please don't just give the answer but rather show me the formula/method to solve the problems.

    1. A tachometer displays engine RPMs from zero to 10,000. If the ADC accepts voltage levels from zero to five volts, in order to achieve full-scale representations from the tachometer, what should the output voltage be at 8500RPM?

    2. An ADC converter has a binary input of 0010 and an analog output of 20mv. What is the resolution?

    3. A given 4-bit digital to analog converter has a reference voltage of 15 volts and a binary inout of 0101. What is the proportionality factor?

    4. The _______ of a DAC is the smallest change that can occur in the analog output as a result of a change in the digital input. (Fill in the blank)

    I have more unanswered question but I'll leave it at that for now.

    Many thanks.
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,638
    2,344
  3. James4553

    Thread Starter Active Member

    Jun 7, 2008
    35
    0
    Thank Bertus but that hasn't helped me.

    Can anyone else please give me assistance? I have an exam coming up shortly...
     
  4. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,157
    What is the name of your coursebook?
     
  5. James4553

    Thread Starter Active Member

    Jun 7, 2008
    35
    0
    We aren't using a coursebook in our classes.


    So nobody can show me how to solve these problems?
     
  6. roddefig

    Active Member

    Apr 29, 2008
    149
    0
    1. To utilize the full range of the ADC 0 RPM should be equal to 0 volts input and 10,000 RPM should be equal to 5 volts. Therefore, we can just divide 5 volts by 10,000 RPM and determine the number of volts/RPM.

    \frac{5\text{ V}}{10000\text{ RPM}} = 5 \text{ mV/RPM}

    To determine the voltage level for 8500 RPM we simply multiply the previous constant by 8500 RPM.

    8500 \text{ RPM}\,\cdot\, 5 \text{ mV/RPM} = 4.25 \text{ V}

    2. The resolution of an ADC is the smallest voltage that it can detect. This is typically expressed as V/LSB where LSB stands for least significant bit and is equivalent to the one. 0010 in binary is equal to two or two LSBs. To determine V/LSB we can simply divide the analog input voltage by the binary output value.

    \frac{20 \text{ mV}}{2 \text{ LSB}} = 10\text{ mV/LSB}

    Note that the input and output that you specified for this problem implied you were using a DAC, but the math works for either device.

    3. I'm not sure what a binary inout or a proportionality factor is so I can't help you there.

    4. You should be able to find this in your notes. I'm not sure what term your professor is looking for, but V/LSB is one answer.
     
    Last edited: Jun 8, 2008
  7. James4553

    Thread Starter Active Member

    Jun 7, 2008
    35
    0
    That's great, thank you so much.

    Is an ADC conversion both more complicated and more time consuming than a DAC conversion?

    And for #3 'binary inout' is a typo. I meant to write 'binary input'.
     
  8. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,157
    Max number for four bits is 15. Your reference is 15 V. Proportionality is 1V/bit.

    Get yourself a recorder and record the lectures.
     
    Last edited: Jun 8, 2008
  9. Dave

    Retired Moderator

    Nov 17, 2003
    6,960
    144
    A lot depends on your ADC and DAC methodology; for example a Flash ADC is very fast however has increased complexity, in terms of numbers of components.

    Have you checked our Digital-Analog Conversion section in the e-book?

    Dave
     
  10. Wendy

    Moderator

    Mar 24, 2008
    20,764
    2,534
    Several analog to digital schemes use a DAC, they feed the DAC a number, compare it to the input, and adjust accordingly. Tends to be slow, but reliable.
     
  11. Dave

    Retired Moderator

    Nov 17, 2003
    6,960
    144
    That is a successive aproprimation ADC, where the speed is limited by the number of bits. Delta-Sigma is the standard these days as a fast and cost effective implementation (particularly common in DSP applications).

    Dave
     
Loading...