To gain access to revision questions, please sign up and log in.
High speed conversion of an analogue signal into a digital signal.
- Used with Video signals.
- Not needed with slower signals like Audio and human interface devices.
- Fast because the speed is limited only by the comparators and exclusive or gates.
- Suitable for digitising video because of its speed
- Fairly easy to understand because it only requires comparators, exclusive OR gates and diodes (used as OR gates).
- High component count (not too much of a problem with custom designed ICs).
- Very hard to build from discrete components because of the large number of components needed.
To calculate the resolution of the ADC ...
- The ADC below detects four different input levels
- The resolution is Vref / 4
- An 8 bit ADC can resolve 28 levels = 256 levels so the resolution is Vref / 256
To calculate the number of comparators (or exclusive OR gates) needed ...
- 2N - 1 comparators are needed where N is the number of bits in the output.
- In the example below, there is a two bit output so 22 - 1 = 3 comparators needed.
- For a four bit converter, 24 - 1 = 15 comparators needed.
- For an 8 bit converter, 28 - 1 = 255 comparators needed. (hard to implement)
- For a 16 bit converter, 216 - 1 = 65535 comparators needed. (Some CPU processors have millions of gates so this is entirely possible.)
For the Falstad Circuit Simulation, CTRL+Click Flash ADC
In options, check European Resistors and uncheck Conventional Current.
Adjust the slider to change the analogue input voltage.
Alternatively view A_to_D_Flash.txt.
Save or copy the text on the web page. Import the saved or copied text into the Falstad simulator.
Here is the new HTML5 Simulator Site.
This is a two bit flash ADC (not very useful!) 8 or 16 bits would be much more use but also very much more complex.
- The circuit above has three comparators.
- Each comparator is fed a proportion of the reference voltage from Vref.
- If the input voltage (Vin) is too low, all the comparators will be turned off.
- If Vin is a little higher, only the bottom comparator will turn on.
- If Vin is a little high still, the bottom two comparators will turn on.
- If Vin is high enough, all the comparators will turn on.
- If all the comparators are off, the output will be 0 0. This is zero in binary.
- If the bottom comparator is on, the output will be 0 1. This is one in binary.
- If the bottom two comparators are on, the output will be 1 0. This is two in binary.
- If all the comparators are on, the output will be 1 1. This is three in binary.
- Adding extra bits is simple. More comparators are needed and the output logic gets more complex too.
Look here for a very similar three bit comparator that has about double the component count. Each extra bit added doubles the component count again. An 8 bit converter would need 255 comparators and exclusive or gates.
Half Flash ADC
An 8 bit converter would need 28 - 1 comparators = 255. This complexity makes the chip expensive and more difficult to make.
With some loss of speed, it's possible to use two 4 bit flash converters with some additional circuitry. This is the circuit diagram.
- The analogue input is converted using the first 4 bit Flash ADC.
- This gives an approximate binary value ( bits 4, 5, 6 and 7 ) which is converted back into an analogue voltage by the DAC.
- This approximate value is subtracted from the original signal and what's left is converted to digital by the second Flash ADC ( bits 0, 1, 2 and 3 ).
- This 8 bit value is stored in the latch when the clock line goes high.
- Due to the time delay introduced by the extra circuitry, a sample and hold circuit on the input would be essential.
Contacts, ©, Cookies, Data Protection and Disclaimers
Hosted at linode.com, London