Transcript DAC: Resolution

DAC: Resolution
Clarification on Defining and Calculating Resolution in DACs
©[email protected]
March 2013
Update March 2015
Introduction
• There is some confusion over the determination of the
“Resolution” of Digital-Analog Converters (DAC).
• This confusion over “Resolution” is quite visible on-line also.
Many people have asked for clarification and several different
answers can be found.
• This presentation is mean to help regain some clarity.
Defining Resolution in DACs
• Part of the confusion over “Resolution” relies on what it defines.
Several definitions of “Resolution” have been presented by various
sources.
• Variety of definitions on “Resolution” in DACs, expressed as either a
value or a percentage:
1.
2.
3.
4.
5.
•
The number of bits that represent an analog value or digital input
word (i.e. an 8-bit DAC has a resolution of 8 bits).
The smallest voltage increment of its output corresponding to a 1 LSB
input code change. This is the more “standard” definition.
The number of different analog output values that can be provided by
the DAC. This will include 0 Volts.
The voltage change compared to the full scale voltage (%)
The voltage change compared to the applied voltage (%)
The term “step” is also problematic as it is sometimes used to
describe a voltage value and sometimes used to describe a transition
between two logic levels.
Perspectives
• Resolution can therefore be viewed from:
• The bit width of the input
• Binary input perspective where there are 2n unique inputs
• Output perspective where there are 2n unique voltage outputs
(counting 0 Volts as a voltage output)
• Output voltage step perspective where there are 2n – 1 steps or
voltage increments as one does not count the starting step (0
Volts).
• Resolution can therefore be measured in:
•
•
•
•
Width of the binary input
Binary moduli
Voltage moduli (voltage as a direct result of a binary moduli)
Voltage per step
DAC Circuits and VFSO
• Most DAC circuits provide a full scale voltage output that is less
than the applied voltage.
• The MSB of the binary input represents a voltage that is half of
the applied voltage but the MSB is not at exactly the half-way
mark for the binary input. Example:
• 10002 is step 8 and represents half the VFSO but
• 11112 is step 15 and is one bit less than twice the value of the MSB
• Viewed from the binary perspective the voltage values that
represent the binary input are based on 2n steps but the
maximum output (VFSO) is 2n-1 steps.
Calculating DAC with Unknown VFSO
• If the VFSO is unknown, for instance when predicting the output
voltage of a DAC, the calculation is based on one more step
than is provided by the binary input. This will affect the scaling
of the steps as the VFSO will be one step below the Vapplied.
Calculate based on the applied voltage:
Vapplied / 2n = Vstep
This will equal the step size of the DAC’s output
Calculate VFSO: Since there are actually 2n-1 steps, the VFSO will be
one step below the Vapplied:
VFSO = Vapplied – Vstep (based on a scaling of one more step)
Calculating DAC Output Voltage Steps
• If the VFSO is known:
• Output voltage per step is based on 2n-1 steps
VFSO / 2n-1 = Vstep
• If the VFSO is unknown but the Vapplied is known:
• Output voltage per step is based on one more step:
Vapplied / 2n = Vstep
Example: Vapplied Known
• Determine the voltage resolution of a 2 bit DAC with an
applied voltage of 12 Volts.
• Calculate step voltage size based on one more step:
• Vstep+1 = Vapplied / 2n = 12V / 4 = 3 Volts/step
• The VFSO will be VFSO = Vapplied – Vstep +1 = 12V – 3V = 9 Volts
Binary 11
9 Volts (VFSO)
Binary 10
6 Volts
Binary 01
3 Volts
Binary 00
3 Volts
3 Volts
3 Volts
0 Volts
Note the maximum Vout of the DAC is 1 step below Vapplied
Example: VFSO Known
• Example: Determine the voltage resolution of a 2 bit DAC with a
full scale voltage output of 9 Volts.
• There are 2n-1 steps: 22-1 = 3 steps
• The voltage resolution is calculated as VFSO / (2n-1) : 9 V / 3 steps = 3 Volts/step
Binary 11
9 Volts (VFSO)
Binary 10
6 Volts
Binary 01
3 Volts
Binary 00
3 Volts
3 Volts
3 Volts
0 Volts
Note the maximum Vout of the DAC is 1 step below Vapplied
Percent Resolution
• As with Resolution there are different perspectives with
calculating percent resolution
• From an electrical (VOUT) perspective, the percent resolution is
the percent of voltage change for each input LSB bit change. It
is another way of demonstrating the significance of a voltage
step compared to the maximum (full scale) voltage.
• From a binary perspective the percent resolution is also the
percent of voltage change for each input LSB change.
% resolution = VSTEP / VFSO
or
% resolution = 1/(2n-1)
Example:
Calculating % Resolution with Vapplied
• Using our previous example of a 2 bit DAC with a Vapplied of 12V:
VSTEP = Vapplied / # Steps+1 = Vapplied / 2n = 12V / 4 steps = 3 Volts/step
VFSO = Vapplied – VSTEP +1 = 12V – 3V = 9V
% Resolution = VSTEP / VFSO = 3 Volts / 9 Volts = 1/3 = 33.3%
-or% Resolution may be calculated in this manner:
1/(2n-1) = 1/3 = 33.3%
• Each step represents a 33.3% change in voltage
Verify: VFSO x %Resolution should equal the Volts per step
9 Volts * 33.3% = 3 Volts/step
Example:
Calculating % Resolution with VFSO
• Using our previous example of a 2 bit DAC with a VFSO of 9V:
VSTEP = VFSO / # Steps = VFSO / (2n-1) = 9V / 3 steps = 3 Volts/step
% Resolution = VSTEP / VFSO = 3 Volts / 9 Volts = 1/3 = 33.3%
-or% Resolution may also be calculated in this manner:
1/(2n-1) = 1/3 = 33.3%
• Each step represents a 33.3% change in voltage
Verify: VFSO x %Resolution should equal the Volts per step
9 Volts * 33.3% = 3 Volts/step
Calculating % Resolution with Binary
• Percent resolution is simply calculated this way:
% Resolution = 1/(2n-1) = 1/3 = 33.3%
• Each step represents a 33.3% change in voltage
Verify: VFSO x %Resolution should equal the Volts per step
9 Volts * 33.3% = 3 Volts/step
Another Perspective…..
(because it wasn’t complicated enough)
• Some may state that the percent resolution should be based
on the applied voltage and not the full scale voltage output of
the DAC. In this case the values will be different.
% Resolution = Vapplied / VSTEP
or
% Resolution = 1/2N
Example: A 2-bit DAC with a Vapplied of 12V:
% Resolution = 3V/12V = 25%
or
1/4 = 25%
Sample Calculations
• The following slides demonstrate that there are two
methods for calculating voltage resolution (voltage per LSB)
Vapplied and VFSO
• Both methods work. It depends on what value is available
(Vapplied or VFSO)
• EWB was used to demonstrate how the two calculations
and the electronic simulation all agree.
EWB Values for 8-bit DAC
Calculation vs Measurement
DAC model with 8-bit input
• Vapplied
• 8-bit DAC calculation with known Vapplied
• # steps (Vapplied) = 2n = 28 = 256 steps (to Vapplied)
(Note the number of steps used in the calculation is one more than the actual output steps)
• VSTEP = Vapplied / # Steps = 5V / 256 steps = 19.53mV per step
Measured in EWB: 19.53 mV per step
Measured in EWB: VFSO = 4.98V
• VFSO
• Note the VFSO for all 1’s input is 4.98 Volts, not 5 Volts, because it is
one step below the applied voltage.
• If we calculate with the VFSO of 4.98 Volts:
• 4.98 V / 255 steps (not 256) = 19.53mV per step, as measured.
EWB Values for 4-bit DAC
Calculation vs Measurement
DAC model with 4-bit input
• Vapplied
• 4-bit DAC calculation with Vapplied
• # Steps (Vapplied) = 2n = 24 = 16 steps (Vapplied)
(Note the number of steps used in the calculation is one more than the actual output steps)
• VFSO
• For this DAC’s configuration the maximum output is based on an input
of 1111 0000. The anticipated VFSO needs to be calculated:
• 1111 0000 = 240th step
• (240 / 256) * 5V = 4.6875 Volts = VFSO
(Note this takes into account one more step than the actual output steps, and the fact that this DAC
will provide a maximum voltage that is one step below the applied voltage)
• VSTEP = VFSO / #Steps = VFSO /(2n-1) = 4.6875 V / 15 steps = 312.5mV/step
Conclusion
• There are various perspectives as to what “resolution” means
in reference to DACs and this can be confusing.
• When it comes to the output voltage resolution, the
calculation selected is based on the known voltages:
• In the case of known VFSO the number of steps is 2n -1 because
the first step (the reference for all other steps) is not counted.
• In the case of known Vapplied , for most DACs the number of steps
is 2n because the scaling of the steps is based on one more step,
but that last step will not be attained by the DAC. The VFSO is
calculated as the applied voltage minus one step.
END