Download DAC,Diodes and TRIACS

DAC, Diodes, and Triacs
Siri Belton
Jeremy Hill
and Brandon Whitt
• What is DAC?
• Applications
• Types of DAC
– Binary Weighted Resistor
– R-2R
• Specifications
–
–
–
–
–
–
Reference Voltage
Resolution
Sampling Rate
Settling Time
Linearity
Errors
• Diodes
– Overview
– Real v. Ideal
– Types: Zener, LED
• Triacs
Outline
Siri Belton
What is DAC?
• Device that converts digital numbers into an
analog output.
• Output can be a voltage or a current
1
0
0
1
0
1
0
1
0
0
1
1
0
1
1
1
1
0
0
1
1
0
1
0
1
0
1
1
DAC
Siri Belton
What is DAC?
• Each binary number corresponds to a specific
voltage output
Siri Belton
Reference Voltage
• Use input reference voltages to determine the
analog output.
• Output voltage, unipolar 0 to V
• Output voltage, bipolar -V to V
ref
ref
ref
N-Bit Binary Word
Analog Reference
Voltage (Vref)
Digital to
Analog
Converter
Analog Output
(Vout)
Siri Belton
Applications
• Used anywhere a digital signal is used to
create an analog output
• Audio/Video
– CDs, Cell Phones, Speakers, DVDs, Youtube
• Signal Generator
Siri Belton
Types of DAC
•
•
•
•
•
•
Binary-weighted resistor
R-2R ladder
Pulse width modulation
Oversampling (Delta Sigma)
Cyclic
Hybrid DAC
Siri Belton
Binary-Weighted Resistors
• Adds resistors in
parallel to divide
voltage on each
branch by a power
of two
• Transistors act as
switches
Siri Belton
Binary-Weighted Resistors
Equivalent Circuit:
𝑉𝑜𝑢𝑡 = −𝑉𝑖𝑛
𝑅𝑓
𝑅𝑖𝑛
𝑉𝑖𝑛 = 𝑉𝑟𝑒𝑓
1
𝑅𝑖𝑛
=
1
𝐴
𝑅
+
1
𝐵
2𝑅
+
1
𝐶
4𝑅
+
1
𝐷
8𝑅
Where A, B, C, and D are 1 or 0
A
B
C
D
Siri Belton
Binary-Weighted Resistors
• Advantages:
– Useful for conversions up to 8-bit
– Simple
– Fast
• Disadvantages
– Need large range of resistor values (2048:1 for a 12-bit
conversion) with high precision resistor values
– Need small switch resistances
– Op-amp can have trouble producing low currents at
the lower range of a high precision DAC
Siri Belton
R-2R Ladder
𝑉𝑟𝑒𝑓
𝑅𝑓
• Each bit controls the switch to the op-amp
or ground (grounded if zero)
Siri Belton
R-2R Ladder
𝑉𝑟𝑒𝑓
𝑅𝑒𝑞 =
1
1
1
+
2𝑅 2𝑅
𝑅𝑓
V0 
V1 
R
1
V1  V1
RR
2
R
1
V2  V2
RR
2
R
1
V2 
V3  V3
RR
2
𝑉3 = 𝑉𝑟𝑒𝑓
1
8
𝑉0 = 𝑉𝑟𝑒𝑓
𝑉𝑜𝑢𝑡 = −
𝑅𝑓
2𝑅
𝑉0 =
𝑅𝑓
1
− 𝑉𝑟𝑒𝑓
16
𝑅
=𝑅
Siri Belton
R-2R Ladder
• Advantages
– Need only 2 resistor values
– Lower precision is acceptable
• Disadvantages
– Slower conversion rate
Jeremy Hill
Specifications of a DAC
•
•
•
•
•
•
Reference Voltage
Resolution
Sampling Rate
Settling Time
Linearity
Errors
Jeremy Hill
Reference Voltage (Vref)
• The reference voltage determines the output
voltage range.
• For Non-multiplying DAC:
– Vref is set internally by the manufacturer
– Constant Value
• For Multiplying DAC:
– Vref is set externally
– Can be varied during operation
• Full Scale Voltage (Vfs)
– Voltage when all digital inputs are 1’s
Vref (2 N  1)
Vfs 
N
2
Jeremy Hill
Resolution
• Resolution is the amount of output voltage
change in response to a least significant bit
(LSB) transition.
Vref
Resolution  N  VLSB
2
• Smaller resolution results in a smoother
output
• A common DAC has a 8-16 bit resolution
Jeremy Hill
Sampling Rate (fs)
• Sampling rate is the rate at which the DAC can
convert the digital input to an output voltage
• The Nyquist Criterion is used to ensure the
output correctly represents the digital input
f sampling  2 f max
• fmax is the max frequency of the analog signal to
be reconstructed
• fs is limited by the clock speed of the input signal
and the settling time of the DAC
Jeremy Hill
Settling Time
• DAC needs time to reach the actual expected
analog output voltage
– The time required for the output voltage to settle
within +/- ½ of VLSB of the expected voltage
Jeremy Hill
Linearity
• The difference between the desired analog
output and the actual output over the full
range of expected values
0000
Analog Output Signal
Non-Linear
Analog Output Signal
Linear (Ideal)
0001
0010
0011
Digital Input Signal
0100
0101
0000
0001
0010
0011
Digital Input Signal
0100
0101
Jeremy Hill
Errors
•
•
•
•
•
•
•
Gain Error
Offset Error
Full Scale Error
Non Linearity
Non-Monotonic
Resolution Errors
Settling Time and Overshoot
Jeremy Hill
Gain Error
• Deviation in the slope of the actual transfer
function from the ideal transfer function
– Can be determined by measuring the output
voltage for a digital input of all 1’s
Jeremy Hill
Offset Error
• Occurs when there is an offset in the actual
output voltage from the ideal output
– Can be determined by measuring the output
voltage for a digital input of zero
Jeremy Hill
Full Scale Error
• Combination of gain and offset error
Jeremy Hill
Differential Non-Linearity (DNL)
• The difference between two successive digital
output codes is ideally 1 VLSB
• DNL error is the deviation from a step of 1 VLSB
•
Manufacturers
will specify a
maximum DNL
error
Jeremy Hill
Integral Non-Linearity (INL)
• The difference in the ideal linear voltage and
the actual output voltage for a given digital
code
– Manufacturers will specify the max INL error
Jeremy Hill
Non-Monotonic
• Occurs when an increase in digital input
results in a lower output voltage
– If the DNL error is less than +/- 1 LSB the DAC is
guaranteed to be monotonic
Jeremy Hill
Resolution Errors
• Resolution will determine how close the
output voltage matches the desired signal
1 Bit Resolution
3 Bit Resolution
Jeremy Hill
Settling Time and Overshoot
• Any change in the input time will not be
reflected immediately due to the lag time
• Overshoot occurs when the output voltage
overshoots the desired analog output voltage
Brandon Whitt
Diodes
What are they?
• A diode is a two terminal electric component which
conducts current more easily in one direction than in
the opposite direction.
• The most common usage of a diode is as an
electronic valve which allows current to flow in one
direction but not the opposite direction.
Brandon Whitt
Diodes
How do they work?
• A diode is created when a p-type semiconductor is
joined with and n-type semiconductor.
• At the boundary a depletion region will form within
the diode. Here the p-carriers will diffuse into the ntype region and vice versa.
Majority carriers
p
n
Depletion Region
Brandon Whitt
Diodes
Real vs Ideal
I
conduction
region
non-conduction
region
V
Ideal Curve
Ideal Diode – no resistance to current flow
in the forward direction and infinite resistance
in the reverse direction.
Brandon Whitt
Zener Diode
• Every p-n junction (i.e. diode) will break down in reverse bias
if enough voltage is applied. Zener diodes are designed to
operate in this breakdown region.
• Zener diodes have a specified voltage drop when they are
used in reverse bias. They are able to maintain a nearly
constant voltage under conditions of widely varying current.
Brandon Whitt
Other Diodes
• Light Emitting Diodes (LEDs):
Photons are emitted when the
carriers pass through the
junction and recombine with
the doped region.
• Photodiode: Photons hitting the
doped regions cause charged
carriers to form. These can be
used to sense light in and Optoisolator.
Brandon Whitt
TRIAC
Triode for AC current
• The TRIAC is an electronic component that
can allow current to flow in EITHER direction
when triggered (bidirectional).
• TRIACs make good switches for AC current.
• They can handle hundreds of amps and
thousands of watts of power.
Brandon Whitt
TRIACs
They’re made of smaller components
• TRIACs are composed of Transistors and Thyristors.
• Two Transistors (PNP and NPN back-to-back) are combined to
make a Thyristor. Current can only go one direction
(Unidirectional).
• With forward voltage, small gate current pulse turns on the
device. Once on, each transistor supplies gate current for the
other so the device stays on.
Brandon Whitt
TRIACs
They’re made from two Thyristors
• A TRIAC is a 3-terminal switch
composed of 2 thyristors facing
opposite directions
• It can conduct current bidirectionally
• MT1 and MT2 are current carrying
terminals while the Gate terminal is
used for triggering by applying a small
voltage signal.
• Once triggered, it continues to
conduct current until the current falls
below a threshold – known as holding
current
Brandon Whitt
TRIACs
Circuit Example
• Simple Triac Switch
• Small control
current/voltage
• Eliminates Mechanical
wear in a Relay
• Much Cheaper
Brandon Whitt
TRIACs
Summary
• TRIACs start conducting when a minimum
current (gate threshold current) flows into or out
of its gate sufficient to turn on relevant junctions
in that quadrant of operation
• Device remains in “on” state even after gate
current is removed so long as current through
the device remains above holding current
• Once current falls below holding current for an
appropriate time period, device switches “off”
Brandon Whitt
TRIACs
Pros and Cons
Pros:
• Can handle much more current than a
transistor
• Much cheaper than relays
Cons:
• Can not stop the current from flowing by using
the gate. The current must be stopped at the
terminal.
Brandon Whitt
TRIACs
Applications
High Power: Switches in AC circuits using milliamp control
currents to turn on kilowatt power flows.
Low Power: Dimmers for light bulbs, speed controls for
electric fan motors, control circuits in appliances
Specs to consider when purchasing a TRIAC:
• Gate signal requirements
• Voltage drop
• Steady-state/holding/peak current specifications