r-2r binary ladder dac
DESCRIPTION
Digital electronicsTRANSCRIPT
et438b-3.ppt 1
R-2R Binary Ladder DAC
Vref = 10 Vdc
R= 10k
2R = 20k
Req3Req2
Req1
Circuit analysis
et438b-3.ppt 2
R-2R ladder analysis (cont.)
10k 10kV=10 Vdc
20k20k20k20k
V1 V2 V3
I1 I2 I3
I4
I I’
Voltages and
currents
R-2R network produces binary weighted currents from
2 values of resistance
et438b-3.ppt 3
Example: Find the output voltage for the R-2R
DAC shown below. The digital input is 1102.
Vref = 5 Vdc
R= 15k
2R = 30k
Rf = 15k
IT
I0I1I2
Find currents I0, I1, I2 and use formula V0 = - ITRf
Req
et438b-3.ppt 4
Commerical DACs
DAC0800 Family
Devices used in practical designs use intergrated
R-2R networks and transistor switching. TTL
compatiable.
8-bit binary code converted to 256 levels of I0Full scale value set by reference current.
1 bit change produces change of 1/256 in I0
4
2
14
Iref
Equations: D = decimal equivalent of binary input
IV
Rref
ref
ref
= I ID
ref0256
=
I
V
Rfs
ref
ref
=
255
256
Full scale output
et438b-3.ppt 5
IC DAC Example: Use OP AMP to convert current
to voltage. The reference voltage is +10 Vdc and the
Reference resistance is 5kΩ. The value of Rf = 2.5kΩ
a.) Digital input 000011012b.) Digital input 100011012
et438b-3.ppt 6
IC DAC example (cont.)
input 100011012
et438b-3.ppt 7
Analog-to-digital Conversion
Convert continuous signals to
digital values
Requires 3 steps
1.) Sample analog signal
(Nyquist rate)
Need a minimum of 2 x max
frequency of interest
Higher sample rates, easier to
reconstruct signal
2.) Hold analog sample value while
conversion in progress
3.) Convert analog value to digital
value
et438b-3.ppt 8
Types of Analog-to-Digital
Converters
Integrating
• High accuracy
• Low speed
• low cost
Not used in data acquisition systems
Tracking (Counter)
• High speed in tracking mode
• Slow conversion time (some sub-types)
• susceptible to noise
Successive Approximation
• conversion time independent of value
“Flash” Parallel
Multicomparator type of conversion
• Highest speed
• HIgh cost
et438b-3.ppt 9
Counter Analog-to-Digital (A/D)
Converters
Clock
AND
C
O
U
N
T
E
R
D
A
C
+
-
When V+ = V-
V+
V-
AB
V
V
1.) Input a constant value. Requires a sample and hold
circuit. Not shown.
2.) AND gate passes clock signal when pt. A logic high
3.) Counter incremented by signal B
4.) DAC output increases as counter output increases
Digital output
et438b-3.ppt 10
Clock
AND
C
O
U
N
T
E
R
D
A
C
+
-
input V+
V-
AB
V
Counter-type A/D converter
5.) Counter stops when DAC V exceeds input V
input V
time
input
DAC output
Conversion time depends on the input level
Tracking A/D converters us up/down counters to
minimize conversion times
et438b-3.ppt 11
Successive Approximation A/D
Converters
Counter A/D converters conversion time proportional
to the input level
Improve conversion speed using a binary search
technique.
Procedure
1. Set MSB to 1
2. Test input, Vin, against DAC output, VDAC
3. If VDAC > Vin, reset bit to 0, else bit = 1 (VDAC < Vin)
4. Move to next bit and repeat steps 1 - 3
The input is converted to digital value in n
steps, where n = the number of bits in digital signal
et438b-3.ppt 12
Successive-Approximation A/D
Converters
et438b-3.ppt 13
Example: an 8-bit successive approximation ADC is
presented with a analog value that has a value of
143. Use the successive approximation algorithm
given previously to determine the binary value.
Assuming that each test takes a single clock cycle,
determine the maximum conversion time for the ADC
if it is clocked at 4.77 MHz
et438b-3.ppt 14
Example continued
et438b-3.ppt 15
Digital Input/Output Signals
(Digital I/O)
Boolean logic level 1 = logic high 0 = logic low
Boolean levels form basis of numbering system and
computer structure.
1 and 0’s bits
groups of bits 8-bits = byte
16-bits = 2 bytes = 1 word
All collections of bits are power of 2
Addressing bits in a byte
Address location0123456
Bit0110001
Weighted number system - conversion from binary
to decimal
n = 1x27+1x26+0x25+0x24+0x23+1x22+1x21+0x20
n = 1x128+1x64+1x4+1x2 = 198
7
1
198 decimal equivalent of binary number
et438b-3.ppt 16
Digital Hardware Standards
Specifies voltage levels of logic high and logic low
current output and input levels. Makes chips from
same family compatible with each other
Transistor-Transistor Logic (TTL)
Nominal 5 Vdc logic high and 0 Vdc logic low
Practical TTL output port characteristics
Source current 200 µASink current 1.7 mA
Logic 1 Threshold voltage V>= 2.4 Vdc
Load
Switch source of current
Load
+5 Switch sink of current
et438b-3.ppt 17
Simple TTL gate Sink and Source
Vcc = + 5 Vdc
RL
Vin
Rb
IL Vin = 5 V for Logic high and
0 V for logic low
IL flows to ground when
Vin = logic 1
Switch model of gate
Load
RL
IL
Vin
When Vin = 5 Vdc Vo = 0 V
When Vin = 0 Vdc Vo = 5 V
Vo
Vo
Logic Inverter
ILL
Vcc = + 5 Vdc
input output
1 0
0 1
Load will draw current went
Vin = 5 Vdc switch (Transistor) must
sink load current
et438b-3.ppt 18
Switch model of gate - current sourcing
IL
Vin
Vo
Load
ILL
Vcc = + 5 Vdc
When Vin is logic 1 load is de-activated (switch
closed)
When Vin is logic 0 load draws current from
source
With Transistors
current
sourcing
Vin
TTL Inverter
symbol
Load
ILLsourcing
current
Load
ILL
et438b-3.ppt 19
TTL Sinking LEDs
Output byte port feeding loads from inverters (buffering)
Resistor
used to
limit current
through
gate
When input is logic 1 (+5 Vdc) output is logic 0 (ground)
Limits of TTL buffers - 1.7 mA
For high current output - use discrete transistor
or relay (electromechanical or solid state)
et438b-3.ppt 20
Solid State Relay
Ac supplies only
Optically coupled device
AC
AC
+5 Vdc
Ground
Typical Application
M
Motor Load
1/4 HP 120 Vac
120 V
Grd
+5 Vdc
Output Bit
Output bit low motor is on
Output bit high motor is offMust remember output
logic level to get
desired action in device
et438b-3.ppt 21
Electromechanical Relays
To output
bit
+ 12 Vdc
Typical
NPN transistor
To high power
load AC or DC
diode
voltage
spike
protector
Relay coil
dc resistance
load to transistor
Resistor
sized to allow
TTL level to turn
on relay
Relay interface using TTL gate (open collector)
Lower current handling ability
+ 12 Vdc
Typical
To output
bit
LS7406 chip
To high power
loads AC or DC
Current limited
by the TTL
chip
et438b-3.ppt 22
Digital Input Interface
Limit current and change voltage levels
Using mechanical switches
+ 5 Vdc
2.2 k
Mechanical
switch
To
data
acquisition
board
Input port
constructed of TTL
chips
Resistor
sized to limit
current to
value
below port
sinking limit
Digital interface for a
byte of inputs.
+ 5 Vdc
All resistors sized to limit sink
current
Switches can be either
momentary contact or
toggle
et438b-3.ppt 23
Digital Interface
Non-TTL Levels
+24 Vdc
Optocoupler
2.2k Typical2.2k Typical
+ 5 Vdc
TTL inverter
gate (7404 or 7414)
To TTL
input
R
Switch closed - LED de-energized
Switch open - LED energized
LED and Optical transistor integrated on same chip
energizing LED causes optical transistor to conduct
Transistor off - inverter input +5 Vdc output 0 V
Transistor on - inverter input 0 Vdc output 5 V
et438b-3.ppt 24
Example: determine the logic levels and currents in
the digital interface circuit shown below
Assume that the optocoupler diode has an on-state
voltage drop of 1.4 V and the optical transistor has a
on-state drop of 0.4 V.
+24 Vdc
2.2k 2.2k
+ 5 Vdc
To TTL
input
R = 100
Logic
open switch = 0 = 0 V
close switch = 1 = 5 V
et438b-3.ppt 25
Example (cont.)
et438b-3.ppt 26
Relay Interface Example
+24Vdc
Rb2N3904
Q1
Relay
Coil model
500 Ω
10 mH
Size Rb such that
Q1 will activate with
a TTL input
Vs
Transistor Parameters
hfe = 200 (nominal)
Vce(sat) = 0.2 V
Vbe(sat) = 0.8 V
Ib
Ic
Assume transistor is in saturation and compute value of
Ic
+
+
--Vbe
Vce
et438b-3.ppt 27
Relay Interface Example (cont.)
+24Vdc
Rb2N3904
Q1
Relay
Coil model
500 Ω
10 mH
Vs
D1
Freewheeling
diode
Diode D1 provides a path for the current induced
when the transistor is switched off. If also clamps the
induced voltage to the forward drop of the diode.