Download Current-Switched R-2R DAC

Current-Switched R-2R DAC
Voltage-Switched R-2R DAC
DAC Non-Linearities
Differential & Integral Non-Linearities
Ideal
Actual
7
Output (Voltage) Value
6
5
4
3
2
1
0
0
1
2
3
4
Digital Input Value
5
6
7
DAC Gain & Offset Errors
Gain and Offset Errors
Ideal
Gain Error Example
Offset Error Example
Output (Voltage) Value
7
6
5
4
3
2
1
0
0
1
2
3
4
Digital Input Value
5
6
7
Direct (Flash) ADC
Successive Approximation ADC
n-bit conversion in n clock cycles
(n+1) bit conversion if comparator
output used
Comparator
Analogue
Input
Logic
DAC
Digital
Output
100(1)
Clock
Successive Approximation ADC
110(0)
101(1)
Integrating ADCs

There is a whole family of these circuits:
o
single-slope
•o
dual-slope
•o
multi-slope
•o
charge balance, PWM
•o
sigma-delta (-) (order 1 to m)
•
None need a T(S)/H, (but may be useful)
•
All integrate the input signal for a fixed time and then digitize it
[conversion time up to 2 x 2n = 2n+1 clock periods]
• However, the last 2 types integrate continuously
•
All allow increased resolution but are slower than the - type
Dual-Slope ADC
Integrator Output Waveforms
VC
2n clock periods
(2nT)
m clock periods (mT)
Time
V
-VREFt/R
-VINt/R
V = (2nT)VIN/R =
mTVREF/R
 VIN/VREF = m/2n
Dual-Slope
Multi-Slope
(n-bit conversion)
Integrator Output Voltage Waveforms
Multi-Slope ADC
•
To increase resolution, the comparator threshold becomes the limiting
factor
•
Multi-slope uses smaller and smaller reference values to progressively
approach the comparator zero at a slower rate
•
Each reference period ‘de-integrates’ the remaining error
•
Very much more complex circuit and costly
•
Much faster than Dual-Slope for the same resolution - used in some
DVMs
•
May also be used with Charge-Balance
Charge-Balance ADC
Integrator Output Waveforms
T
mT
Clock
-(VIN/R)t/C
Integrator
Output
Comparator
Output
D-Type (Q)
Output
-[(VIN/R) + I0]t/C
V
Charge Balance Performance
Integrates signal and ‘reference’ signals
continuously
I0 = VREF/R then VIN = VREF.Count/CountMax
Reduces integrator capacitor error of DualSlope
Capable of 10-8 (26-bit) performance, if you
can wait - speed v resolution
DVM type ADC
[PWM variants]
Sigma-Delta ADC (1)
•
•
•
•
•
•
•
Recent variant of Delta modulator and
Charge-Balance concepts
Originally used for audio only, now used
from dc to rf
Very high resolution up to 22bits and
very fast … 1000x faster than dual-slope
Very complex internal operation but quite
simple analog circuitry
Uses a special (decimating) digital filter
Can be integrated with µCs etc
Minimal chip cost but support circuits
still expensive for ultra-high performance
Sigma-Delta ADC (2)
Functional Diagram of 3rd order Sigma-Delta ADC LTC2440