Working with ADCs, OAs, and the MSP430 - Texas Instruments

40 Pages·2007·0.26 MB·English

Working with ADCs, OAs
and the MSP430
Bonnie Baker
HPA Senior Applications Engineer
Texas Instruments
© 2006 Texas Instruments Inc, Slide 1
Agenda
• An Overview of the MSP430 Data Acquisition System
• SAR Converters
(cid:131) The INS and OUTS of the SAR converter
(cid:131) Useful Applications
• Using Op Amps
(cid:131) Op Amp Configurations
(cid:131) Driving SAR Converters
© 2006 Texas Instruments Inc, Slide 2
Where to Find ADCs and Op Amps
Voltage Reference Source
OP
OP
AMP
AMP
FILTER A/D
MUX FILTER A/D
MUX
REF
REF
Anti-Alias Filter
Band-pass Filter
Sensor Interface
Programmable Gain Amp
Voltage Reference Source
Instrumentation Amp
Buffer
A/D Converter Driver
Gain
μ C
μ C
Difference Amplifier
Instrumentation Amplifier
Filter Voltage Reference Source
Level Shift DDS Synthesis
POWER
POWER
D/A
D/A
AMP
Actuator Driver AMP
Valve
Line Driver
4-20mA Driver
© 2006 Texas Instruments Inc, Slide 3
ADC Architectures
• There are many different ADC Architectures
(cid:131) Successive Approximation (SAR)
(cid:131) Sigma Delta (SD)
(cid:131) Slope or Dual Slope
(cid:131) Pipeline
(cid:131) Flash...as in quick, not memory
• All converters in the MSP430 chips are SAR and
Sigma Delta types
• SAR determines the digital word
(cid:131) By approximating the input signal
(cid:131) Using an iterative process
• How the Sigma Delta converter determines the digital
word
(cid:131) By oversampling
(cid:131) Applying Digital Filtering
© 2006 Texas Instruments Inc, Slide 4
Op Amp Architectures
• The Different Types Op Amp Architectures
(cid:131) Single Supply
– Rail to Rail In
– Rail to Rail Out
– CMOS or Bipolar
(cid:131) Dual Supply
• All Op Amps (OAs) in the MSP430 chips are Single
Supply, CMOS
• Our CMOS Op amp
(cid:131) Easily Configured with the MSP430 Controller
– General Purpose, Buffer, Comparator, PGA, Differential Amp
(cid:131) Easily Programmed for
– Optimized Gain
– Bandwidth
– etc
© 2006 Texas Instruments Inc, Slide 5
Agenda
• An Overview of the MSP430 Data Acquisition System
• SAR Converters
(cid:131) The INS and OUTS of the SAR converter
(cid:131) Useful Applications
• Using Op Amps
(cid:131) Op Amp Configurations
(cid:131) Driving SAR Converters
© 2006 Texas Instruments Inc, Slide 6
The SAR ADC
• Most Serial ADCs are SARs or Sigma Deltas
• The MSP439 SAR Converter
(cid:131) SAR ADC = Successive Approximation Register, Analog-to-Digital
Converter
(cid:131) ADC12 – 12-bit Analog-to-Digital Converter
• SARs are Best for General Purpose Apps
(cid:131) Very Prevalent for Signal Level Applications: Data Loggers, Temp
Sensors, Bridge Sensors, General Purpose
SAR
• In the Market SARs
Analog to
(cid:131) Can be 8 to 18 bits of resolution
Digital
(cid:131) Speed range: >10 ksps to < 5 Msps Converter
• Usually require a Low-pass Filter before Analog Input
© 2006 Texas Instruments Inc, Slide 7
System Integration Using an A/D
MSP430
AAmmpp
FFiilltteerr
SAR Micro-
Input
Analog to
Analog to
Digital Controller
Signal
CDoingviteartle r
Engine
Source
Converter
Filter
DAC or
Output
PWM
© 2006 Texas Instruments Inc, Slide 8
SAR Converter – Block Diagram
V
S
S
Shift Register
1 Cap array is both the sample
cap and a DAC
R
IN
(2 kΩ) +
S
S
C
A
16C 2C C C
_ R
1/2 V
REF
Control
Logic
V V
SS REF
© 2006 Texas Instruments Inc, Slide 9
Successive Approximation Concept
FS
Bit = 0
3/4FS
Bit = 0 Analog
Bit = 1
input
V Bit = 1
IN
1/2FS
TEST TEST TEST TEST
MSB MSB -1 MSB -2 LSB
1/4FS
0
Time
DAC Output Digital Output Code = 1010
© 2006 Texas Instruments Inc, Slide 10