Download Measuring

Measuring ?
„Wer misst, misst Mist.“
Digitizing in
analogue measurements
Physical/Chemical
Property
Sensing,
Signal Processing
Measurement
Digitizing in a
digital measurement
digital data acquisition
5,53
Evaluation
numeric result
numeric result
DIGITAL RESULT
1
The Analogue//Digital Interface
Digital circuits for the analogue//digital interface are typically realised in
TTL components and operational amplifiers working with 3 to 5 V DC and
currents up to 20mA.
State of the art instruments are mostly based on commercial PC compatible
computer components.
Add on components are available to make a standard PC a digital recording
instrument for analogue data.
A single bit on a digital output can directly be utilised as switch
Digital words can be converted into analogue output signals via a D/AC
Analogue values can be converted into digital words using a D/AC and a
comparator in a trial and error process (A/DC).
The analogue to digital conversion is thus a more complex and time
consuming process than a comparable D/AC step.
digital data acquisition
2
Analogue/Digital Converter
programming input
conversion
selection of signal source andStart
amplification
analogue
signal(s)
MUX
S/H
“polling”
A/DC
digital
data
PGA
measurement cycle Dt
settling time
10 to 100 µs
0
sample hold - A/D conversion
10 to 30 µs
1 to 5 µs
integration time t
digital data acquisition
t
3
.. or using VCO
A Voltage Controlled Oscillator (VCO), also called Voltage to
Frequency Converter (V/FC) converts an input voltage Vin into an
oscillating TTL-signal with the frequency fout proportional to Vin.
counter reset and start
counter
command
stop command
VCO
Vin
digital
counter
digital
data
measurement cycle Dt
counting time, gate time
µs to s-range
0
integration time t=Dt
digital data acquisition
t
4
A/D
SAMPLE
PGA-settling
conversion
digital readout
analogue signal
Registering Digital Data
VCO averaging
Dt
digital data acquisition
5