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analogue <> digital conversion (DAC (imperfection (offset, gain…
analogue <> digital conversion
discriminator
threshold
schmitt trigger
DAC
characteristics
resolution
full scale voltage
LSB
imperfection
offset
gain error
monotonic
settle time
glitches
design
binary weighted resistor
max 2^(n-1)
dis
R-2R ladder
only R and 2R
resistance between any node and ground is 2R
output voltage is weighted sum of inputs
dis
ad
inverted R-2R ladder
two resistor values
constant current drawn from Vref when the binary input changes
bipolar coding
offset binary
2's complement
ADC
imperfection
non-monotonic
offset error
gain error
missing codes
design
ramp-up converter
5 steps process
ad
dis
successive approximation converter
4 steps process
ad
dis
integrating converter
single slope
dual slope
4 steps
ad
dis
parallel converter
fast but expensive
duplicated circuit
3 components
ad
dis
inexpensive ADC
ADC #1
microcontroller with dedicated digital input and output ports
2 steps
time proportional to sensor resistance
rely on input port switching at well defined voltage and conversion is not linear
ADC #2
an I/O port
2 steps
t ∝ R
sensor R must never be so low that the I/O port cannot discharge C
rely on input port switching at well defined voltage and conversion is not linear
ADC #3 (Schmitt oscillator)
T ∝ R
sample and hold circuit
sample
switch is closed and C charges to the voltage of the analogue input
problem
acquisition time
aperture time
charging time
solution
amplifier provides appropriate impedence
choose correct capacitor
discharging
pedestal error
hold
switch is opened to hold the analogue input and the capacitor maintains the output at the voltage of input just before the switch is opened