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3.5
Single-Slope A/D conversion
V
V
= V
in
ref
0 V
t
1
Fig. 5: Single-Slope
The simplest method is the single slope conversion. A sawtooth
is generated by integrating a reference voltage V
two comparators, one compares the ramp with 0 V, the second
with the unknown input voltage V
ses 0 V, a counter is started which is stopped when the second
comparator switches at V
. The accumulated count is propor-
in
tional to the input voltage V
in
accuracy as it is directly affected by R and C of the integrator.
3.6
Dual-Slope A/D conversion
V
r
t
= const.
1
0 V
t
1
Fig. 6: Dual-Slope principle
With the dual slope method the accuracy is not dependent
on R and C of the integrator, both and the counter frequency
must only be constant during a complete conversion cycle. The
measurement starts at time t
input voltage Vin is integrated. The integration stops when the
counter reaches its maximum count, the integration time Δt
is thus constant, the input voltage is disconnected from the
integrator. Now the reference voltage Vref which is of opposite
polarity is connected to the integrator. At time t
starts to count again. The ramp changes its polarity and runs
towards 0 V. The counter stops at t
The time span Δt
= t
– t
is proportional to the input voltage.
2
3
2
If the input voltage was high, a higher ramp potential will re-
sult at the end of Δt
as if the input voltage was small. A small
1
Name: Single Slope
t
t
2
. There are
ref
. As soon as the ramp cros-
in
. The disadvantage is the limited
t
2
t
t
3
t
t
2
3
: a counter is started while the
1
the counter
2
when the ramp reaches 0 V.
3
M e a s u r e m e n t P r i n c i p l e s a n d B a s i c s
input voltage will yield a lower slope and a lower ramp voltage
(see V
). As the reference voltage which is connected to the
r2
integrator at t
is constant, the downward slope is constant,
2
hence the time for disharging the integration capacitor differs.
It takes more time to discharge the higer ramp voltage V
for discharging the smaller ramp voltage V
Vin can thus be determined from the respective discharge time
span Δt
= t
– t
and the constant reference voltage.
2
3
2
Advantages:
The accuracy is no longer dependent on the accuracy of the RC
of the integrator, nor on the counter frequency. all 3 must only
be constant during a complete cycle Δt
change over time, this will only affect the slopes of both ramps.
If the slope of the upward ramp becomes higher, a higher ramp
voltage V
will be reached. But the downward slope will also be
r
steeper such that the ramp will cross 0 V at the same point in
time t
as before.
3
V
r
t
= const.
1
0 V
t
1
Fig. 7: Dual Slope: Change of time constant by component drift
As this type of converter does not measure the instantaneous
value of the input voltage but its average during the upintegra-
tion time Δt
, high frequency ac voltages are attenuated. If the
1
frequency of the superimposed ac voltage is equal to 1/Δt
multiple thereof, this frequency will be completely suppressed.
If Δt
is made equal to the line frequency or multiples thereof,
1
hum interference will be rejected.
3.7
Multi-Slope A/D conversion
The Multiple Slope method is based on the Dual Slope method.
Several measurements are performed with the Dual Slope
method, their results are averaged. This calculated value wil
V r
#
Auto-Zero
V
dt
i
1
0 V
Phase 1
Phase 2
t 0
t 1
Fig. 8: Multi-Slope
r1
. The input voltage
r2
+ Δt
. If their values
1
2
t
2
t
t
3
t
t
2
3
1
#
V
dt
Auto-Zero
ref
Phase 3
4
5
Phase 1
t 2
t 3
t 4
t 5/0
t 1
Subject to change without notice
than
or a
t
43
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