43
Subject to change without notice
3.5 single-slope A/D conversion
Name: Single Slope
V
r
t
0 V
V
in
= V
ref
V
t
1
t
2
Fig. 5: single-slope
The simplest method is the single slope conversion. A sawtooth
is generated by integrating a reference voltage V
ref
. There are
two comparators, one compares the ramp with 0 V, the second
with the unknown input voltage V
in
. As soon as the ramp cros-
ses 0 V, a counter is started which is stopped when the second
comparator switches at V
in
. The accumulated count is propor-
tional to the input voltage V
in
. The disadvantage is the limited
accuracy as it is directly affected by R and C of the integrator.
3.6 Dual-slope A/D conversion
V
r1
V
r2
t
0 V
V
r
t
1
t
2
t
3
t
3
t
2
t
1
= const.
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
1
: a counter is started while the
input voltage Vin is integrated. The integration stops when the
counter reaches its maximum count, the integration time
Δ
t
1
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
2
the counter
starts to count again. The ramp changes its polarity and runs
towards 0 V. The counter stops at t
3
when the ramp reaches 0 V.
The time span Δt
2
= t
3
– t
2
is proportional to the input voltage.
If the input voltage was high, a higher ramp potential will re-
sult at the end of Δt
1
as if the input voltage was small. A small
input voltage will yield a lower slope and a lower ramp voltage
(see V
r2
). As the reference voltage which is connected to the
integrator at t
2
is constant, the downward slope is constant,
hence the time for disharging the integration capacitor differs.
It takes more time to discharge the higer ramp voltage V
r1
than
for discharging the smaller ramp voltage V
r2
. The input voltage
Vin can thus be determined from the respective discharge time
span Δt
2
= t
3
– t
2
and the constant reference voltage.
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
Δ
t
1
+
Δ
t
2
. If their values
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
r
will be reached. But the downward slope will also be
steeper such that the ramp will cross 0 V at the same point in
time t
3
as before.
V
r1*
V
r1
t
0 V
V
r
t
2
t
1
= const.
t
1
t
2
t
3
t
3
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
Δ
t
1
, high frequency ac voltages are attenuated. If the
frequency of the superimposed ac voltage is equal to 1/Δt
1
or a
multiple thereof, this frequency will be completely suppressed.
If
Δ
t
1
is made equal to the line frequency or multiples thereof,
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 r1
V r1
t
0 V
V
r
Phase 1
Phase 2
Phase 3 4
5
Phase 1
t
1
t
1
t
2
t
4
t
5/0
t
0
t
3
Auto-Zero
#
V
i
dt
#
V
ref
dt
Auto-Zero
Fig. 8: multi-slope
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
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
