Hameg HM8112-3 Digital-Multimeter, DMM, 1 200 000 Counts 25-8112-0302 Datenbogen
Produktcode
25-8112-0302
44
Subject to change without notice
then be displayed. The number of measurements for averaging
decides how well interference will be suppressed. Because the
input voltage is continuously being integrated upwards and
then the reference voltage downwards, three further steps are
necessary. In the following the individual steps for converting
one measurement value are described. For averaging a number
of measurement results is required.
Phase 1: Autozero – constant time span Δt
1
The duration of the autozero phase is, in general, identical to
the integration time of the input voltage V
in
. This is to ensure
that all errors to be expected will be caught. The errors caused
by the offsets of the comparators and the integrator will be
compensated by adding a definite offset (which is mostly stored
on a separate capacitor).
Phase 2: Integration of the input voltage V
in
Constant time span Δt
1
.
Phase 3: integration of the reference voltage V
ref
Δt
2
depends on the amplitude of the ramp voltage V
r
at time t
2
.
The duration of this time span must be measured with great
accuracy, because the digital value of the input voltage will be
determined from this time span.
Phase 4: Overshoot Δt
3
Due to delays in the integrator and the control signals (e.g. by
a microcontroller) an overshoot is generated. The integrator
capacitor charges in negative direction. This charge is elimi-
nated in phase 5.
Phase 5: integrator output Zero Δt
4
The charge caused by the integrator overshoot will be
discharged.
3.8 Accuracy specifications
The accuracy specifications of multimeters consist of diverse
numbers and units.
The measurement deviation is specified as:
± (xx % of measurement + xx % of range) at a temperature
of xx °C ± xx % ; this will apply for a time span of (xx hours, xx
days, xx years)
example: measuring range 10 V:
± (0.004% of rdg + 0,001% of f.s.) valid for 24 h at 23 ±1 °C
The temperature coefficient specifies the deviation per degree
C valid in a specified temperature range.
example: measuring range 10 V:
± (0.001% of rdg /°C) within a temperature range of (10 ... 21°C).
the long term stability indicates the irreversible drift of the
instrument for a given time span. standard time intervals are:
30 days, 90 days, 1 year, 2 years.
example: long term stability better than 3µV for 90 days at
23 ±2 °c.
The short term stability indicates how far a measuring in-
strument is useful for comparative measurements with other
measuring instruments. This is valid for a short time span within
a limited temperature range.
example: short term stability better than 0.02 µV within 24 h
at 23 ±1 °c.
to be calculated:
the possible total deviation at 16 °c in the 10 V
range.within a time span of 14 hrs. the measure-
ment result shown is 6.000000 V?
calculation:
± (0.004% of 6.0 V + 0.001% of 10 V)
for 24h at 23 ±1 °
Result: 0.00034 V.
± (0.001% of 6.0 V / °C) x ΔT
within a temperature range of (10 ... 21 °C)
with ΔT = (23-1 °C) – 16 °C = 6 °C
Result: 0.00036 V
The possible total deviation is equal
to the sum and amounts to
0.00070 V.
HINT
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