8 Construction
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8-3
The total of all pulses counted during the entire measuring time is proportional to the
average value of the input voltage.
In this kind of voltage to time conversion the result is not falsified by the loss factor
of the capacitor or by drift of its capacitance value.
The result is also independent of the frequency of the clock oscillator because the
times for up and down integration are determined with the same frequency.
The precision of this method essentially depends on two circuit components, the re-
sistor in the input and the reference voltage source U ref.
For the reference voltage source it is not the absolute accuracy which is important but
the long term stability and low noise performance which can be optimized by selec-
tion and long term tests (aging).
The converter is composed of PREMA’s customized IC, the external reference, the
series resistors, and the integration capacitor.
The use of ASICs represents an important advantage here, since space is saved on the
one hand, and the possibility of component failure is further reduced on the other
hand.
Mains Synchronization
With an integrating converter, the synchronization between the measurement time and
the period length of mains frequency is of great importance.
The reason is that, at resolutions of more than 4½ digits, power line noise becomes
noticeable in the measurement result.
With sampling converters, this line noise is often eliminated by averaging over a
number of measurement results. This method greatly increases the measurement time,
however, and also suppresses short-time changes in the measurement signal.
In PREMA’s patented Multiple-Ramp Technique, the duration of the measurement
time, as well as the phase position, is coupled to the mains voltage. This coupling is
effected through a PLL Circuit (Phase-Locked-Loop), which insures that phase posi-
tion and length of measurement time are always adapted to the frequency and period
length of the mains voltage.
This process forms the basis for optimal suppression of power line frequency in the
measurement signal.
