YOKOGAWA WT210 User Manual page 39

1.1 System Configuration and Block Diagram
1-2
Signal Flow and Process
The sections that make up the WT210/WT230 are VOLTAGE INPUT, CURRENT
INPUT, DSP, CPU, display, and interface.
In the voltage input circuit, the input voltage is normalized by a voltage divider and
operational amplifier, then sent to the A/D converter. Shunt resistors form a close circuit
at the current input circuit. The voltage across shunt resistor is amplified and normalized
by the operational amplifier and then input to the A/D converter. This method enables
switching of the current range without opening the current input circuit, so the current
range can be switched while power is supplied to the circuit. This also enables remote
control via communications outputs. The WT210 equipped with two shunt resistors, one
for minute currents and another for large currents,.
The output from the A/D converter in the current input and voltage input circuits is sent to
the DSP (Digital Signal Processor) via a photo-isolator (ISO), which is used to provide
insulation between the current input circuit (or voltage circuit) and the DSP.
During normal measurement, the DSP converts digital values sampled at a period of
approximately 20 µS to voltage, current, and active power; sums the values over a
predetermined period; and then divides the sum by the number of samples to derive the
measured value of voltage, current, and active power. In addition, to achieve high
accuracy, the start and end points of sampling are determined by synchronizing to the
zero-crossing point of the voltage or current signal that you select to be measured. From
the voltage, current, and active power, the DSP computes the apparent power, reactive
power, power factor, and phase angle and performs processing such as scaling and
averaging. During harmonic measurement, the DSP performs FFT based on the digital
values sampled at an integer multiple of the period of the PLL source signal (period of
the clock that the PLL circuit generates) and determines the measured values of various
items of harmonic measurement.
The measured value is transmitted to the CPU section. Various computed values such
as Σ and MATH are determined from the measured values. These measured values and
computed values are displayed (on the 7-segment LED), output through a D/A converter,
or output through communications.
During normal measurement, the processing on the DSP and CPU is pipelined, and the
DSP processing is performed in real-time. Therefore, measurements with few data
dropouts can be achieved against the input signal.
IM 760401-01E