XT2640 Operating Manual
13 July 2016
Page 161 of 187
25.2
CHANNEL
CIRCUITRY
The diagram below is a simplified block diagram of the circuitry for each channel. Up to 4 channels can be installed in a XT2640.
Although not shown in this diagram, there is digital filtering on the sample data for both voltage and current as required for the
BANDWIDTH configuration of the controlling VPA and the data is scaled and offset according to the channel calibration data and to
user set scale and offset factors. The analog filtering shown in the diagram is simply to reduce sampling artifacts.
25.3
SAMPLING
CONTROL
There are 3 sampling controls in a XT2640. Typically each is associated with a VPA, however if you configure the LF/PERIOD setting
in a VPA to synchronize that VPA to a lower numbered VPA then the sampling control of that VPA is used for the channels in the VPA
so configured.
Sampling control performs the following–
Calculation of the measurement period for measurement results.
The measurement period is adjusted to be an integer number of fundamental cycles which is the closest to the configured
measurement period for the controlling VPA.
A half‐cycle period is also generated for controlling the period of half‐cycle valley measurements.
Calculation and control of the sampling period of channels.
The required sampling period is calculated to yield an integer number of samples in a measurement period which gives the
closest sampling period to the nominal sampling period for the channels being controlled. The sampling period is also
adjusted to not yield an integer number of samples in any harmonic (including the fundamental) over 500Hz. The nominal
sampling period for the channel types are as follows (this does not limit the HF amplitude performance of the XT2640) –
L, S or A types: 245KSPS
W type: 910KSPS
Each actual sampling period is controlled with 2.5ps average resolution and 2.6ns incremental resolution with a quasi‐
random distribution of incremental sample periods within a range sufficient to avoid Nyquist sampling limits.
This yields an effective sampling rate of 384MSPS and synchronization to the measured fundamental to within <1ppm.
Starting and stopping measurement periods.
If the XT2640 measurement mode is configured to anything other than SYNC‐VPA then each sampling control always
immediately starts a measurement period when the previous one ends. In this manner all channel and VPA measurements
are totally ‘gapless’.
If the XT2640 measurement mode is configured to SYNC‐VPA then each sampling control will start all measurement
periods if all measurement periods have ended. In this manner all channel and VPA measurements are synchronized, but
only the longest one is ‘gapless’.
25.4
CHANNEL
MEASUREMENTS
For each of the channels the following measurements are performed –
Voltage and current DC (the average of the samples in the measurement period).
Voltage and current maximum peak (the highest sample in the measurement period).
Voltage and current minimum peak (the lowest sample in the measurement period).
Voltage and current half‐cycle based valley measurements.
Voltage and current rectified (the average modulus sample in the measurement period).
SHUN
T
S: 22bit ADC
A: 24bit ADC
W: 22bit ADC
ISOL
AT
IO
N
H: 0.0025Ω
D(hi): 0.004Ω
D(lo): 0.5Ω
X(hi): 20.5kΩ
X(lo): 10.25kΩ
ATTEN
S: 22bit ADC
A: 24bit ADC
W: 22bit ADC
ISOL
AT
ION
S: 1.2MΩ
A: 1.2MΩ
W: 400kΩ
L: 121kΩ
CURRENT
VOLTAGE
DATA
DATA
SAMPLE