SBAS582C – JULY 2014 – REVISED APRIL 2015
Typical Applications (continued)
The phase angle of the electrical signal on the power network buses is a special interest to power system
engineers. The primary objective for this design is to accurately measure the phase and phase difference
between the analog input signals in a multichannel data acquisition system. When multiple input channels are
sampled in a sequential manner as in a multiplexed ADC, an additional phase delay is introduced between the
channels. Thus the phase measurements are not accurate. However, this additional phase delay is constant and
can be compensated in application software.
The key design requirements are given below:
•
Single-ended sinusoidal input signal with a ±10-V amplitude and typical frequency (f
IN
= 50 Hz).
•
Design an 8-channel multiplexed data acquisition system using a 16-bit SAR ADC.
•
Design a software algorithm to compensate for the additional phase difference between the channels.
9.2.1.2 Detailed Design Procedure
The application circuit and system diagram for this design is shown in
. This design includes a complete
hardware and software implementation of a multichannel data acquisition system for power automation
applications.
The system hardware uses the ADS8688, which is a 16-bit, 500-kSPS, 8-channel, multiplexed input, SAR ADC
with integrated precision reference and analog front-end circuitry for each channel. The ADC supports bipolar
input ranges up to ±10.24 V with a single 5-V supply and provides minimum latency in data output resulting from
the SAR architecture. The integration offered by this device makes the ADS8684 and ADS8688 an ideal
selection for such applications, because the integrated signal conditioning helps minimize system components
and avoids the need for generating high-voltage supply rails. The overall system-level dc precision (gain and
offset errors) and low temperature drift offered by this device helps system designers achieve the desired system
accuracy without calibration. In most applications, using passive RC filters or multi-stage filters in front of the
ADC is preferred to reduce the noise of the input signal.
The software algorithm implemented in this design uses the discrete fourier transform (DFT) method to calculate
and track the input signal frequency, get the exact phase angle of the individual signal, calculate the phase
difference, and implement phase compensation. The entire algorithm has four steps:
•
Calculate the theoretical phase difference introduced by the ADC resulting from multiplexing input channels.
•
Estimate the frequency of the input signal using frequency tracking and DFT techniques.
•
Calculate the phase angle of all signals in the system based on the estimated frequency.
•
Compensate the phase difference for all channels using the theoretical value of an additional MUX phase
delay calculated in the first step.
9.2.1.3 Application Curve
The performance summary for this design is summarized in
and
. In this example, multiple
sinusoidal input signals of amplitude ±10 V are applied to the inputs of the ADC. The initial phase angle is the
same for all signals, but the input frequency is varied from 45 Hz to 55 Hz. The phase error in the last column of
reflects the measurement accuracy of this design.
Table 17. Theoretical and Measured Phase Difference
THEORETICAL PHASE
MEASURED PHASE
PHASE ERROR AFTER
INPUT TEST CONDITION
ERROR
(1)
ERROR
(2)
COMPENSATION
(3)
Phase difference
0.036°
0.036145°
0.000145°
(consecutive channels)
Phase difference
0.252°
0.249964°
0.002036°
(farthest channels, channel 0 to channel 7)
(1)
Theoretical phase difference introduced by multiplexing is calculated based on the formula:
Δφ
= (f
IN
/ f
ADC
) × N × 360°, where N =
integral gap between two channels in the multiplexer sequence; f
IN
= input signal frequency; and f
ADC
= 500 kSPS, maximum throughput
of the ADC.
(2)
Measured phase value (before compensation) includes phase difference between any two channels resulting from multiplexing ADC
inputs.
(3)
The algorithm subtracts theoretical phase difference from the measured phase to compensate for the phase difference resulting from the
MUX inputs.
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