In my previous post, we started drawing the schematic of the diagnostic test-bed. Today, we will extend the schematic with signal acquisition, so the analogue voltages and currents at points and get digitized and transferred to the Arduino Due that we will use as a data collection hub and a controller.
Without further ado, the ADC connections:
The ADS131E04 is a 24-bit delta-sigma Analog to Digital Converter (ADC) that supports rates of up to 64kSPS. It can sample simultaneously from 4 channels. This ADC has been designed by Texas Instruments for industrial power monitoring.
We plan to sample at 6 KHz. Our main frequency is 60 Hz (the AC frequency), so we plan to over-sample 100 times.
Consider, first, the power supply requirements of the ADC. We need analog and digital power lines. The analog one can be single-ended or double-ended. As we do not want to design our own power supply we will use the one of the excess power of the Arduino Due. The linear regulator of the Arduino Due provides 5V and 3.3V. The ADC requires 3V to 5V for the analogue supply and 1.8V to 3.6V for the digital one. We will supply both with 3.3V from the Arduino Due, but to reduce the RF noise, we will do the splitting on the Arduino Due shield. Similarly, we will have two ground planes (AGND and DGND) that we will wire separately and ground near the power supply of the Arduino Due.
If you read carefully the datasheet of the ADS131E04, you will notice that the design decision to use the Arduino Due 3.3V power supply (single-ended) results in decreasing the useful resolution of the ADC. As the ADC is designed to be differential (for monitoring AC signals in a plant), not providing a negative power supply means that we have to waste half of the dynamic range. Providing 3.3V instead of 5V for the analogue supply also limits us on the upper range of the input signal. As a result, we will use only 22-23 bits of the 24-bit ADC.
All capacitors are ceramic (except the 22 μF that can be tantalum) and are used to filter-out unwanted noise. Talking about capacitors, one shall not forget the power supply ones, which should be placed close to the power supply pins of the ADC:
The final part of the data acquisition is the voltage and current conditioning. For the voltage part, it is easy. We have to scale the input AC range with a simple resistor divider:
For the current measurements, we need to convert the current to voltage. To do that, we use the miniature 200 mA CSLW6B200M Hall-effect current transducer designed by Honeywell. The current sensors have a built-in common-base amplifier (actually two transistors) and the output voltage is between 0V and 5V which is somewhat unfortunate as we cannot use part of the ADC's fully-differential range.
Notice the use of the resistor/capacitor pairs R3, C12; R7, C8; R10, C9; and R13, C15 as anti-aliasing filters. We have used the wonderful online RC low-pass filter design tool to calculate the values of a low-pass filter capable of attenuating frequencies greater than 6 KHz.