One of my projects requires continuous current and voltage logging (to a database) for further analysis. As I could not find a ready-made solution that satisfies my requirements, I have designed my own board. Here is the first version of the schematics:
For current measurement I am using the LTSR 15-NP Hall effect current sensor. The voltage is down-converted by a power transformer. The ADC conversion is done by two 24-bit LTC2440s (a rather amazing ADC design). The two ADCs are clocked on the same CS line so the power and voltage measurements are synchronized.
This is the PCB layout:
The power logger is designed to connect to an Arduino Mega 2560 board and the initial experiments seem to be successful at 500 Hz sampling rate and a small mostly resistive load (approx. 40/80 W heat-gun at 130V).
The design has a number of problems that I intend to solve in a subsequent design. These are listed in order of importance:
- The LTSR15 current sensor generates 0-5V at its output with 2.5V at 0A. Right now this voltage is divided by R1 and R2 so the range is 0V - 2.5V. This voltage division is imprecise. Use the second LM358 operational amplifier to properly get the LTSR15 output voltage in the range of the ADC;
- The ADCREF output of the LTSR15 should be buffered by another LM358 operational amplifier (this is mentioned in the datasheet);
- Two 1 μF capacitors should be used close to the power supplies of the LTC2440 ADCs to filter noise;
- The SCK digital clock of the two ADCs can be connected together (I am doing this in the cable right now).
I am also planning to add the following new features:
- Jumpers to configure the range of the LTSR15;
- External oscillator for the LTC2440 ADCs;
- External 5V power supply (through a voltage regulator) so I can decrease the power supply noise through the interface cable.