Dear EEVblog forum,
I am working on a university project where I need to measure the power consumption of a computer (total power as well as power on HDD and SSD). Unfortunately, I have encountered a problem that is beyond my expertise and was hoping that I could get some help for sorting it out. In theory the task seems simple, I need to measure the voltage of three separate power rails (12V, 5V and 3,3V) as well as the voltage drops across some small current shunts using a PCIe DAQ card made by National Instruments (
https://www.ni.com/docs/en-US/bundle/pcie-usb-6353-specs/page/specs.html).
Here are some pictures as visual reference:
(you can make out the shunts on the bottom right from the big fan in the computer as well as the shunts on the HDD next to the electrical tape.)
(shielded twisted pair cables soldered onto shunts on a PCIe riser for the SSD)
The problem is that "Maximum working voltage for analogue inputs (signal + common mode)" of my DAQ is +-11V so when I try to measure the 12V rail or the drop on the shunts the measurement just clips. So the 5V and 3,3V rail seem to work fine, it is only the 12V rail that I need to scale down somehow for the DAQ card.
Here's a simplified schematic of the original approach I wanted to take to get around this issue:
(as you can see ai1-3 are single ended voltage measurements and the others are differential measurements across the shunts)
Unfortunately, as before 5V and 3,3V but 12V is incorrect. Since I am a beginner in this field I did not realise that the resistance of the input load into the DAQ will be far too large this way. I did not consider the fact that since this is a multiplexing amplifier it also has a significant input capacitance next to the large input impedance. When the DAQ is multiplexing between the inputs, that capacitor needs to charge and discharge to the new input value before it can be measured. This takes time and that time depends on the output resistance of the measured load. Figure 1 of the specification of the DAQ shows data for this.
Now with this in mind here's a schematic that shows more data about what needs to be done showing just the problematic 12V rail:
As you can see all I need is three amplifiers that can change the differential voltage across the three shunts to values that my DAQ card can measure. People over the National Instruments forums recommended that I should use "high side current shunt amplifiers" but nobody was yet able to recommend me something specific that is guaranteed to work in this application.
Please bear in mind that I'm a computer science student trying to finish a delayed project that outgrew me and I don't really have much experience selecting such components and I am under extreme time pressure. I don't wanna risk building up something that is not gonna work. Considering this, could somebody please recommend me and help me pick a suitable amplifier? Is it possible to build up such a circuit on a matrix board without much trouble? Please also bear in mind that I do not have enormous budget or industry skills, so probably the simplest way to make it work is the best even if it slightly sacrifices accuracy.
My rough requirements are the following:
- I cannot make my shunts bigger easily so I have to stick with these.
- The schematic above shows the currents I'm expecting (70A total can be reduced to 49A if necessary). With this, the voltage drops on the shunts could be anywhere between 0V to 0,161V
- The common mode is about 12V
- I am taking continuous samples with a rate of 10 measurements per second per channel. But this could be reduced if it is necessary.
- The amplifier needs to come in a package that I can hand solder wires to, either by itself on a matrix board or with some dip converter board. If I understand everything correctly I would need to connect three of these one to each shunt (or find a three channel one).
- Are there any other factors that I should consider for picking the right component? Something I did not think of yet?
With these in mind, I have stayed up all night researching amplifiers as I am not very knowledgeable at this topic. I looked at the TSC103IPT and to me it seems all right (even has features I don't need like adjustable gain) but maybe a bit too small to hand solder wires to. Two other potential candidates I found are the INA282AIDR from TI and MCP6C04 from Microchip and maybe something from analogue devices maxim amps (
https://www.analog.com/en/technical-articles/highside-currentsense-measurement-circuits-and-principles.html#/ if you look at the sidebar they have many options listed) although I'm sure there are even many more options I did not even consider. Do you, forum members, think any of these could satisfy my demands or can you recommend an alternative even better option that I did not consider?
Thank you,
Károly