Multifunction DAQ
Getting incorrect voltage measurements overt resistor divider bridge
@Károly wrote:
I have a problem where I need to measure power on three separate lanes with different power supplies (12V, 5V and 3,3V), however, my PCIe DAQ can only measure voltages up to 10V. To get good measurements I need to know the exact voltage on all lanes as well as the voltage drop over some shunt resistors. As advised in: DAQ change input voltage range I have built up the following resistor divider bridges:
I am measuring the voltage in single ended ground referenced mode and the voltage drops over the shunts in differential mode. Problem is that the 5V and 3,3V seem to provide sensible readings while the 12V is inaccurate. I am expecting voltage drops in the order of 0,01V and when I measure with finite samples I get a sinusoidal with a mean value of 0,1V so 10 times the expected. If I change to continuous mode it becomes even worse with a measured voltage drop of about 0,5V so 50 times the expected. If I change both ai5+ and ai5- to single ended ground referenced and subtract them in software I seem to get a reasonable value but I am unsure if it is actually correct since I don't know what causes the problem. Can anybody give me an insight?
As an electrical engineer I can say that your issue is two things. First is the current in the 10M voltage divider, you are expecting 12V/20Mohm = 600nA through your voltage divider to drive the input of the DAQ but you pick up noise or input offset current that is > 600nA in your circuit. Second is the input impedance on your DAC, it should be >> than 10M or the input offset current in your first gain stage will dominate the measurement you are trying to take with this particular circuit, but with these kinds of impedances you are more likely to pick up radio stations than your measurement if you are using test leads (antennas).
The lesson here is:
1) This is not the best circuit to do what you want, it works in SPICE but not in real life, check this out for some tips.
2) When you design a circuit, you need to think about the entire circuit, as in; what is going on inside the DAQ, if you attach it to your circuit, it will have some effect on it.
Additionally, this is not a Labview question. There is one more thing you need to figure out to make this work, If the above info is not enough to get you going on that, I have an EE consulting business that can show you how to take these measurements : )
I have move the thread.
@Károly wrote:Do you think if I use lower resistor values for the bridge (let's say 1M) it might work, or is it a totally lost cause and I must use an external amplifier?
The input impedance of the DAQ is specified as >1G so I assumed it would not be a problem as it is orders of magnitude bigger than that of my bridge.
Also thank you for the offer, however, this is a dissertation project that I was supposed to finish months ago to graduate from university but it is still not working, that's why I decided to consult these forums. As such I cannot afford to use a consulting service.
In your opinion then, is there any way to do what I want without adding many more external components?
In the past, we used a total of 1MOhm divider and found that it did not work well for the DAQ. We had to add voltage follower op-amp circuits for the divider readings. Super simple circuit.
For the current shunt measurements, I found it better to use current shunt amplifiers that output a single ended value, such as this chip: TSC103IPT.
There are two different issues goong on here. If you would have a steady input to a DMM then the 10 MOhm << 1G would be all that matters. But you also have a multiplexing ampliifier and the amplifier input is always having a capacitance (and even some inductance). Before the amplifier input can measure the real value the external signal must (dis)charge that capacitor from the voltage level of the previous multiplexed input value. Even with only 10pF the time constant gets really significant at an input impedance of 10 MOhm!!!
