Hello,
This probably isn't a labjack unique concept but it's causing me some issues recently. I'm interested in taking voltage measurements across some resistors, which may range anywhere from +/- 10, but likely are under 5. I was using the LJ_rBIP10V in stream mode, but I'm not sure what this is doing or what it'll affect if I change to LJ_rgBIP5V.
I'd like to imagine that Labjack will autocorrect, but my sanity check has shown that it can greatly change the measurement. For instance, I tried to measure the resistance of a tungsten wire with BIP10V and it came out to be ~12 Ohms, but when I changed the range to BIP5V it went to ~4.7 ohs, the value I got from my hand held DMM.
Any advice would be helpful.
Thank you,
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The max input range of the U6 is +/-10V. This is with the internal gain set to x1. The other available gains are x10, x100, and x1000, corresponding to ranges of +/-1V, +/-0.1V, and +-0.01V.
If you request a range that does not exist, the driver will use closest range that includes the entire range you want, so if you request +-5V the driver will use +/-10V, and thus you should see no difference whether you use BIP10V or BIP5V. Makes me think something else is going on. Perhaps tell us more about your wiring and setup. Also, do you need to use stream mode because you are doing some sort of dynamic measurements?
Hello, thanks for the reply.
I'm doing a transient analysis where I record an unsteady current signal which pulses in the microsecond range for as long as 2 seconds. I've got a DC supply put across a series of resistors, which is measured in channel AIN3 (set from 3 to 199), I've got a negative voltage (on the other side of the path) measured across a similar resistor in AIN1 (From 1 to 199). I expect these values to be very similar apart from the resistances of the resistors when the current signal is off. The current signal is read through the same resistor in AIN1.
When we measured the tungsten wire we used log mode. We put it in through AIN3, supplied 200 uA, and put it to ground (199). We then divided the voltage read in by the 200 uA to get the resistance. After doing the 1 measurement with BIP10V we got >12 ohms, then switching the range to BIP5V we got >4.7 Ohms.
Something is definitely going wrong, but not sure what. Can you do some testing with a simple fixed resistor, say 4.7 ohms or 10 ohms, so you know what value to expect all the time and then you know which measurements are working right and which are not?
I grabbed a 10 ohm resistor and did a quick test myself. I connected 200UA to AIN3 to Resistor to GND. You can see in my LJLog screenshot that I am reading AIN3 using +/-5V and +/-10V and they all read the same (within the noise level, which would be much better if I used a small range for this small signal).
So, we did as you suggested and we've learned some things.
When using one laptop, all the resistances (we used your exact Log Setup) were above by 7 ohms when testing a 10 ohm resistor. We switched laptops and got the correct answer using only one channel at BIP1V. When we switched to BIP5V, it regained those 7 "ghost" (Happy Halloween btw) ohms. We toggled between BIP10V and BIP5V and kept getting the 17 ohms. We switched to AIN2, and saw the same issue.
It appears that only BIP1V gives us the correct answer, but if we need to be able to read in a signal up to 10 V, but mostly up to 5 V, we can't use BIP1V correct?
Is there a solution, or is our Labjack busted?
Does not sound like a symptom of device damage at this point. I suspect something else is going on.
I edited my previous post because I described my connections wrong. Should be:
200UA -> AIN3 -> Resistor -> GND
So I have a jumper wire from 200UA to AIN3, and the resistor connects AIN3 to GND. Like Figure 2.5-1 here:
https://labjack.com/support/datasheets/u6/hardware-description/10ua-200ua
You have the same wiring? Testing with LJLog configured with 4x AIN3 readings like in my screenshot?
The absolute accuracy of the U6 on the +/-10V range is +/-0.01% full-span, which is +/-2mV. Note that 7 ohms with 200uA is 1.4 mV. Thus if you are seeing a fairly consistent 7 ohm error on the +/-10V (or +/-5V) range that could just be expected accuracy tolerance.
https://labjack.com/support/datasheets/u6/appendix-a
We have the same wiring.
This is weird, so even if the 10 ohm resistor is correct, then maybe the channels are working? The voltage necessary with a 2 uA current to measure 4.7 ohms is 94 uV, where as to get 12 ohms you need 2.4 mV. Since these aren't 2 mV's different, they're within the labjacks accuracy range?
Sorry but I'm not following your numbers. Are you trying to measure current or resistance or both? If your current is about 2 uA, then 4.7 ohms would give you just 9.4 uV and 12 ohms would give you just 24 uV. You have mentioned readings possibly going over 5 volts, and if your resistance is 5 ohms that would mean a current of 1 amp.
Yeah, I'm losing it myself. I have no idea what the problem is. Just that My resistances are all over the place for the tungsten wire and I want to make sure thats not causing me issues for my current measurement in my actual application.
I'm going to spend some more time debugging and then if I find anything I'll come back and update.