05-24-2012 10:35 AM
Does anybody know what all of the internal timing aspects that have to be taken into consideration for a PXI-4072 and what the values are? I found a white paper (Digital Multimeter Measurement Cycle) that talks about the different steps in that measurement cycle, but it doesn't talk specifically about what the switching time is. This is important to me becasue I'm trying to collect current measurements and synchronize them with some voltage measurements from a PXI-6259 and it appears that the dt from the 6259 waveform is OK, but without being able to take into account the swithing time, I won't be able to subtract out all of the measurement delays to come up with the time between measurements that the multipoint measurement setup VI is looking for. Any help is appreciated. Thanks,
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05-24-2012 10:43 AM
By way of further explanation, I'm trying to make current measurements in the ~100 uA range. I tried using a PXI-6239 but that doesn't appear to be able to measure such small currents. So, I've got the dt from my AO voltage and AI voltage that I can use and so far have subtracted the apeture time and settle time, turned the ADC Calibration and Auto Zero to off and looked as much as I could to find a way to get the switch time specified in that white paper but cannot find it. Once I have that, I think I can get my captured waveforms to line up, but at this point they aren't. I just have a resistor in the circuit for now so the voltage and current waveforms should line up fairly closely and they are not.
I appreciate any thought or insight anyone can provide.
05-24-2012 12:07 PM
The switching time varies based on the current device configuration and the configuration you are switching to. Trying to synchronize the devices by making things last the same on two separate devices is futile. Even if we published every possible switching time for the 4072 (a gargantuan task), you must understand that the internal clock on both boards will be different so the granularity in times will likely not match. Besides, the two instruments are running off independent clocks which means that there will be some small amount of drift between your them. I would not even try to synchronize two identical 4072s this way because of that reason.
The way to synchronize the two instruments is through the PXI trigger bus. We design our products so that you can send and receive triggers through this bus in order to synchronize them. Please look at the documentation for your devices. If you need further help, feel free to hit this message board again.
05-24-2012 12:41 PM - edited 05-24-2012 12:42 PM
I'll throw out there that the 6259 does not capture analog input channels simultaneously (your post hints that you might already know this); it uses a multiplexer to scan through all channels in rapid succession. You can look at the 6259 specifications to see how far off each channel from the others. If you need true simultaneous sampling, I recommend looking at some of out simultaneous sampling boards, such as the 6143, 6123, 6356, etc.
As Marcos mentioned, synchronizing the DMM with other measurements is going to be very difficult if they all need to happen at the same time. If a few hundred microseconds time difference between measurements is acceptable, then we can use the DMM's triggers to synchronize the measurements. If that is too long, then you might look into using a current sensor that outputs a voltage that you can measure with your DAQ board (e.g. series current sense resistor, hall effect sensor, etc).
Let us know your timing requirements and we can comment further.
05-24-2012 01:34 PM
I hear what you guys are saying and have already tried to go down the path of using the PXI trigger lines to get the closest synchronization that I can. I have a PXI-6602 that I can use but it's proving to be a very difficult task to actually get the triggers to the 4072 and have it recognize them and get any meaningful data. It seems like the values I get from drop down boxes don't line up with one another very well. I've been looking through all of the documentation and getting from the timing card to the DMM is not obvious from that, at least not in practice.
05-24-2012 02:34 PM
Sorry to hear you're having trouble routing the triggers. For a good overview of how triggering works on the DMM, I recommend looking at the DMM triggering example VIs (Example Finder»Hardware Input and Output»Modular Instruments»NI-DMM»Triggers). In particular, I recommend looking at Acq Multiple Samples - Ext Sample Trig.vi
For example, I'll assume you want to send the 6602's sample trigger onto PXI Trigger line 0. There are various names for this line, depending on the product (RTSI0 or Trig0 or TTL0, etc); they all refer to the same physical trigger line 0 on the backplane... for the DMM, we use 'TTL0', so select that line in the example code above.
Next, we need to actually send the trigger on the 6602. I'm assuming you're using an 8 slot chassis that shares a single trigger bus. If you're using a larger chassis, it can have up to three separate backplane busses (such as slots 1-6, 7-12, 13-18). On these larger chassis, the busses have internal bridges that allow signals to span multiple busses. If you're using a larger chassis, for now I'll recommend placing all three cards in slots 2, 3, 4, so that we're in guaranteed to be in one backplane bus. once we get everything working, then we can tackle the different busses, if needed.
So, to send a trigger out of the 6602, I need to look and see what internal 6602 routes I can route onto PXI Trigger line 0. To do this, fire up MAX and click on the 6602. One of the bottom tabs is 'Device Routes'. Click on it. Then find PXI Trigger line 0 (in this case, it's named 'PXI_Trig0':
The image above only shows a few of the available routing options... scroll in MAX to see all available options.
I'm assuming you've already figured out which signal you want to export onto TTL0... in the picture above, I've assumed that you want to output counter 0 onto the PXI trig 0 line as the sample clock... you'll want to select whichever line you intend to use after verifying it is supported (green box).
Note that you could also use an onboard counter on the 6259 so that you don't need the 6602... etc.
Hopefully this clears some of the confusion. If not, post up and we'll be in touch.
05-24-2012 03:01 PM
Ok, so I can get the counter lined up and that seems to be doing what it should, but the DMM driver is acting funny. I can set it up exactly, or close to exactly like it is in the example for taking multiple samples but the DMM does not seem to be responding to the triggers, it just goes as fast as it can and is done way before the task delivering the finite number of pulses is finished. Is there any reason that would be the case?
I've tried a number of things that seem to indicate that the DMM card is running by itself and not off of the triggers. If I change the frequency of the trigger task by a factor of 1/2, 2 or 4 I get the same waveform from the DMM. I'll post a copy of my code with the caveat that I'm in full debug mode and haven't done anything to break up or pretty up the code, I just want to get it working some way, somehow, first.
05-24-2012 04:18 PM
Have you considered taking the current measurements with the 6259? Depending on your signal levels, you would need only a 1kΩ resistor to turn your 100µA current into a 100mV signal that the board could measure directly.
05-24-2012 04:31 PM
I would do that except the item we are measuring is essentially a wire with a little capacitance from some fluid and adding 1k would throw off the whole voltage waveform. So I'm really trying to avoid putting in a resistor at all costs because it will throw off the whole system we're trying to measure the response of.
05-24-2012 05:04 PM
Well, the 4072 measures current by measuring the voltage across a shunt resistor. It's just that it's a lower value resistor. Depending on how much noise and inaccuracy you can tolerate, you may be able to find a resistor that will work for you. 100Ω? 10Ω?