Hi,
I'd like to better understand what is going on in the background when I use the VoltDiff command.
Let's use this VOltDiff instruction that I've seen in many programs in my company:
VoltDiff (Vcc_LVDT,1,mV5000,1,True ,0,60,0.001,0)
So reading this line we can see that:
Reps=1
Range=mV5000
DiffChan=1
RevDiff=True
SettlingTime=0 (so 500usec)
fN1=60
Mult&Offset = 0.001 & 0
My understanding is that, behind the scene, an integrating ADC (am I right here on the ADC type ?) will integrate signal for 16.67ms (due to fN1 being set at 60Hz) and returns 1 sample (lets call it Samp1). Am I right ?
RevDiff being activated, the same acquisition will be performed interverting the differential lines (SE 1 and 2). Lets call this second measurement Samp2.
How are Samp1 and Samp2 combined to return a value in my Vcc_LVDT variable ?
How is noise removed if only 1 sample is taken ? Is it by the integration process of the ADC being 'open' for 16.67ms ?
I more used to use SAR ADC. It usually take 10-100 samples and average them in my program to get 1 value at the end.
If some samples present higher/lower amplitude due to spikes/noise on the signal, the average is decreasing their effect.
Here the ADC used does this alone thanks to its 'integration time' ?
Since you mention f notch, I assume you are using a CR6 or CR1000X. The ADC performs many fast samples over the integration time and returns the average to filter the noise. In the CRBasic help for VoltDiff on the CR1000X, you will see 31,250Hz as the highest possible f notch. That is the subsample speed. One full 60Hz AC cycle is 16.67ms.
On older models like the CR1000, an intregation capacitor was used for filtering.
For more information about the RevDiff parameter, I suggest reading "The Benefits of Input Reversal and Excitation Reversal for Voltage Measurements". It is a technical white paper available on the website. A google search will find it.
Hi JDAvis,
Thanks for your answer !
I've got several models: CR6, CR1000, CR1000X mainly (and even some CR10 but this last ref is out of scope for this question).
Lets says I work on a CR6 or a CR1000X, and the instruction detailed upper:
VoltDiff (Vcc_LVDT,1,mV5000,1,True ,0,60,0.001,0)
I don't understand how many samples are taken and used to get an averaged value placed into the Vcc_LVDT variable.
Can you detail that process based on this instruction ?
My current hyspothesis is that, since Reps = 1, it will take 1 sample only and then the fN1 set at 60Hz as not so much an aeffect.
Reading the manual and your comment, fN1 being set at 60, a table in the manual shows that Burst Sampling Rate is 50ms (20 samples per second). So does that mean the VoltDiff instruction will last 1 second, and behind the scene the ADC will get 20 samples over that 1 second and average the values ?
But since in my instruction DiffChan is a positive value, I guess it doesn't enter in burst mode...
As you can see, reading the manual and your comments, it is still not clear to me.
Can this be clarified ? :-)
Thanks !
Cyril
On a CR6, reps set at 1 and fNotch at 60 you will get 1 value back. That value is the average of hundreds of samples taken by the ADC over 16.67ms. Total measurement time, which includes internal channel switching, is about 20ms.
As seen in the CR6 help for the VoltDiff instruction, the ADC runs at 93750Hz.
93750/60 = 1562.5 samples
The logger rounds to the nearest, so 1,563 samples go into that 1 value that comes back.
This post is under review.