PCSGU250_ PcLab2000LT_DC Mean?

Instruments PC Oscilloscopes and Recorders
1

I need formula for DC Mean. Please help
(PcLab2000LT.exe ( PCSGU250) - View -Waveforms parameters - DC mean)
Now I have Voltage value = fullscale *Databuf(n)-GND_level )/ 255, but it different.

2

Here is an example:

The full scale on 1V/Div range is 8V.

Here is the beginning of the data file:

[code]TIME STEP:
125 = 1ms

VOLTAGE STEP:
CH1: 32 = 1V
CH2: 32 = 1V
GND 75 86
N CH1 CH2

0 153 87
1 153 87
2 153 87
3 153 87
4 154 87
5 154 87
6 153 87
7 153 87
8 153 87[/code]
To calculate the DC mean for CH1 you can use following formula:

DC mean = fullscale * (Databuf(n)-GND_level )/ 255

Example:
Databuf(0) = 153
GND_level = 75
fullscale = 8V

DC mean = 8V * (153 - 75) / 255 = 2.447V

If you have a varying voltage, then you have to calculate first an average of the Databuf(n) values.

3

Hello, thank you for fast answer.
But: ( for DC)
Why voltage value from voltmeter (FLUKE 87) is not equals Voltage value = fullscale *Databuf(n)-GND_level )/ 255.
But DC Mean (from PcLab2000LT) is equals values from voltmeter (FLUKE 87) (it’s ok)

I need formula that equals values ​​from voltmeter ( for DC).
May be need add setup ?
Please help
Thank you

4

Can you please post the values you get with the Fluke and the DC value from the “Waveform Parameters” window.
Also please post the beginning of the stored data file.
If your voltage is clean DC, then only about ten data values is enough to post.

5

Hello,

Source: Fluke 741 ( calibrator) NEW: 0.8 V
Measure: Fluke 287 NEW: 0.8001 v
Measure: PGSU250 DC MEAN: 0.84
Date from PGSU250:
CH2

Sample rate [Hz] 125000 125000
Full scale [mV] 8000 8000
GND level [counts] 127 127

Data (0) 127 155
Data (1) 127 155
Data (2) 128 155
Data (3) 128 154
Data (4) 127 155
Data (5) 127 154
Data (6) 128 154
Data (7) 128 154
Data (8) 127 154
Data (9) 128 155
Data (10) 127 155

DC Mean ( calc) = = 8 ((1556+154*5)/11-127 )/ 255 = 0.864 V

6

There seems to be strange difference.
The reason to this may be that the PCSGU250 calculates the DC mean over all the data.
Have you tried if the calibration helps: select Options -> Calibrate. ?

7

Yes, I did calibrated every time before. I have other device (new PCSGU250), I will try with new PCSGU250… I will let you know…

8

Hello,
I tried more times, but same results…
Can I get formula DC Mean from PcLab2000LT or get part of programme code from PcLab2000LT.exe.
Because formula for DC Mean (average of the Databuf(n) values) not equal DCMean from PcLab2000LT.
Please…

9

I think this is the main difference between your calculation and the oscilloscope’s calculation:
The oscilloscope uses values between 0 and 255 in calculation.
The sum of 4096 samples is divided by 4096 to get the average value.
Both the dividend and divider are integers.

      average := sum div 4096;

The result is an integer between 0 and 255.

I think, you are using floating point value to calculate the average of the 4096 samples.
This gives more accurate result.

This is the cause to the difference between the displayed value and the calculated value.
The calculated value is more accurate.