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Converting mV value back to ADC count

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Converting mV value back to ADC count

Postby einar.nygard » Mon Jun 13, 2016 10:44 am

I'm using picoprobe/scope for simple X-ray spectroscopy and I'm automatically saving amplifier traces for trigged events using the alarm feature and dumping the buffer. I save thousands of such events; then I convert the .psdata to cvs (works nice but a bit slowly); then, using some bash scripts, I collect one sample from each file (the sample with the highest amplitude). Clumsy procedure, but useful for me right now.

Now I want to use those thousands of picked peak-samples (each representing the wavelength/energy of an X-ray) and plot them in a histogram to create the X-ray spectrum. However, making those kind of spectroscopy-histograms requires precise binning of the data, and here lies my problem:

I cannot convert back the Picoscope represented mV values to exact ADC counts. I.e. I do not get integer numbers if I divide the mV value by (scope-range)/ 32768. It may be caused by truncation of the mV value, but each step of the actual 8 bit mV values is also not linear. The (8 bit) steps actually varies between 140 and 150 times the (16 bit) LSB value, i.e. quite strange values (8 bit steps should be 256 16 bit steps).

Would you have some additional information for me that could help me getting my (back-projected ADC count) binning right ?

Thank you in advance.
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Re: Converting mV value back to ADC count

Postby Hitesh » Wed Jun 15, 2016 11:36 am

Hello Einar,

This post will help to explain why we scale to 16-bit data from 8-bit. It is also to allow for calibration purposes.

The formula you need to use is:

ADC count = (reading_millivolts / voltage_range_millivolts) * max_ADC_count;

where max_ADC_count is the maximum ADC count for your device (not 32768). If you can indicate the device you are using, I can let you know the max_ADC_count value to use.

Regards,
Hitesh

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Re: Converting mV value back to ADC count

Postby einar.nygard » Thu Jun 16, 2016 9:43 am

Dear Hitesh,

Thanks very much for your answer. Your formula will yield homogeneous LSB steps as it should be when the voltages are representing a binary 'staircase'. But the problem is that the resulting voltage scale output is not yielding a homogeneous 'starcase'. The column of data below are all the, unique, voltage values that I get as output after having collected about 6000 data points. I get data in most ADC bins after that amount of statistics, in this case except 3-5 bins. If you plot these data as Y-values against a linear (e.g. integer value) X-axis, you will see what I mean. It is virtually impossible to revert these voltage values back to homogeneous binary bins.

I wish that the Picoscope 6 program could have had the very simple feature of dumping binary values rather than only values converted to voltage. I realize that I can get the ADC values if I read-out using my own program, but it is also useless for me because there seems to be no trigger-listening feature in the API that I could use as alternative to the 'Alarm' feature in the Picoscope 6 (or is there ?). That would be necessary for my kind of (asynchronous) spectroscopy needs.

I'm sorry that I've gotten stuck with this, I have a higher-spec bench-top oscilloscope in my lab and I bough the Picoscope as it seemed to offer me more of a general purpose "data-acquisition" system, but I'm about to conclude now that no one in the gamma-/X-ray spectroscopy community currently can get the maximum out of your system. The only thing that you need to do to change that is to offer the optional binary ADC values output.

Best regards, Einar

Below are the voltage 'staircase' values that I get from your system:

The data is taken at the +-1V oscilloscope range and the values are scaled by 10e8 just to get integer values (thus the first value, e.g, was originally 0.08865627)

8865627
9192175
9753716
10089420
10641800
10983610
11532940
11880860
12421030
12778100
13309120
13672290
14197210
14569540
15085300
15463730
15973390
16360970
16861480
17749570
18152410
18637650
19046600
19525740
19943850
20413830
20838040
21301920
21735280
22190010
22632530
23081150
23526720
23969240
24423960
24857330
25318150
25745420
26215400
26633500
27109590
27521590
28006840
28409680
28901030
29297770
29798270
30185860
30692460
31073950
31962030
32486950
32850120
33381150
33738210
34278390
34629350
35172580
35517440
36405530
36964020
37293620
37861260
38181710
39069800
39957880
40546890
40845970
41444130
41734060
42341380
42622150
43235570
43510240
44398330
45027010
45286420
46177560
46818440
47065650
47953730
48609880
48841820
49729910
50401320
50618000
51298560
51506090
52195810
52394180
53282270
53987240
54170350
55058440
55778680
55946530
56837670
57570120
57725760
58464310
58613850
59361550
59501940
60390030
61152990
61278120
62050240
62166200
62944420
63054290
63942380
64830470
65633110
65718560
66527300
66606650
67494740
68385880
69273970
70113220
70162050
71050140
71938230
72826320
73696090
73714410
74602500
75490580
76378670
76381730
77266760
77278970
78154850
78173160
79967650
81710260
81759090
82653280
83486430
83550520
85262610
85341960
86150700
86236160
87133400
88027590
88814970
88924830
89703050
89822080
90591140
90716270
91482280
91613510
92370370
92507710
93258460
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Re: Converting mV value back to ADC count

Postby Hitesh » Thu Jun 16, 2016 10:50 am

Hi Einar,

Would it be possible to send the corresponding PicoScope data file so that I can see the values collected?

With regards to the trigger, what sort of condition are you looking to trigger on? The API functions from the driver allow a greater range of trigger options relative to the PicoScope software.

Regards,
Hitesh

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Re: Converting mV value back to ADC count

Postby einar.nygard » Thu Jun 16, 2016 12:21 pm

Hello Hitesh,

Thanks again for fast reply. The acquired voltage values I sent you was generated from traces in 6000 psfiles converted to csv (using the DOS command Picoscope /c *.psdata /f csv /b all). In a Linux bash script I then selected the maximum value of each file. This gave me 6000 values that included multiple instances of nearly all possible voltage levels. Than I ran the bash commands "sort" before "uniq" that gave me the single instances of nearly all possible voltage levels, i.e. the range of data that I sent to you. None of the above Linux operations altered any of the values, but I did multiply them by 1e8 before I sent them to you.

I also have tried to use the measurement "Maximum" feature, and upon each trigger Alarm, run the script Picoscope.com /a Measurements.CSV? >> mydata.csv. This is more effective and would be my preferred method, but in that case the voltage homogeneity between ADC levels is even worse. A set of these data (~34000 acquisitions) I have attached 1: rawdata directly after the Picoscope script, appended values, 2: only the max values (with removal of other columns, removal of the "mV" string and deletion of all header lines), and 3: only single instances of the max values. The last one is similar to what I sent you before and if you plot these values against a linear X, you will again see the inhomogeneity between the steps.

Regarding API & trigger. This is not so much about type of trigger, it is about recognizing the trigger remotely so that buffers can be read only upon each new trigger. You may imagine that if triggered events are rare, as in my case now, I don't want to read the same buffer over and over again. I want to wait until I get a new trigger and then read. Maybe this is possible (?), but it is more preferred anyway for me to just get correctly binned data directly generated from Picoscope 6 (programming is time consuming).

Thank you again very much for your assistance.

Best regards, Einar
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Re: Converting mV value back to ADC count

Postby Hitesh » Mon Jun 20, 2016 10:10 am

Hi Einar,

Could you please post a data file here with one waveform so I can see your settings and the range of values?

A trigger will only occur when the condition has been met.

Regards,
Hitesh

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