Bernhard Holzmayer wrote:

   ...

> Nice result to start into weekend..

At least I was right about one thing. I couldn't help.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������

Thanks for scanning brains, fellows.

I probably found a coding error which provoced the strange peaks,
although I'm still guessing how it happened:

I didn't pay attention to SIMD mode, which had been activated while 
interrupts where serviced.
Indirect data movements seem to have damaged ADC data values.

Obviouslly DMA events happened during a timer initiated routine 
which worked in SIMD mode. 
Therefore the ugly thing happened periodically with timer.

Was difficult to trace down, bc. the effect kept hidden unless 
interrupts nested and a special program flow ran.

I'll have to clean the code now to confirm the effect has vanished,
but I'm rather sure that it'll be gone then.

Nice result to start into weekend..

Bernhard

   
 

Bernhard Holzmayer wrote:

> Hi all,
> 
> for too long already, I'm fighting against a spurious peak on a 
> signal which comes from an AD1836 SigmaDeltaConverter.
> 
> I hope that somebody can help me to trace this down or at
> least find out where I'd look for the origin.
> 
> Basically I have a noisy signal with very low levels . 
> This signal is converted and brought into a SHARC 21161 DSP.
> Method is the same as in the talkthru example ADI provides.
> 
> Sample rate is 48kS, hardware is EZ-LITE-Board.

   ...

Bernhard,

You know too much to let me think that I will help, but I hope a few of
my thoughts might accidentally uncover the cause. When you write "peak",
I assume it is what I would call a "spike". For me, peaks are in the
frequency domain and spikes occur in time. I may be alone in that usage.

The original signal needs to be amplified. Is it possible that a
transition on an external timer pin emits noise that interferes with the
low-level signal?

I assume that the timer is serviced by an interrupt. Is it possible that
the timers ISR forces a one-sample dropout or time displacement? I don't
see how that could mimic a spike, but ...

Have you liikes at the signal before and after the amplifier with an
oscilloscope? What you see could help to identify the noise mechanism.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������

Hi all,

for too long already, I'm fighting against a spurious peak on a 
signal which comes from an AD1836 SigmaDeltaConverter.

I hope that somebody can help me to trace this down or at
least find out where I'd look for the origin.

Basically I have a noisy signal with very low levels . 
This signal is converted and brought into a SHARC 21161 DSP.
Method is the same as in the talkthru example ADI provides.

Sample rate is 48kS, hardware is EZ-LITE-Board.

Depending on external interrupts and a timer based processing,
a resulting signal stream leaves the DSP, again via SPORT link.

For debuggin purposes I switched the processing off, except
amplification by a factor of 800 to make the issue visible on a 
scope.

In software, there's the DMA interrupt (SP0) which transports the 
signal streams in and out.
Then there's an external interrupt IRQ1, which is triggered once per 
millisecond.
Third, there's a periodic timer (TMZL), which is retriggered every 
30ms.  

The strange thing is, that the spurious peak appears exactly at the 
frequency of the timer, this is every 30ms.
If I change the timer frequency, the peak changes its frequency 
accordingly. If I switch the timer off, the peak fades.

I cut away all processing which directly influences the signal, 
so that the signal is only amplified and passed through the ISR.

Most of the othe processing, I did move it to a main loop, so that 
the three ISRs would cause least interference.  

Strange thing is that the problem is not only on the output,
which might mean it has to do with program failures like faulty 
memory or register sharing.  Instead, the fault seems to enter from 
the ADC (or at least from DMA).

If I invert the signal directly after it comes in, peak is inverted, 
too. 

However, despite what I try, the peak persists.

Does anybody have similar experiences?
There should be an explanation, though I can't find it.
 
Quite a little in despair, eagerly waiting for some hints...
Bernhard

Here a series of samples taken around the peak.
The usual values should be around -0.04....-0.07
peak (undesired) where level is below -0.1

@@Floating Point 32 bit
-0.058313467
-0.054451086
-0.048174717
-0.042091466
-0.053678609
-0.060437776
-0.054257967
-0.03929124
-0.060341217
-0.055127002
-0.037842847
-0.11055217
-0.12223587
-0.13053998
-0.14878973
-0.14251336
-0.14106497
-0.15052781
-0.14888629
-0.14183745
-0.14347896
-0.14145121
-0.1596044
-0.14627919
-0.15332803
-0.14965877
-0.14541015
-0.15352115
-0.15892848
-0.13498172
-0.13855442
-0.14270648
-0.15052781
-0.15815601
-0.15728697
-0.14850006
-0.14685854
-0.15641794
-0.14048561
-0.14541015
-0.13633355
-0.14521703
-0.14038906
-0.11422143
-0.1388441
-0.15390739
-0.14367208
-0.14463767
-0.16626701
-0.15583858
-0.15265211
-0.16182527
-0.14270648
-0.13671979
-0.13237461
-0.1316987
-0.12155996
-0.12745008
-0.12387738
-0.12059436
-0.17640576
-0.13614044
-0.077432252
-0.10533796
-0.076080419
-0.11306272
-0.089695312
-0.099930622
-0.11509047
-0.13382301
-0.083901741
-0.21541581
-0.23820385
-0.10794506
-0.10591731
-0.099640943
-0.12020812
-0.10263429
-0.11818037
-0.1245533
-0.120015
-0.11373863
-0.13469204
-0.16703948
-0.1479207
-0.13990626
-0.13469204
-0.13227805
-0.12107716
-0.13005719
-0.14203057
-0.13652667
-0.13633355
-0.12938127
-0.12947783
-0.13092622
-0.12793288
-0.13324365
-0.12300835
-0.1347886
-0.14154777
-0.12300835
-0.12648448
-0.12378082
-0.12629136
-0.13498172
-0.13053998
-0.13160214
-0.13797507
-0.13633355
-0.13845786
-0.15313491
-0.14994845
-0.14405832
-0.14038906
-0.13797507
-0.14212713
-0.12851223
-0.14598951
-0.14009938
-0.13720259
-0.14193401
-0.13710603
-0.13034686
-0.14164433
-0.15284523
-0.1438652
-0.13758883
-0.14492735
-0.13237461
-0.1484035
-0.13749227
-0.13681635
-0.14096841
-0.12600169
-0.13015375
-0.13324365
-0.14241681
-0.15352115
-0.13391957
-0.12542233
-0.13536796
-0.1357542
-0.13420925
-0.15236244
-0.136237
-0.14318928
-0.136237
-0.14019594
-0.12677416
-0.13227805
-0.1316987
-0.11982188
-0.128126
-0.1316987
-0.1276432
-0.13894066
-0.12339459
-0.14328584
-0.136237
-0.12918815
-0.1343058
-0.12358771
-0.12387738
-0.1302503
-0.12426362
-0.1352714
-0.12271867
-0.12754664
-0.13295397
-0.12194619
-0.12368426
-0.13556108
-0.13343677
-0.12378082
-0.13343677
-0.16858444
-0.12783632
-0.11972532
-0.1240705
-0.11006937
-0.12020812
-0.11064873
-0.12213931
-0.11016593
-0.12629136
-0.12349115
-0.14927253
-0.14077529
-0.13488516
-0.12725696
-0.12600169
-0.10920034
-0.13160214
-0.11084185
-0.12368426
-0.11750446
-0.12165651
-0.091143705
-0.11489735
-0.1047586
-0.11441455
-0.11933909
-0.12291179
-0.10031686
-0.12474642
-0.10977969
-0.1316987
-0.1245533
-0.12551889
-0.13140902
-0.12996063
-0.11422143
-0.1383613
-0.10485516
-0.11383519
-0.096068241
-0.12735352
-0.10968313
-0.12976751
-0.10640011
-0.12300835
-0.12484298
-0.13160214
-0.11566982
-0.1174079
-0.11509047
-0.11624918
-0.10591731
-0.11151776
-0.10891066
-0.12822255
-0.11769757
-0.1321815
-0.13923034
-0.13700947
-0.1343058
-0.13305053
-0.11470423
-0.12484298
-0.11557326
-0.12262211
-0.1123868
-0.10050998
-0.10862098
-0.11373863
-0.10668979
-0.11895285
-0.12493954
-0.10437236
-0.11489735
-0.10678635
-0.11103497
-0.089791872
-0.12127028
-0.098675348
-0.1011859
-0.10331021
-0.10977969
-0.095682003
-0.10871754
-0.093943931
-0.10369644
-0.095971681
-0.11006937
-0.10060654
-0.10891066
-0.085157014
-0.097130395
-0.087860681
-0.096551038
-0.097902872
-0.10292397
-0.096551038
-0.094330169
-0.082549907
-0.096840717
-0.08003936
-0.11074529
-0.086798526
-0.11113153
-0.090757467
-0.11180744
-0.08197055
-0.10485516
-0.075211383
-0.085350133
-0.074632026
-0.10688291
-0.085157014
-0.1092969
-0.083322383
-0.092302419
-0.075694181
-0.097033836
-0.079363443
-0.099351265
-0.066907264
-0.086701967
-0.071059324