DSO 150 : Accuracy part 2, what is happening, how to fix it

The root cause is the TL084.
It is part of several stages in the input chain.

It is powered by +-5V and if we look at the datasheet, the output voltage is capped depending on the power supply :

With +-5v, the TL084 can actually only output about +-3V according to that spec
The real value is much worse, it is actually VCC-4v to be safe (!) So only +-1 swing !

Raising the power supply is not an option, that would not be compatible with unmodified hardware

Now let's compute the max value at every point of the input chain

		MAX @ G3		Max @ G2		Max @ G1B			Max IN
Max ADC	G4		G3		G2		G1A	G1G1B		Friendly name		Actual MAX IN			SAT VALUE	SAT ADC		TL084 SAT
										V/DIV
1.50	8.00	0.1875	1.00	0.1875	2.00	0.09375	0.91	1.00	0.103125	20mv	20							2
1.50	8.00	0.1875	0.50	0.375	2.00	0.1875	0.91	1.00	0.20625	40mv	40							2
1.50	8.00	0.1875	0.25	0.75	2.00	0.375	0.91	1.00	0.4125	80mv	80			Max V/Div				2
1.50	8.00	0.1875	0.10	1.875	2.00	0.9375	0.91	1.00	1.03125	200mv	200							2
1.50	8.00	0.1875	0.05	3.75	2.00	1.875	0.91	1.00	2.0625	400mv	400	213.3333333		200 mv	1.1	1092.266667		2
1.50	8.00	0.1875	0.03	7.5	2.00	3.75	0.91	1.00	4.125	800mv	800	213.3333333		200mv	1.1	546.1333333		2
1.50	8.00	0.1875	1.00	0.1875	2.00	0.09375	0.91	0.01	10.3125	2v	2							2
1.50	8.00	0.1875	0.50	0.375	2.00	0.1875	0.91	0.01	20.625	4v	4							2
1.50	8.00	0.1875	0.25	0.75	2.00	0.375	0.91	0.01	41.25	8v	8							2
1.50	8.00	0.1875	0.10	1.875	2.00	0.9375	0.91	0.01	103.125	20v	20							2
1.50	8.00	0.1875	0.05	3.75	2.00	1.875	0.91	0.01	206.25	40v	40	21.33333333		20V	110	1092.266667		2
1.50	8.00	0.1875	0.03	7.5	2.00	3.75	0.91	0.01	412.5	80v	80	21.33333333		20V	110	546.1333333		2

The important value are in red. They mean that  for that range, in  one of the intermediate amplifying step the TL084 will saturate (the table is using 2v as saturation voltage for the TOL084, which is not too aggressive, but not guaranteed to work every time).

So let's not use the last line of each group, it is useless
Let's also only use partially the swing for the line before , i.e only half its swing so it does not saturate.


With that new mapping between volt/div and the hardware range we use, the end result is
(all in mVolt)

Vin	DSO	Error (%)
20,1	20	0,5
79,8	79	1
95,5	95	0,5
95,5	95	0,5
190	189	0,5
386,7	386	0,2
541,2	540	0,2
860	860	0
1658	1660	0,1
1658	1660	0,1
3600	3600	0
4377	4390	0,3
7458	7480	0,3
8163	8130	0,4
12080	12040	0,3
18330	18290	0,2

Much better !
The noise is still high though, about 2%