Replacement R26 - R27 by regulator

[Diabolo]

Hello,
To improve the supply voltage in the analog board of the DSO150 I replaced R26 by a regulator 78L05 TO92 and C11 by 100nf, and R27 by a regulator 79L05 TO92 and C14 by 100nf.
The GND pins of the regulators will be welded on C14 screen after having scratched the protective varnish.

Regards.

[Petroid]

So how's the result? What exactly do you improve by these changes?
On the side note, the new version (E) of analog board has two LDOs in place of R26 & R27

[Diabolo]

Hello,

This provides a precise and stable 5 volt positive and negative voltage independent of variations in the source voltage.
With a power supply from a 3.7V li-ion source with booster to get 9V, the voltages are rid of any pollution.

The analogue channel is sensitive to power supply noise. Peaks are more likely to be seen when a dirty switch mode is used.
Examine the original schema to understand.

Sorry for my English, I use a translator.
Regards.

Diabolo

[surenz]

I've made the same modification but unfortunately it didn't make any difference.
Might be useful when I add Li-ion with voltage booster to make the device autonomous.
Be careful when adding the 79L05 as it has different pinout from 78L05 (very weird).
Added some SMD capacitors instead of C11 and C14 on the solder side.

[Diabolo]

Hello,

With this change you get a steady and steady voltage even if the 9V power source varies, while R26 and R27 give any voltage depending on the load and source voltage. ICL7660 produces noise on the negative line. The addition of the two regulators can only bring good and remove noise on the measurement of weak signals.

Sorry for my English, I use a translator
Regards

[zmurcht]

I read about a similar mod in the German Elektor magazine. They didnt change the capacitors but instead set the Z-diode to 5V.
Any reason why the GND of the voltage regulators is not set to GND? The AGND would be just there on the board

[MorganFlint]

I read about a similar mod in the German Elektor magazine. They didnt change the capacitors but instead set the Z-diode to 5V.
Any reason why the GND of the voltage regulators is not set to GND? The AGND would be just there on the board

Do you mean this review? (English version)

In this case, what they say is: "The supply to the Zener diode was transferred to the stabilized 5 V" (different from setting D1 to 5V). I understand this can be done disconnecting R28 (old version schematic) from V+ and connecting it to AV+, once R26 and R27 have been changed for 78L05 and 79L05, respectively. This may be tricky because R28 is SMD, but it makes sense as it avoids possible noise in V+ from getting into ADCIN through U4D-U4B. In the new version this problem is completely avoided as the negative bias to the last stage before ADCIN (U1B in this case) is taken from AV-, regulated by 79L05 in this version. UPDATE: Instead of re-routing R28 (SMD) to AV+, it might be easier to change R13 to 130 ohm and R15 to 3K, connecting the side of R15 that goes to pin 14 of U4 to regulated AV- instead. This way this part of the circuit (U4B) will be the same as the new version (built around U1B).

I also agree with Elector's reviewer that it may not be needed to replace C11 and C14 by 100 nF, but I'll check if it makes a difference to add 100 nF (SMD solder side as surenz suggested) in parallel with C11 and C12 once I make the mod (I have still to get the regulators).

[zmurcht]

Yes thats the one!
I dont mind that R28 is a SMD. But I'll take a look at what you suggested as well.

[MorganFlint]

... I understand this can be done disconnecting R28 (old version schematic) from V+ and connecting it to AV+, once R26 and R27 have been changed for 78L05 and 79L05, respectively. This may be tricky because R28 is SMD, but it makes sense as it avoids possible noise in V+ from getting into ADCIN through U4D-U4B...

Yesterday I tried this mod and I have to say it didn't work wwll for me at the beginning, because the zener diode didn't reach 3 V. I think it was because it didn't get enough current, as the resistor feeding it from unstabilized ~8V is 1K, (approx 5 mA), so when you transfer it to 5V the zener would draw only 2mA. I changed the resistor to 330 ohm and after that the zener again reached 3V properly.

So instead of re-routing R28 to regulated AV+, it's better to disconnect it from AV+ and connect a new 330-390 resistor from AV+ to the anode of D1. I still think it would be better and easier to change R13 to 130 ohm and R15 to 3K, connecting the side of R15 that goes to pin 14 of U4 to regulated AV- instead, leaving this part of the circuit like in the new version, but I couldn't try it as I didn't have those resistors on hand.