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RafaelLeeImg
@RafaelLeeImg
Is there any good way to use a different DAC in this project?
The DAC I'm using is MCP4822 which is a 2-channel DAC with SPI bus.
# pslab-python/pslab/instrument/power_supply.py
        self._mcp4728 = I2CSlave(self._ADDRESS, self._device)
        self._pv1 = VoltageSource(self._mcp4728, "PV1")
        self._pv2 = VoltageSource(self._mcp4728, "PV2")
        self._pv3 = VoltageSource(self._mcp4728, "PV3")
        self._pcs = CurrentSource(self._mcp4728)
6 replies
RafaelLeeImg
@RafaelLeeImg
The DAC with in the current configuration, the supply voltage is 3.3V, the full-scale voltage swing by the current supply can be 20mV~3.260V, but the DAC values cannot cover the full-range of the output voltage.
20220301-141802-pslab-DAC-swing.png
RafaelLeeImg
@RafaelLeeImg
The 5V rail is a little bit noisy.
20220301-165421-pslab-v6-5V-rail.png
That makes the oscilloscope noisy with high gain. The image is showed with the gain of 16.
20220301-162724-pslab-v6-gain-16.png
apoorva-raj
@apoorva-raj
Hi, are the Open Hardware and PSLab meetings happening nowadays?
RafaelLeeImg
@RafaelLeeImg
Why does the LM324 use ±10V voltage supply rail while the output voltage swing is only ±5V?
Can I connect it to ±7.5V?Since the LM324 output range is from Vee to Vcc-2V.
1 reply
20220304-162933-pslab-v6-+5V.png
+5V is significantly affected by the switching cap voltage inverter. Since the operating frequency of 7660 is too low. There is no good way to filter this ripple with small capacitors.
I want to use some other ways to generate negative voltage. I want try buck-boost.
RafaelLeeImg
@RafaelLeeImg
The most obvious noise source on each power rail is from the switching capacitor power inverter.
Since the semiconductor industry is evolving rapidly, the era that use low frequency power converter and big capacitors are already faded away.
RafaelLeeImg
@RafaelLeeImg
The calculation value of voltage ripple is quite near the measured result. If I increase the cap to 100uF, the noise will be several mV, that is achievable but the problem is not solved elegantly.
The current of +10V rail is 15mA, measured with DMM.
Voltage ripple = 1510^-3 mA(1/20000 Hz)/(1010^-6 F)=0.075V.
RafaelLeeImg
@RafaelLeeImg
I'm going to modify the ±6V to ±7V by adding a diode to 6.8V zener diode. The good side is to add a reverse PN node which has a negative temperature coefficient to compensate the
I'm considering to change the U10 OPAMP which used for wave generator to ±7V. Thus, I can remove ±10 rail for OPAMPs voltage supply. The output voltage of ±3V can be covered with this configuration.
RafaelLeeImg
@RafaelLeeImg
I didn't find other replacement of MCP4822. I tried to find some MCU with 2 12bit DAC, for example, STM32L431RCT6 the price is OK (around ¥25), but the stock is only like 100pcs. Or, sample-and-hold amplifier is another option, with that, we can reduce the DAC channel to 2 or 1. That leads to some work on firmware.
RafaelLeeImg
@RafaelLeeImg
The MCP4728 has 4 DAC channels, if we can reduce it to 2, we can still have 2 voltage source and a current source, one current source and one voltage cannot controlled independently. Is that OK?
Mario Behling
@mariobehling
Thanks for the suggestion. What is your opinion @CloudyPadmal @bessman @CyReVolt:matrix.org ?
6 replies
RafaelLeeImg
@RafaelLeeImg
I think the reason to have 4 DAC channel is only because MCP4728 has 4 channels and easy to buy. Now, even the price of MCP4822 has rised a lot. In my career, I didn't find any common condition that 4 channel of DAC is required.
There are exceptions, for example, use 2 audio DAC connected as bipolar (BTL) to drive speakers. For 2 channels, it needs 4 DAC. That's only used in very high-end audio amplifier systems.
Daniel aka CyReVolt
@CyReVolt:matrix.org
[m]

Sorry for not having joined the last meetings. I've been a little busy again, about to present something fancy again, today noon at CLT: https://chemnitzer.linux-tage.de/2022/en/programm/beitrag/226

During FOSSASIA Summit, I might be in Berlin, not 100% sure yet. Let me know if you'd like to meet. I'd arrive at the Wednesday, and stay until Sunday evening. Will let you know when it's for sure. :-)

1 reply
RafaelLeeImg
@RafaelLeeImg
I got the new PCB last weekend, this week, I measured all the waveforms and performance on each power rail. Most of the part works pretty well. Including the negative voltage generation part, to replace 7660.
-6.bmp.png
-6V rail, the noise is really low.
5V,10V_SW.bmp.png
The problem is here, the boost converter is working under discontinuous mode, that causes 5V tail near 100mV ripple. This leads to the noise of 3.3V rail and VDDA.
VDDA.bmp.png
RafaelLeeImg
@RafaelLeeImg
This is VDDA, there are around 10mV ripple, not quite good. Since there is no additional voltage reference, this causes noise on the voltage samples of oscilloscope, this will not cause noise on DAC, since the DAC uses internal voltage reference.
The 10V rail consumes 15mA of current, with that condition, I shall use an inductor with larger value. I'm using a 4.7uH now, and I'm waiting for the components to arrive.
RafaelLeeImg
@RafaelLeeImg
DSC_7304_retouch.jpg
I remove all the big capacitors, after I solve the discontinuous oscillation problem. I think the overall performance will be good.
Daniel aka CyReVolt 🐢
@CyReVolt:matrix.org
[m]

One issue I ran into with this I2C sensor:
https://github.com/tasmota/docs/pull/954/files

The docs led me to believe the boards had the same pin order.
I.e., I had GND and SDA swapped.
Anyhow, there's still nothing detected. I will see if it works with an ESP board here when I have the time.

Alexander Bessman
@vgfsrhkbgd:matrix.org
[m]

Could be because of:

Only a single AHT10 can be connected to the I^2^C bus and no other I^2^C devices can be connected.

The pslab's I2C bus also has the mcp4728 on it, as well as the rtc if you're using the v6 prototype.

Daniel aka CyReVolt 🐢
@CyReVolt:matrix.org
[m]
ah dang, right... I think I've actually tried having only the AHT10 on the v5, will recheck
I2C things can be so stupid sometimes xD
Daniel aka CyReVolt 🐢
@CyReVolt:matrix.org
[m]
Can't get it to work. But I do have some progress with the libusb WASM port and managed to build it somehow. Not sure how to integrate it just add.
https://twitter.com/OrangeCMS/status/1507728538782384135?t=mH_mtIOMvLb4zebfKWwRQw&s=19
For FOSSASIA Summit, I will merely have a draft, not even a PoC, I think. I know that it's doable, it's just complex unless you've done it already. ^^
RafaelLeeImg
@RafaelLeeImg
in-,out.bmp.png
The actual wave form output I measured for the LM833, the last stage for variable gain amplifier.
20220326-234303-simulation1.png
RafaelLeeImg
@RafaelLeeImg
This a simplified amplifier simulation graph, the main cause of oscillation is C4, which is introduced by MAX5400 pin input capacitance. This leads to uneven frequency response. Add with other distributed parameters, this will cause oscillation.
20220326-234334-simulation2.png
Modify C4 to 0pF, the frequency response is flat. But this cannot be done in physical world.
RafaelLeeImg
@RafaelLeeImg
20220326-234602-simulation3.png
Add R4 and C3 to simulate another stage of phase shifting, the oscillation starts.
RafaelLeeImg
@RafaelLeeImg
20220327-000348-simulation4.png
Add C5 to 5pF to compensate the C4. The voltage devider of R3//C5 is the same with C4//R1, in this situation, the frequency response is flat.
Daniel aka CyReVolt 🐢
@CyReVolt:matrix.org
[m]
And I've released a new version of Fiedka the firmware editor, https://fiedka.app/
it can now edit and save UEFI based firmware, and also analyze AMD PSP and coreboot filesystems 🐙
Michael Deffner
@Co_eus:matrix.org
[m]
Hey everyone, I'm having some troubles with a project & pslab python library, is it fine to ask some questions here or can you point me to the right place? You work is awesome, by the way 👍️
Alexander Bessman
@vgfsrhkbgd:matrix.org
[m]
This is the right place to ask.
Daniel aka CyReVolt 🐢
@CyReVolt:matrix.org
[m]
I've updated the issue on WebUSB, libusb, Sigrok, monorepo and what I'm experimenting with:
fossasia/pslab-desktop#697