Recent Posts

1

Beginners / Re: LC filtering for combined Vref/VDD of ADC

« Last post by temperance on Today at 10:54:59 pm »

An LM4040 shunt reference maintains a dynamic resistance below 0.25 mOhm with a bias current of 1 mA up to about 350 Hz. With a 270 R bias resistor this equates into a 60 dB PSSR. Below 200 Hz the dynamic impedance is even lower and close to 100 mOhm or about 68 dB PSSR.

Does it outperform an LDO? When bypassed with a 10 µF ceramic capacitor is does for freq. above 1...10 K. But some LDO's are better than others. Some modern ones are very good while some are just bad. The REF2030 I mentioned earlier is a series reference. (My mistake) When it comes to high frequency noise, the LM4040 when bypassed with 10 µF outperforms the REF2030 for freq. above 1 K.

Besides, at low currents, the PSSR plot might still be valid because the LDO is not operating close to or in saturation. But the way they specify drop out can be tricky. I usually calculate the MOSFET on resistance. That seems a more valid approach to me. But that only works for LDO's with output voltages > 2.5 V.

A shunt ref with a very low dynamic resistance is the AZ431L. 50 mOhm from DC up to almost 50 KHz. This would translate onto 75 dB PSSR from near DC up to 50 K without a bypass capacitor. Not bad.

But your question is valid because this turns out to be an edge case.

2

Projects, Designs, and Technical Stuff / Re: Passing 2 wires into a hot water cylinder

« Last post by tom66 on Today at 10:53:03 pm »

Why not just put a temperature sensor on the hot water output of the cylinder. It will be very close to the actual temperature of the cylinder water, since there will be some conduction even without active hot water flow.  If you insulate that area well, it will be even more accurate. 

3

Projects, Designs, and Technical Stuff / Re: (Yet Another) DIY Multislope ADC

« Last post by PCB.Wiz on Today at 10:48:29 pm »

NNNI may test that easily (including the new rundown) while running off the stock 12MHz crystal against the PLL version. A stable canned XO/TCXO would be even better.

An external Osc is certainly easy to try.
TCXO's need care, as some use digital corrections that give small jumps in frequency as the temperature varies.
Some vendors now call their better TCXOs analog compensated - eg
https://ecsxtal.com/news-resources/what-are-the-benefits-of-using-analog-compensated-tcxos/

I'm thinking the ideal is an oscillator that is (very) stable over the integration time.
That means long term drift is less important, & it is best to focus to avoid effects like micro-jumps or other disturbances.
The close-in phase noise is looking like the best indicator of that.

A bit of thermal mass around the oscillator can help reduce temperature slope effects, and a local low noise regulator can isolate supply variations.

The 48MHz is needed, however, as you want to upload the binary to the 2040 easily, and you will do it many times, sure.

You just need to be able to generate 48MHz for the USB, so more clock choices exist.
I've not found a very low phase noise oscillator at 48MHz yet, tho NDK say they can cover 20-50MHz in their NZ2520SDA family. Just needs some MOQ 

4

RF, Microwave, Ham Radio / Re: fake MPF102s from AliExpress

« Last post by Gerhard_dk4xp on Today at 10:43:48 pm »

Just curious. Is there anything special about MPF102? Could J304 be a suitable replacement?

MPF102 is traditionally the bin for the FETs that fail every test
but being a fundamentally working transistor.

Gerhard

5

Beginners / Re: LC filtering for combined Vref/VDD of ADC

« Last post by HwAoRrDk on Today at 10:43:45 pm »

How much bandwidth do you need?

I'm only measuring voltage rails on a device-under-test to determine whether their levels are in-spec, not any kind of varying signal, so hardly any bandwidth at all.

If not much, you might average it all out on the digital side and kinda not care.  It's not a great solution, as the noise will alias and there is an asymmetry to it (it's ratiometric, in particular in the denominator, not additive), which will cause some wandering baseline that doesn't average out, but it might be small enough not to mind (i.e. consider the effect of 3.35 vs. 3.25V ref).

I might do some averaging on the digital side, not sure yet. Although, I want to ensure the voltage rails under test aren't fluctuating so averaging over too much of a period will probably mask what I'm trying to measure. What I'm probably going to be looking at is not only the mean value of samples but also their std. deviation too.

What spectrum is the noise?  If it's switching noise, an LC filter is fine.  If it's all kinds of crunchy, from various causes, erratic time constants, likely you need an LDO.

I think it's switching noise from a DC-DC converter mainly. No idea what frequency the switcher is running at. I should probably try and analyse it in more detail than I've done so far: that is, poke it briefly with the 'scope and go "ew, that's dirtier than I was expecting".

Note the LDO can be very thin: say 3.0V, so logic levels aren't violated.  You don't want e.g. 3.6V+ beginning to forward-bias the ESD clamp diodes into a device at 3.0V supply, or 3.3 into 2.7, but 3.3 into 3.0-3.1 would be perfectly fine.  Downside: LDO PSRR is typically trash to begin with, and it's even worse operating so close to dropout.  Again, it's only a thing if you need low frequency filtering.

Bonus: LDO at least gives [the chance for a] more accurate supply/ref, potentially reducing calibration error.

I suppose an additional benefit of running the ADC and op-amps from a 3V supply is that I'll also get slightly higher resolution, as the input will be larger relative to the reference. But that's minor, and I don't particularly need super accuracy - even this 12-bit ADC is a bit overkill for my application.

I wouldn't know how to pick an LDO regulator with good PSRR, though. It seems difficult to make comparisons because the manufacturers all give specs according to different conditions. Some give it at 1kHz, some at 100kHz, etc. I'm not even sure what magnitude of PSRR is good. Is 50 dB good or poor? 70 dB? 90 dB?

6

Projects, Designs, and Technical Stuff / Re: Passing 2 wires into a hot water cylinder

« Last post by 2strokeforever on Today at 10:42:32 pm »

Find something like this
https://www.ebay.ca/itm/285509878582

its just a tube with the sensor in it, and a compression fitting that seals it up. they are very proven and not much to go wrong.

7

General Technical Chat / Re: Cable Management

« Last post by thermistor-guy on Today at 10:40:51 pm »

Each individual cable loosely coiled inside a ziplock bag, with a descriptive label on a card insert. All the bags stacked filing cabinet style (on edge) so the labels are visible, in a drawer. Apart from mains leads, which breed when you don’t look at them.

I do something similar:

 - coil the cables with generous radius in large ziplock bags;
 - place the bags in stackable transparent storage boxes; label the boxes (3 labels: side, front, top of box);
 - have separate labelled boxes of cables meant for repair or scavenging;
 - have separate labelled boxes for connectors and unterminated cables;
 - have a catch-all box labelled "misc" for odd items.

The idea is to have stacked array of boxes, where it's easy to tell what's inside each one from a glance.

8

Beginners / Re: How Current Limitation is happening in the circuit???

« Last post by Xena E on Today at 10:40:24 pm »

Hello All,

I started constructing prototype of the below circuit, But 68ohm,2Watts resistance went to breakdown.
again i changed to 470ohm, 2Watts , again it started smoking.


I also changed the MJE2955, but 470 ohm started smoking, What is the problem in the circuit?

How to find my transistor has went to breakdown?

Thank you all.

Incredible!

Taking 470 ohms as the value of the resistor, you have to have over 30 Volts across it to even max out its design dissipation of 2 Watts.

Now, assuming that the base emitter junctions of the pass transistors are intact that means that the output would have to be shorted to ground and in that case almost all of that hypothetical voltage has to appear across the 0.1 Ohm emitter degeneration resistors, that will cause them to try to dissipate over 8.5kW each.

So as this isn't possible, (the transformer isn't that big is it?), it's either ignorance or Bullshit.

The circuit diagram as drawn won't work, in fact if the practical circuit is wired as drawn it would be the transformer that has smoke coming out of it, (you show the output of the rectifier on the 5.1Volt supply shorted to ground for starters).

If you really want help you need to furnish facts and details.

Regards,
Xena.

9

Projects, Designs, and Technical Stuff / Re: Control of 2 CAN Open DS402 motors on a single shaft

« Last post by moffy on Today at 10:40:09 pm »

The crux will be getting the torque motor to react quickly enough to fool the speed controlled motor into thinking that it has twice the torque or rather that the load is half of what it is as it will not be aware of the second motor. So the speed controlled motor will need to have a slower torque ramp rate than the torque control motor so that whatever torque it uses is quickly matched by the Torque controlled motor so that it does not see the full load. And hopefully this all happens without the two falling ass about face or oscillating.

I was thinking the same thing, two torques rather than one, but I assume since it is a semi intelligent controller there must be something like PID parameters that it uses and if so then you can adjust these to reflect the new reality and hopefully prevent overshoot oscillations. Can you get a regular report back from the controllers on motor current? That would provide a good insight into what is happening torque wise.

10

Metrology / Re: ADR1399 reference

« Last post by EC8010 on Today at 10:39:54 pm »

Thanks for your thoughts. That's what I'm thinking; it's too close to the limits of my present measurement ability. I keep wondering about an 8 digit DMM. Or better, an array of at least four temperature controlled voltage references that can be compared using existing kit.

The 1399 is in the cardboard box with foam the evaluation board came in, but not properly sealed; I have done much better thermal control on other experiments. This is early days. I'm also wondering about that plastic hat the device is in. For instance, is the plastic triboelectric where it wobbles and scrapes on the leads? Is the plastic hat there simply because it's cheap and easy to add by the manufacturer, but not genuinely optimised? Would foam be better? The wobblyness and rattling concerns me, too. At this level, microphony is often an issue.

The LM334 is pretty much as per data sheet. I have tried tweaking the ratio of the resistors but it didn't help, suggesting that its remaining tempco was actually down to the (metal film) resistors themselves. I could perhaps do better using low-tempco wirewound resistors and trimming, but I think that's a bit of a dead end. I only put the compensated LM334 in because I had one ready-made and set for 3mA. I think the bootstrapped op-amp solution for the Zener should be better.