Author Topic: LM399 based 10 V reference  (Read 319819 times)

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Online imo

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Re: LM399 based 10 V reference
« Reply #875 on: February 10, 2019, 02:12:41 pm »
You may utilize the second 2057 as a 2nd order low pass (ie 10Hz below). And relax the first filter if necessary..
« Last Edit: February 10, 2019, 02:15:02 pm by imo »
 

Offline Andreas

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Re: LM399 based 10 V reference
« Reply #876 on: February 11, 2019, 08:26:12 pm »
Hello,

@IMO: nice schematics. From where did you get the model of the ADG419?

I checked some variations of the cirquit by simulation:

- R1 100K splitted into 3 different 50K resistors
  I hoped that this would reduce the ripple after the 100 R resistor due to less influence from the charge injection.
  But obviously the effect is only a minor. not worth on getting different T.C.s for the 3 resistors.

- Reducing the 100R resistor. Same hope as above since I thougt that the 100R resistor together with the 100K build a voltage divider with the influence lower for lower 100R resistor.
  But obviously the ripple is only a thing between discharging C4 over the 100K Resistor R1.

- Finally exchanged C4 with 10uF: this gave the most reduction on ripple for the output voltage.

interestingly C3 does not play a large role for the filter. I have reduced C3 to 10nF without significant change on output ripple at VOUT2 in simulation. Now trying influence of C2.

@Branadic: can we change C4 up to 10uF in the layout?

with best regards

Andreas
 

Offline branadic

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Re: LM399 based 10 V reference
« Reply #877 on: February 11, 2019, 08:44:13 pm »
Quote
@Branadic: can we change C4 up to 10uF in the layout?

Currently C4 is a 1µF/63V MKS2 type (7.2mm x 5.0mm x 10mm). Changing it to 10µF means replace by 10µF/50V MKS2 type and requires quite some space (7.2mm x 11mm x 16mm). Have to check if I can make this space available.

-branadic-
Fluke 8050A | Prema 5000 | Prema 5017 SC | Advantest R6581D | GenRad 1434-G | Datron 4000A | Tek 2465A | VNWA2.x with TCXO upgrade and access to: Keysight 3458A, Keithley 2002, Prema 5017 SC, 34401A, 34410A, Keithley 2182A, HDO6054, Keysight 53230A and other goodies at work
 

Online Kleinstein

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Re: LM399 based 10 V reference
« Reply #878 on: February 11, 2019, 08:59:09 pm »
Leakage of C4 would only be effective through the 100 Ohms resistor and if the output signal is taken from C4, the leakage would not even matter. So it might be possible to use an electrolytic cap for C4, preferable low ESR and maybe with some 100 nF in parallel.

Splitting the 100 K to 50 K before and after the switch does not look like a good idea, I think we had that idea before.

For me C3 still has an effect. It may not be visible in the simple simulation with only 10 kHz. It can be important for the lower frequency components due to the modulation of the PWM signal. Even with 10 kHz main PWM there is a little amplitude down to the 100 Hz range.
For me it's more like C2+C3 are important, but C2 and C3 could be same value.
 

Offline serg-el

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Re: LM399 based 10 V reference
« Reply #879 on: February 12, 2019, 05:38:15 pm »
I am still missing your source code implementation for the 79 kHz.

My code is not optimized.
Enough only for PWM.

But in the source code there are errors.
1 - PWM start before memory initialization.
2 - load from memory before initialization of buttons.
3 - the length of the variables is more than necessary.

Just compare mine and your code.
Successes!   ;)

 
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Offline pigrew

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Re: LM399 based 10 V reference
« Reply #880 on: February 12, 2019, 10:18:09 pm »
I've started working on a PCB design for this project, but with few tweaks. First, I plan to use the STM32G071 CPU, which is fast and pretty low power. I've gotten some reasonable results (though too much HF switching noise due to breadboard construction) with a STM32F103  (PLL enabled), so I'd like to see how it performs. Also, I propose to build a 3-level bit DAC using two synchronized PWM. Since this is geared towards 10V, we can do better if we PWM between 5V and 10V instead of between 0V and 10V. For the 5V output, I will flip-flop between switches (use one for the "low" branch and use the other one for the "low" branch the next cycle). In my current implementation, I've also implemented random frequency variation which greatly spreads out the frequency response, but it may be bad due to offsets caused by unequal rise and fall times of the switch (though this does eventually average out).

I've drawn up an "analog" schematic and I've appreciate feedback.

I've marked all the capacitors as poly-film or tantalum, I figure they are less risky for piezo-electric badness, but I likely will just use X5R (and components I already have) on the first build.

I've tried to incorporate most of the features I've seen here. I was thinking about using Kelvin force/sense, or designing a BJT output stage, but I will add those after making a first board work decently.


EDIT: Adding the following specific questions, also replaced the schematic with an updated one.

Things in particular to decide before layout:

  • ADG419, ADG1419, or ADG5419. I'm leaning towards ADG1419 due to lower on resistance, but it has larger parasitic capacitances though much lower charge injection
  • Switch to LTC2058 (at least for prototyping) since it is cheaper (per op-amp), nearly as good?
  • Use ceramic X7R 0.1uF caps for decoupling?
  • Add ferrite at switch output (next to R1)??? Perhaps need to build the circuit and try with/without?
  • SMD resistors have fewer parasitics, but perhaps we want the series inductance of THT resistors?
  • Looking back into the thread, I screwed up the positioning of the 10V node and 100 ohm R4. Fixing that, will upload 20190212b soon.
  • Switch to Sallen-Key LPF on the output?
  • I want to use the STM32G0 because it's low power, but it seems that the 64-pin package is the best choice. Ugh.
« Last Edit: February 13, 2019, 02:22:40 am by pigrew »
 
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Offline Andreas

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Re: LM399 based 10 V reference
« Reply #881 on: February 13, 2019, 11:51:22 am »
I've started working on a PCB design for this project, but with few tweaks. First, I plan to use the STM32G071 CPU, which is fast and pretty low power.

 Also, I propose to build a 3-level bit DAC using two synchronized PWM. Since this is geared towards 10V, we can do better if we PWM between 5V and 10V instead of between 0V and 10V. For the 5V output, I will flip-flop between switches (use one for the "low" branch and use the other one for the "low" branch the next cycle). In my current implementation, I've also implemented random frequency variation which greatly spreads out the frequency response, but it may be bad due to offsets caused by unequal rise and fall times of the switch (though this does eventually average out).

I've drawn up an "analog" schematic and I've appreciate feedback.

Hello,

Fine to see a different approach with different ideas. How much current is drawn by your CPU?
I have to admit that I do not understand how your PWM works. (also not familiar with the STM CPU).
Do you have something like a timing diagram?
When combining 2 different PWMs I would have expected a e.g. 100:1 ratio (coarse/fine) between the mixing resistors.
How many bits are used from the PWM?

I´m trying different PWM frequencies at the moment.
The frequency has (as expected) a influence on the output voltage in the mV range.
Interestingly the frequency also has a influence on the T.C. of the whole cirquit.
The sweet spot (at least for my sample of ADG419 seems to be between 9.6 and 19.2 kHz (more near the upper frequency)).
Of course this may also depend on if you use 3.3V or 5V input levels.

Things in particular to decide before layout:

  • ADG419, ADG1419, or ADG5419. I'm leaning towards ADG1419 due to lower on resistance, but it has larger parasitic capacitances though much lower charge injection
  • Switch to LTC2058 (at least for prototyping) since it is cheaper (per op-amp), nearly as good?
  • Use ceramic X7R 0.1uF caps for decoupling?
  • Add ferrite at switch output (next to R1)??? Perhaps need to build the circuit and try with/without?
  • SMD resistors have fewer parasitics, but perhaps we want the series inductance of THT resistors?
  • Looking back into the thread, I screwed up the positioning of the 10V node and 100 ohm R4. Fixing that, will upload 20190212b soon.
  • Switch to Sallen-Key LPF on the output?
  • I want to use the STM32G0 because it's low power, but it seems that the 64-pin package is the best choice. Ugh.

For the switch: we started with an existing design (ION) so no idea which is really the best. (we need more experience so perhaps it would be good to use a different one).

LTC2058 is the dual version of LTC2057. So should give no differences. But usually single OPs make the PCB layout easier.

I see no problem with ceramic CAPs on low impedant power supply lines. In the analog path I would use Film capacitors in the final layout.

for the ferrite: as R1 is rather high impedant 150K in parallel with 0.x pF a ferrite at this point will not have too much effect.
I also had the idea of using a ferrite/inductor but more from SA input of the switch to the C16 (VRef10V) side.
But LTSPICE simulation had up to now not the effect that I have expected. I guess I still have a problem with my Switch model.

What we did not up to now is a series resistor between digital PWM output and switch for dampening digital noise.
but of course this may give additional influences on rise/fall time symmetry.

I would not rely on the inductive behaviour of resistors. As this may vary from batch to batch.
I use through hole only if I need long term stability.

To the schematics:
C11 C12 could be larger in value to reduce ripple on the output. Also C16 plays a large role for the remaining ripple.
I would place C10 between PIN6 and PIN2 of the OP-Amp to allow capacitive loads on the output.
If you actually place a series resistor between the voltage regulator and heater of the LM399 you might need a additional electrolytic capacitor for the LM399 heater (see LM399 data sheet).

with best regards

Andreas






 

Online Kleinstein

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Re: LM399 based 10 V reference
« Reply #882 on: February 13, 2019, 04:24:55 pm »
The ARM based circuit is not that much different. The PWM is still in the same way, just 2 channels in parallel. As shown the two channels have the same weight and one would get about the average PWM value that count. With 2 equal PWM settings this reduces the ripple to start with a little, as in most case the PWM would be more like out of phase.

Going from 7 V to 10 V has a PWM setting of about 70% and thus a considerable reduction in ripple to start with. However the loading problem to the first OP is still there. A lower resistance switch would help a little with "linearity", but could cause more load on the input side. It could help to have switches that a specified for only 16 V or so, as these might have low R_on with still low spikes.

The STM32 µC should be able to use 16 bit or similar PWM, which can be a slight advantage with a higher base clock. So less low frequency component. to filter.

Still I am not so sure using 2 switches is worth the extra effort (e.g. 2 x DG419 or similar). Especially with a slightly higher PWM base frequency the low frequency ripple should not be that critical. It is more about parasitic effects and  higher frequency spikes that go around the filter.
 

Online imo

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Re: LM399 based 10 V reference
« Reply #883 on: February 13, 2019, 07:18:20 pm »
@IMO: nice schematics. From where did you get the model of the ADG419?
Bordodynov library. It includes various switches.
https://forum.allaboutcircuits.com/threads/the-libraries-and-components-models-of-ltspice-free-download.133690/
 
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Online imo

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Re: LM399 based 10 V reference
« Reply #884 on: February 13, 2019, 07:34:09 pm »
The STM32 µC should be able to use 16 bit or similar PWM, which can be a slight advantage with a higher base clock. So less low frequency component. to filter.
Above are the tables with stm32 @72MHz and 16bit hw pwm ranges. When not using PLL and targeting "low power" a realistic clock could be something like 24MHz. With 24MHz clock you may get 10kHz PWM with ~11bit resolution.
PS: a 10kHz PWM with full 16bit resolution would require 655MHz clock :)
« Last Edit: February 13, 2019, 07:45:52 pm by imo »
 
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Online Kleinstein

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Re: LM399 based 10 V reference
« Reply #885 on: February 13, 2019, 08:07:01 pm »
The point was not so much really using 16 Bit PWM, but having HW that supports at least 12 Bits, so that something like 11 bits can be used. So it's about 3 more bits's of PWM resolution and still 10 KHz. So something like an amplitude for the modulated lower frequency part that is only 1/8 and does not extend all that far down as the original solution.

So the higher PWM clock is already more efficient than using 2 channels, which kind of halves the amplitude.
 

Online imo

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Re: LM399 based 10 V reference
« Reply #886 on: February 13, 2019, 08:07:12 pm »
STM32x HW PWM. The "max duty" is the max value (100%) you may load into the pwm register for a specific pwm frequency.

For example at 10kHz pwm frequency the max duty is "2400" that is something like 11.x bits.

You may have many 16bit pwm channels active, btw (up to four independent 16bit pwm channels for each single 16bit Timer_x used).

Theoretically you may design a 44+bit DAC with four pwm channels at 10kHz pwm frequency.. :)
« Last Edit: February 13, 2019, 08:30:09 pm by imo »
 

Offline Magnificent Bastard

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Re: LM399 based 10 V reference
« Reply #887 on: February 13, 2019, 08:26:30 pm »
Microchip has a new zero-drift op-amp: MCP6V51

It has very low input current noise, which allows for a very high input impedance without spoiling the rather good input voltage noise spec.  Of course, all of the horrors of using a chopper need to be addressed-- such as clock feed-through, etc.-- but with proper circuit design, this is a real contender to the offerings out there right now.

-MB
 

Online imo

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Re: LM399 based 10 V reference
« Reply #888 on: February 13, 2019, 09:10:54 pm »
Switch to Sallen-Key LPF on the output?
There is around 800uVpp (@10KHz pwm) ripple at the output of the ADG switch (in my simulation above). You have to decide what is the targeted ripple amplitude at the ION's output. That gives you an idea how to filter it out..
PS: Getting a little bit different results when the V- of the DG switch is connected to -15V instead to the GND.
« Last Edit: February 13, 2019, 09:20:22 pm by imo »
 

Offline pigrew

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Re: LM399 based 10 V reference
« Reply #889 on: February 13, 2019, 10:00:53 pm »
I´m trying different PWM frequencies at the moment.
The frequency has (as expected) a influence on the output voltage in the mV range.
Interestingly the frequency also has a influence on the T.C. of the whole cirquit.
The sweet spot (at least for my sample of ADG419 seems to be between 9.6 and 19.2 kHz (more near the upper frequency)).
Of course this may also depend on if you use 3.3V or 5V input levels.

In my simulations (and in real life with the ADG1404), my major issue has been charge injection into the "common" terminal of the switch, causing the common node's voltage to swing widely for a few hundred ns. I figure that this is due to charge injection coupled with the "slow" switching time. The Vishay DG419LE may give somewhat better performance as it is much much faster (it even has a few body snatchers in it, see datasheet.). The Renesas ISL43210A also looks good. The V_IN,H of both of these are a bit high (3.0V), but I think it's workable. I think the turn_on/turn_off times are why the voltage is so frequency (and temperature) dependent.

This transient issue may also drive me to use a lower switching frequency (with a larger number of channels.) Another option would be to reduce the value of the resistors at the switch's common.

- Reducing the 100R resistor. Same hope as above since I thougt that the 100R resistor together with the 100K build a voltage divider with the influence lower for lower 100R resistor.
  But obviously the ripple is only a thing between discharging C4 over the 100K Resistor R1.

If the 100R is reduced, perhaps we would run into stability issues due to the large capacitors to ground?


Fine to see a different approach with different ideas. How much current is drawn by your CPU?
I have to admit that I do not understand how your PWM works. (also not familiar with the STM CPU).
Do you have something like a timing diagram?
When combining 2 different PWMs I would have expected a e.g. 100:1 ratio (coarse/fine) between the mixing resistors.
How many bits are used from the PWM?


The STM32F103 I used is about 15 mA. The prototype only has a single channel (unlike what I'm proposing). My final design will use the STM32G071 which is much lower power, probably ~3 mA. One can actually run the timer clocks faster than the core CPU which saves power, too. The PWM has shadow registers which apply at the start of each cycle, so one has the entire clock cycle of time to calculate and update the HW registers and there is no edge case where you have to apply the PWM settings after the pulse. Each timer has a few PWM generators, so it could easily support 4 or 8 channels.

The disadvantage of mixing resistors is that the resistor ratio must remain stable. In my proposal, we don't get additional resolution, but the resistor ratio can drift significantly.

I am attaching a LTSPICE simulation of the dual-PWM scheme. Notice that large spikes on most of the voltage nodes. Not good. As I mentioned, I think they are a combination of charge injection and slow switching times.

ADDENDUM: The MCP6V51 looks reasonable, but it has a larger max offset voltage (15uv vs the 4 uV of the LTC2057), and double the offset drift. Other than that, the parts look very similar, even sharing the same chopper frequency (100 kHz).
« Last Edit: February 13, 2019, 11:54:16 pm by pigrew »
 

Offline serg-el

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Re: LM399 based 10 V reference
« Reply #890 on: February 13, 2019, 10:58:50 pm »
Add 10 more phases and get russian V1-18  :-DD

https://www.qrz.ru/schemes/download/4445
 

Offline bsw_m

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Re: LM399 based 10 V reference
« Reply #891 on: February 14, 2019, 12:39:51 am »
With the only difference that in V1-18, 12-phase voltage pulse divider (as originally named PWM DAC in documentation) used for divide ref output (about 12.92V) to the output range 0-12v with steps in 1uV.  And this is not used in feedback, but what's the difference :)
 

Offline Andreas

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Re: LM399 based 10 V reference
« Reply #892 on: February 14, 2019, 09:23:55 am »
There is around 800uVpp (@10KHz pwm) ripple at the output of the ADG switch (in my simulation above).

PS: Getting a little bit different results when the V- of the DG switch is connected to -15V instead to the GND.

Hello,

you mean the node A2 in your simulation above?
https://www.eevblog.com/forum/metrology/lm399-based-10-v-reference/?action=dlattach;attach=648510

This ripple mainly depends on the 1uF capacitor together with the 100K at the common of the switch (R4/C4) versus PWM frequency.
Thats why I asked branadic to increase the cap to 10uF in our test-board.

of course you will have to filter the remaining ripple anyway to reach something which is below the noise of a LM399 or a LTZ1000.

If the 100R is reduced, perhaps we would run into stability issues due to the large capacitors to ground?

Depends on the Op-Amp which is used. For the LTC2057 I have tested that 22R are stable up to 10uF foil capacitor if there is a large enough (10-100nF) feedback capacitor (here the 1uF directly at the LTC2057).

I am attaching a LTSPICE simulation of the dual-PWM scheme. Notice that large spikes on most of the voltage nodes. Not good. As I mentioned, I think they are a combination of charge injection and slow switching times.

Thanks for that.
Now I understand what you want to do.
So you are reducing the PWM amplitude at the switch from 10 to 5V by interleaving the PWM signals.
But I think you will have to take care that the ratio of R1a+R1b is stable over temperature and time.
(besides the R,on in high/low state of the switches).

For the spikes: I am still not shure if they are real or a simulation artefact.
I am not shure wether the ADG413 model supports break before make feature.

A STM32G071 with 3 mA would also be in my target spec.
(although a SO-8 package is more easy to solder).

with best regards

Andreas
 

Offline Andreas

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Re: LM399 based 10 V reference
« Reply #893 on: February 14, 2019, 10:24:11 am »
Hello,

attached diagram from measurements of VOut (@25 deg C) and
resulting average (linear) T.C. of my ADG419 switch over PWM frequency. (without R15 tap).

so around + 40.5 uV/kHz near linear change in Vout
and  +12.7 ppb/khz change in T.C.

so interpolating: Zero T.C. somewhere near 18.5 kHz

with best regards

Andreas
« Last Edit: February 14, 2019, 10:27:30 am by Andreas »
 

Offline pigrew

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Re: LM399 based 10 V reference
« Reply #894 on: February 14, 2019, 03:43:34 pm »
attached diagram from measurements of VOut (@25 deg C) and
resulting average (linear) T.C. of my ADG419 switch over PWM frequency. (without R15 tap).

so around + 40.5 uV/kHz near linear change in Vout
and  +12.7 ppb/khz change in T.C.

so interpolating: Zero T.C. somewhere near 18.5 kHz
Andreas

Interesting, I hadn't considered using the frequency as the TC-adjustment knob.

I just got some parts in the mail today. I'll compare my existing switch (ADG1404) with the DG419LE to see if I see less variation of DCV with switching frequency, on the breadboard.

Thanks!
 

Online imo

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Re: LM399 based 10 V reference
« Reply #895 on: February 14, 2019, 05:37:43 pm »
There is around 800uVpp (@10KHz pwm) ripple at the output of the ADG switch (in my simulation above).
..you mean the node A2 in your simulation above?
This ripple mainly depends on the 1uF capacitor together with the 100K at the common of the switch (R4/C4) versus PWM frequency.
Thats why I asked branadic to increase the cap to 10uF in our test-board.
I think a better solution would be a good higher order low pass filter. Even the 10Hz 2nd order one in my schematics should filter the 10kHz+ perfectly out.
 

Online imo

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Re: LM399 based 10 V reference
« Reply #896 on: February 14, 2019, 05:44:21 pm »
Add 10 more phases and get russian V1-18  :-DD

https://www.qrz.ru/schemes/download/4445
Shots made by Max Otto von Stierlitz :)
« Last Edit: February 14, 2019, 05:48:31 pm by imo »
 

Online Kleinstein

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Re: LM399 based 10 V reference
« Reply #897 on: February 14, 2019, 05:55:41 pm »
The filter at the first OP is already 3 rd order and there is another 1st order filter before the 2nd OP, that could be changes to 2nd order. For the filtering the 10 kHz or so main PWM frequency should ideally not be a problem anymore. However in real life there may be limited gain / output impedance of the first OP that can limit the effectiveness of the real filter.  In a similar way the 2 nd order Sallen-key filter is in theory better than just a passive 2nd order low pass with buffer, but with real OPs there may be a little higher frequency signal loading the OPs output and this way bypassing the filter. So not sure if the Sallen key circuit is an improvement in reality.

Due to the modulation, there are also much lower frequencies present, though at a lower amplitude. These could also be a problem, not that visible in the simple simulations.

With the 2 PWM stages it depends on the settings, whether the ration of the two 150 K resistors really matters. If the PWM settings are the same, there would be essentially the same voltage from both sides and than taking the average. Different resistors would not really matter than.
 

Online imo

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Re: LM399 based 10 V reference
« Reply #898 on: February 14, 2019, 07:38:14 pm »
I still see 700uVpp ripple (vref10 node) in the pigrew's simulation.
How should your dual PWM work? What is the trick?

FYI - the best PWM equipped STM32 is the STM32F334 series. It includes 217ps resolution timers.

For example you can generate a 100kHz PWM with ~15.5bit resolution:

144Mx32/100k = 46080  (100% duty)
46080*0.7 = 32256  (70% duty)
« Last Edit: February 14, 2019, 08:09:23 pm by imo »
 

Offline pigrew

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Re: LM399 based 10 V reference
« Reply #899 on: February 14, 2019, 11:05:09 pm »
I made some oscilloscope captures of the ADP1404, ISL43210A, and DG636EEQ. My below plots used Vdd=15V, Vpulse=3.2V, S1=9V 10V, S2=0V. Blue in input, Yellow is output.

The ADP1404 is clearly the worst, with quite visible charge injection during the dead time (60us) and very slow (150ns). The delay of the ADG1404 is a strong function of Vdd, with the "best" response with Vdd=20V.

The other switches were much faster (~15ns delay, <10ns rise/fall time), and have minimal charge injection. The DG633EEQ looks best (less ringing), but that could be just because the switch is too fast for my probing techniques.  |O

For those of you using the ADG419, I'd be interested if you could measure the voltage (versus frequency, constant duty cycle) with the ADG419 vs. the Vishay DG419LE. I've not tried the DG419LE (forgot to order it), but I think it'd be an improvement (unless the supply current goes up too much?).

EDIT: Added photo of breadboard.

-Nathan
« Last Edit: March 20, 2019, 04:14:00 am by pigrew »
 
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