OSHW - 24bit ADC measurement system for voltage references

[branadic]

Almost every CAD for printed circuit boards is able to import Eagle Files.

What can you do with the schematics in PDF format? Almost nothing, just viewing it. Please don't repeat this discussion we already had before. If you don't have Eagle or similar CAD software with import function it's not my problem.

[rs20]

Sorry, but think: phone, tablet, my locked down work computer, internet cafe, borrowed devices. Every one of my devices except my home PC, the only one which I have complete dominion over. All places which have no CAD software, and all support PDFs near-natively. And viewing a schematic is all that 90% of people well ever want to do. You're free to keep your design behind a proprietary wall, just don't pretend that it's sensible or in the spirit of a forum.

To be clear, if you had just said "no", I'd have not said anything. But you said "no, it's easy for you to use what I've provided". And I have to point out that this is massively, massively untrue for many people.

[branadic]

And viewing a schematic is all that 90% of people well ever want to do.

And I'm not adressing exactly those 90% of people

Some design parameters of the pcb to be ordered:

- size: 83 x 50 mm²
- thickness: 1.55mm
- smallest width: 0.2mm (due to guard at reference noise reduction pin)
- smallest via: 0.4mm
- finish: Ni/Au
- no silk screen
- stop mask: green on both sides

[timb]

As an FYI, I would be in the target audience for this device, however I browse the forum (and do research, etc.) almost exclusively on my tablet. Further, I don't use Eagle for design work. If you can't be bothered to provide a PDF, or at the *very least* a PNG of the schematic, then I'm not going to bother with your device.

PDF is the de facto standard for distributing documents on the Internet. If I have to install Eagle on my main computer, just to view your schematic, then I'm not going to even consider your device. Period. What it tells me is you're either lazy or don't have any attention for detail. All you have to do is download Ghostscript... (Or one of the other numerous PDF printers freely available, assuming you're on Windows; if you're on OS X you can natively print to a PDF, so there's no excuse there.)

Sorry for being blunt, but that's just the way things are. (This would be like TI distributing datasheets exclusively in Word or PowerPoint formats. Can you imagine?)

[branadic]

The former version with LT1236LS8 has already been shown. Is it really that complicate for you to imagine that LT1236LS8 was replaced by LT1027LS8 and therefore minor changes on the layout had to be performed? The complete rest is identical to the former version.
If your answer is yes, than you really don't need the pcb, trust me and you are one of the 90% of people out there that don't need a 24bit ADC measurement system for voltage references either.

[timb]

I actually would be in the market for something like this, but since you can't be bothered to keep your documents current, I won't bother either.

[TiN]

It takes 5 seconds to export PDF and attach to files, and saves all that time wasted with valid and invalid complains. If every engineer will post their hobby stuff only in software they used, discussion would be less open and wide. Viewing schematics is all that most of interested folks need, as those who will bother building something likely to use their own chassis, own PCB layout, own components set they want anyway, and PDF-schematics is more than enough to test your idea.

Almost every CAD for printed circuit boards is able to import Eagle Files.

3000 EUR OrCAD can do nothing with Eagle files. Yes, it's not your problem.

I was remotely interested as just got few LTC6655's and ADCs not long time ago and going to bodge little PCB for few tests into my order, so hence came to this thread to check out on similar design ideas.. Baa , can't see files.

[AlfBaz]

Hi Branadic, first let me thank you for posting this and Andreas for allowing it.
I'm in the process of duplicating the circuit using altium, roughly copying your PCB layout and routing.

I'm sure this circuit has been tweaked for minimum noise and as I work through the design I see some of the choices you've made but I still have a few questions

The relatively high series resistance to the optos seems to be to round off the edges on the pulses and the even higher resistances (22k and 47k) to allow the pic to be programmed
The one part I'm stuck on is why the parallel 2.2nF cap across the 3k3 resistor from the PIC to the opto. This seems to consume a large spike of current from VDD on transitions given the typical 10ns rise and fall times, am I missing something here?

B1 and B2, are they ferrite beads? If so i'm curious which frequency you chose 70, 100MHz or something else

The choice of micro, was it based on its low current consumption and adc for the thermistor? Did you create a software UART  and SPI for the interface to the ftdi usb and LTC2400 or is it just some rudimentary bit banging? I ask this because the PIC you have chosen is quite low current and don't know if I'll find anything lower

When you run this in conjunction with the 2-1 divider where do you power the whole lot from, DB9 plug or from the JP1 header on the LTC2400 board?

And finally after you tested did you end up cutting out the LTC2400's mounting square and go dead bug like Andreas did. In other words, did you end up noticing the effects of humidity
 

[Andreas]

The one part I'm stuck on is why the parallel 2.2nF cap across the 3k3 resistor from the PIC to the opto.

And finally after you tested did you end up cutting out the LTC2400's mounting square and go dead bug like Andreas did. In other words, did you end up noticing the effects of humidity

Hello,

intention of the 2.2nF is to speed up the rise time at relatively low power (~1mA).
Otherwise 9600 Baud are marginal with PC817 and 1 mA.

The LTC2400 does not need decoupling from the PCB. (Self adjustment during each measurement).
Only the voltage reference shows a dependancy from PCB stress and should be decoupled.

with best regards

Andreas

[branadic]

Hi,

B1 and B2 were choosen to be 600ohms at 100MHz, if I remember correct. Micro was choosen by Andreas because of battery operation and thus low power consumption required.

Power is always supplied via JP1, DB9 powers additional modules connected to it.

I didn't test the ADC to excess as Andreas did, however I have the reference still mounted directly onto pcb but NTC was changed to 0402 type and this way is much closer to the reference compared to through hole NTCs.
What I wanted to try is to place reference on a flexboard, which is then soldered to the board giving some strain-relief. Since real semi rigid pcb's are barely expensive and dead bug style is not a good production process I thought this tradeoff should give best results to keep board stress due to humidity away from the reference. But haven't tried that yet as some other projects came into focus.

-branadic-

[AlfBaz]

The LTC2400 does not need decoupling from the PCB. (Self adjustment during each measurement).
Only the voltage reference shows a dependancy from PCB stress and should be decoupled.

My mistake, LTC2400 on the brain, meant LT1236

Thank you gentlemen, about to order parts and PCB, will post pics when complete. Lots of measurement fun ahead

Just one more thing, is there any reason to go without solder mask? I seem to recall seeing you guys post pictures of your various pcb's without it and always assumed it was an artifact of an in house manufacturing process. I see Dr Frank's LTZ1000 boards I got from you Branadic have it though

Regards, Alf

[branadic]

Andreas made is first boards at home and I once had a cheap and fast german pcb manufaturer with the only drawback of having no solder stop mask nor silk screen. Unfortunately he's no longer in business.

-branadic-

[AlfBaz]

Ok thanks

[Andreas]

Hello,

Thanks for the summary.
But noise is not all.

For a absolute measurement the offset and gain drift and INL is also important.

That is why I love the LTC2400:
Offset + Gain drift is self adjusted.
INL can be easily modeled and canceled out to below 1 ppm.

with best regards

Andreas

[chickenHeadKnob]

2 of the interesting ADC in the list is ADS125H and ADS8699. it has built in DMM gain scale.
i have also included some relative levels of 6.5D MM noise.

there is a chance i made some fundamental errors ... i have not seen any ADC being able to oversample to hell at nV level of noise ... pls feel free to highlight any corrections.
to note : it would be good to view the chart with lesser importance on rank but to analyze which sampling speed to use to maximise brute force lowering of noise

Thank you for the work you put into this, My first impression is that you have been overly optimistic in two dimensions. Both in using the data-sheet specmanship Enob of the ADC's as well as relying on  getting more than 2 extra bits from averaging. Two extra bits with simple averaging in the presence of gaussian noise, easy. The next bits progressively more difficult, then you will hit some system noise floor that becomes an insurmountable limit. 

The ADS8699/8689 is indeed interesting, I have been planning a design for a year now and  I need to get off my ass. One complicating factor is the bandwidth of the analogue front end. From the data sheet (page 25):
 In order to mitigate the noise of the front-end amplifier and gain resistors of the PGA,the AFE circuit of the device features a second-order,anti-aliasing LPF at the output of the PGA.The magnitude and phase response of the analog anti-aliasing filter are shown in Figure51 and Figure52, respectively.For maximum performance,the –3-dBcutoff frequency for the anti-aliasing filter is typically set to 15 kHz.The performance of the filter is consistent across all input ranges supported by the ADC.

 I read this as you won't get near nyquist performance suggested by the sample rate if you are simply brute force adding these puppies in parallel.

[jeffheath]

I found this thread, files were eagle cad... so I installed eagle cad.... now I can open eagle cad files! It's not that hard...

[CarlO]

Hi All.  I came across this old thread and am very interested in building some of these devices.  I was able to open the PCB files in Eagle and produce Gerber files for PCB production.  I have searched the forum and I have not been able to find any firmware or other software to complete this project.  Can someone please point me to a thread with firmware files for this project?  Or perhaps someone could PM me the files?  Note that I seem to be unable to send a PM on this forum, perhaps because I am a Newbie?  Thanks in advance for any assistance you can provide.
-Carl

[Andreas]

Hello,

find the firmware attached.

you can use the 9 pin DSUB to program the PIC on the board using a cable with corresponding PINs connected between PICKIT3/PICKIT5 and DSUB.
Note that PICKIT5 needs a 5V signal to program the chip. Since the digital 5V are not available on DSUB I use the 5V analog VREF as connection between VCC pin and board.

The "thick wires" in the cable are 470 R protection resistors in case of short cirquits.

with best regards

Andreas

edit:
measurements on a similar board from Branadic (the LT1027 variant) are in this thread:
https://www.eevblog.com/forum/metrology/t-c-hysteresis-measurements-on-brand-new-lt1027dcls8-5-voltage-reference/msg1003827/#msg1003827

[CarlO]

Hello,

find the firmware attached.

you can use the 9 pin DSUB to program the PIC on the board using a cable with corresponding PINs connected between PICKIT3/PICKIT5 and DSUB.
Note that PICKIT5 needs a 5V signal to program the chip. Since the digital 5V are not available on DSUB I use the 5V analog VREF as connection between VCC pin and board.

The "thick wires" in the cable are 470 R protection resistors in case of short cirquits.

Thanks Andreas,

For the firmware and the in-circuit programming cable details and the additional link.

I have the PIC12F675 programmed and connected to the LTC2400.  I am using a AD586LQ as the voltage reference.
I was able to determine that the PIC firmware responds to the ascii character "1" with about 9 bytes of data at 9600 baud.  I am assuming that this represents the raw bits from the LTC2400 and the temp data from the PIC ADC?
I also noticed there seems to be some configuration data in the eeprom memory?

This leads to a couple of questions:
Are there any additional ascii commands that the firmware recognizes?
What is the format for the data the firmware sends to the host?

Thank You for all your help,
-Carl

[dobsonr741]

For the fans of LTC24xx, I did a design, including PCB, and threw in an ADA4255. Sharing the schematics here.

[Andreas]

I have the PIC12F675 programmed and connected to the LTC2400.  I am using a AD586LQ as the voltage reference.
I was able to determine that the PIC firmware responds to the ascii character "1" with about 9 bytes of data at 9600 baud.  I am assuming that this represents the raw bits from the LTC2400 and the temp data from the PIC ADC?

Hello,

so you are gone already very far.
Pictures from the board please.

Note that serial parameters are 96,8,E,1 (even parity). so actually you have a 8 byte answer.
the $31 "1" character is used to determine the baud rate (4800-9600 are possible) after power on and sends the EEPROM data.

For reading the temperature sensor raw value  together with the LTC raw value you need to send $28 in normal operation.

Description is on page 13 of the attached document. Sorry I have it only in german.

with best regards

Andreas

[CarlO]

Description is on page 13 of the attached document. Sorry I have it only in german.

Thank You for the Specification document.  I have not had any time to work on the project.  I will share some results and photos soon.

Attached is an English translation (Google AI) of your Specification document, in case someone will find it useful.

Note to readers: If you open either version of the document with Adobe Acrobat, the first 12 pages form a kind of slide-show.  It's really quite cool!

[CarlO]

Ok, I finally have some time to work on the project and share some initial results.

so you are gone already very far.
Pictures from the board please.

Well...  Maybe not so fast...

I ordered the PCBs as can be seen in the first photo.  Then after reading through these threads I realized that the AD586LQ may still be the best option, and I have a few in old stock.

So I decided to build the circuit on a solderless breadboard to see what else I could learn.
From left to right in the photo:
7815 voltage regulator
AD586 with NTC thermistor
LTC2400
PIC12F675 with 7805 regulator
PC817 Opto-Isolators
FT232 on breakout board

Note: The circuit on the solderless breadboard is totally unsuitable for precision measurement.  It's much too fragile and there are unknown thermal EMF from the contacts/plating etc.
But at least I can see zero and full-scale and start with some software development.

I had a bit of trouble with the opto-isolators.  I had to increase the pull-up resistors on the transistor side to 4.7K to get enough voltage swing for reliable communications YMMV.
These opto-isolators are really running on low current for the diode side?  Might be worth decreasing the series resistance slightly?  A small increase in current for the diodes seems to make a large difference.
The characteristics of the opto-isolators may vary slightly from batch to batch.

The third photo shows the FT232 replaced by a micro-controller.  More on this in the next post.

I have identified the following changes I will make to the circuit and PCB.

1. Change footprint for the reference to 8pin DIL for AD586 using Andreas' "Stressless" method (only 1 pin soldered and oversize holes with fine wire for the other pins)

2. Increase input supply voltage to ~18V.  This might be too high for some accessory boards with OP-Amps?  Possibly add 12V linear regulator for compatibility?

3. Modify voltage regulator feedback for 15V for the AD586.  Is there any opinion or data here on running the AD586 at 12V vs 15V???  The data sheet specifications are for 15V.  Would lower supply voltage cause a shift in the TC?

4. Add a 0.1" (2.5mm) pin header on the host side of the opto-isolators for TTL connections to the micro-controller.  This would be an option to populate the TTL pin header OR the FT232.

5. Increase the pull-up resistors on the opto-isolators to 4.7K or possibly increase the diode current slightly with lower value series resistors.

6. Add a solder link to select +5V or GND on the fo pin of the LTC2400.  60Hz here in the USA.

Time to cook some new PCB's.  Any thoughts or opinions would be appreciated,
-Carl

[CarlO]

I have been using a micro-controller to collect data from my 34401A multi-meters.  I wrote some software to do some averaging, compute P-P and Standard deviation etc. and log the data in CSV format to flash memory or SD card.
Since I already have some software for that application, I decided it should be "Easy" to modify it to collect data from the LTC2400 project.
You can see the micro-controller in my previous post.  Soon it will have an LCD display and a few buttons like my others.

The micro-controller is running "PicoMite BASIC" firmware.  It's a lot like the old "Boot to BASIC" computers of the 1980's but much faster and with many enhancements to the BASIC language.
https://geoffg.net/picomite.html

Edit:  I forgot to mention that the micro-controller is 3.3V logic levels.  I had to reduce the series resistor on the TX side to 1.8K to have about the same diode current as the 5V with 3.3K series resistor.

I already wrote a small test program to decode the ADC output from the PIC12F675.  Now I "Just" need to merge the decoding routines into my other data collecting program.
Here are some initial results:

> run
Press a key for the command

1
Byte Hex Binary
 1 0x31 00110001
 2 0x01 00000001
 3 0x08 00001000
 4 0x41 01000001
 5 0x44 01000100
 6 0x67 01100111
 7 0x00 00000000
 8 0x00 00000000
 9 0x00 00000000
 10 0x00 00000000
 11 0x00 00000000
 12 0x00 00000000
 13 0x01 00000001
 14 0x10 00010000
 15 0x08 00001000
 16 0x20 00100000
 17 0x10 00010000
 18 0x01 00000001
 19 0x20 00100000
 20 0x02 00000010
 21 0x30 00110000
 22 0x03 00000011
 23 0x40 01000000
 24 0x04 00000100
 25 0x00 00000000
 26 0x00 00000000
 27 0x00 00000000
 28 0x00 00000000
 29 0x00 00000000
 30 0x00 00000000
 31 0x00 00000000
 32 0x00 00000000

1
Byte Hex Binary
 1 0x31 00110001
 2 0x01 00000001
 3 0x08 00001000
 4 0x41 01000001
 5 0xF3 11110011
 6 0xF4 11110100
 7 0xFF 11111111
 8 0x1F 00011111

$
Byte Hex Binary
 1 0x24 00100100
 2 0x01 00000001
 3 0x08 00001000
 4 0x41 01000001
 5 0x44 01000100
 6 0x67 01100111
 7 0x80 10000000
 8 0x8B 10001011

(
Byte Hex Binary
 1 0x28 00101000
 2 0x40 01000000
 3 0x8B 10001011
 4 0x41 01000001
 5 0xFF 11111111
 6 0xF4 11110100
 7 0xFF 11111111
 8 0x1F 00011111

            Raw       Volts
PIC ADC =  557      2.720
LTC2400 = -2817 -0.00005247V

(
Byte Hex Binary
 1 0x28 00101000
 2 0x00 00000000
 3 0x8A 10001010
 4 0x41 01000001
 5 0xD9 11011001
 6 0xFB 11111011
 7 0x9F 10011111
 8 0x28 00101000

            Raw       Volts
PIC ADC =  552      2.695
LTC2400 =  144702425    2.69529270V

(
Byte Hex Binary
 1 0x28 00101000
 2 0x40 01000000
 3 0x8A 10001010
 4 0x41 01000001
 5 0x06 00000110
 6 0x21 00100001
 7 0x00 00000000
 8 0x30 00110000

            Raw       Volts
PIC ADC =  553      2.700
LTC2400 =  268443910    5.00015747V

-Carl

[Andreas]

2. Increase input supply voltage to ~18V.  This might be too high for some accessory boards with OP-Amps?  Possibly add 12V linear regulator for compatibility?

3. Modify voltage regulator feedback for 15V for the AD586.  Is there any opinion or data here on running the AD586 at 12V vs 15V???  The data sheet specifications are for 15V.  Would lower supply voltage cause a shift in the TC?

5. Increase the pull-up resistors on the opto-isolators to 4.7K or possibly increase the diode current slightly with lower value series resistors.

Hello,

nowadays there is not only the LTC1050 available (that is why I chosed initially 14V and -1.2V), but also "high voltage" choppers as ADA4522-1.
But if you want to add a LTC1043 2:1 capacitive precision voltage divider you have to stay below 16V.

Self heating will differ somewhat depending on supply voltage. (so T.C. may be influenced slightly.)

Photocoupler current is minimized for battery operation on the isolated side. (Also to minimize noise on the analog side).
So if you change supply voltage or have marginal couplers you might have to adapt the resistors (or reduce the baud rate to 4800 Baud).

with best regards

Andreas