Author Topic: LTC2508 32 bit ADC (Read 32801 times)

Echo88 · « **Reply #50 on:** July 31, 2018, 11:23:18 am »

@Coppercone: See also comment here: https://www.eevblog.com/forum/metrology/32-bit-adc-playground-for-precision-measurement-tasks/msg1237921/#msg1237921

As far as i know these ADCs are also useful for seismic measurements were you need a very high dynamic range, see also this interesting pdf:
https://www.researchgate.net/profile/Nikolai_Beev/publication/325285614_Analog-to-digital_conversion_beyond_20_bits_Applications_architecutres_state_of_the_art_limitations_and_future_prospects/links/5b03dd96a6fdccf9e4f7c361/Analog-to-digital-conversion-beyond-20-bits-Applications-architecutres-state-of-the-art-limitations-and-future-prospects.pdf?origin=publication_detail
and the reference [5]:
SH. Gao, B. Xue, J. Li, Z. Lin, Y. Chen and XY. Zhu, "High-resolution data
acquisition technique in broadband seismic observation systems," ->

"The dynamic range of the currently most widely used 24-bit seismic data acquisition devices is 10–20 dB lower than that of broadband seismometers, and this can affect the completeness of seismic waveform recordings under certain conditions. However, this problem is not easy to solve because of the lack of analog to digital converter (ADC) chips with more than 24 bits in the market. "

coppice · « **Reply #51 on:** July 31, 2018, 11:59:21 am »

Quote from: Echo88 on July 31, 2018, 11:23:18 am

As far as i know these ADCs are also useful for seismic measurements were you need a very high dynamic range, see also this interesting pdf:

I think you might find a slight mismatch between the sampling speed of these ADCs, and the needs of seismic measurement.

Echo88 · « **Reply #52 on:** July 31, 2018, 12:18:15 pm »

I assumed the sampling speed was sufficient for measured waves between 1-100Hz. If thats wrong i stand corrected. Please explain what sampling speed is needed.

b_force · « **Reply #53 on:** July 31, 2018, 12:59:03 pm »

Why can't they use just audio ADCs for seismic stuff?

SNR of 120-130dB is even doable as well a THD+N -106 or better.

coppice · « **Reply #54 on:** July 31, 2018, 01:14:12 pm »

Quote from: Echo88 on July 31, 2018, 12:18:15 pm

I assumed the sampling speed was sufficient for measured waves between 1-100Hz. If thats wrong i stand corrected. Please explain what sampling speed is needed.

The spec for the LTC2508 says 145dB Dynamic Range (Typ) at 61sps and 131dB Dynamic Range (Typ) at 4ksps.

Seismic waves have a low fundamental frequency, but are very rich in harmonics. If you've ever seen something like a seismic plot on the news when an earthquake happens you must have noticed the very sharp transitions which occur. The ADC needs to capture those. I don't know how high a sampling rate they use, but I assume its at least has high as good audio.

iMo · « **Reply #55 on:** July 31, 2018, 04:45:07 pm »

TI's ADS126x claims 26.9bits ENOB (25 noise free bits) at 2.5SPS (datasheet page 28).
Afaik the earthquake freqs of interest are mostly below 3Hz.

splin · « **Reply #56 on:** August 01, 2018, 12:18:38 am »

Quote from: Echo88 on July 31, 2018, 12:18:15 pm

I assumed the sampling speed was sufficient for measured waves between 1-100Hz. If thats wrong i stand corrected. Please explain what sampling speed is needed.

Table 1, page 21 of the datasheet shows that due to the digital filtering and downsampling, the 3dB bandwidth at an *output data rate* of 61 SPS, is only 7.5Hz. At 796 SPS bandwidth increases to 120Hz but the dynamic range drops by 9dB to 136dB.

The LTC2500-32 does about 3dB better and you also get to choose from a range of filters to trade off BW, settling time and noise - see table 2 page 31.

b_force · « **Reply #57 on:** August 01, 2018, 05:53:58 am »

Quote from: coppice on July 31, 2018, 01:14:12 pm

Quote from: Echo88 on July 31, 2018, 12:18:15 pm
I assumed the sampling speed was sufficient for measured waves between 1-100Hz. If thats wrong i stand corrected. Please explain what sampling speed is needed.
The spec for the LTC2508 says 145dB Dynamic Range (Typ) at 61sps and 131dB Dynamic Range (Typ) at 4ksps.

Seismic waves have a low fundamental frequency, but are very rich in harmonics. If you've ever seen something like a seismic plot on the news when an earthquake happens you must have noticed the very sharp transitions which occur. The ADC needs to capture those. I don't know how high a sampling rate they use, but I assume its at least has high as good audio.

Some audio ADCs go up to 768k that should be more than sufficient.
Hell the default 48k is fine as well at these low frequencies.

branadic · « **Reply #58 on:** August 04, 2018, 11:37:57 am »

Quote from: Echo88 on July 31, 2018, 11:23:18 am

@Coppercone: See also comment here: https://www.eevblog.com/forum/metrology/32-bit-adc-playground-for-precision-measurement-tasks/msg1237921/#msg1237921

As far as i know these ADCs are also useful for seismic measurements were you need a very high dynamic range, see also this interesting pdf:
https://www.researchgate.net/profile/Nikolai_Beev/publication/325285614_Analog-to-digital_conversion_beyond_20_bits_Applications_architecutres_state_of_the_art_limitations_and_future_prospects/links/5b03dd96a6fdccf9e4f7c361/Analog-to-digital-conversion-beyond-20-bits-Applications-architecutres-state-of-the-art-limitations-and-future-prospects.pdf?origin=publication_detail
and the reference [5]:
SH. Gao, B. Xue, J. Li, Z. Lin, Y. Chen and XY. Zhu, "High-resolution data
acquisition technique in broadband seismic observation systems," ->

"The dynamic range of the currently most widely used 24-bit seismic data acquisition devices is 10–20 dB lower than that of broadband seismometers, and this can affect the completeness of seismic waveform recordings under certain conditions. However, this problem is not easy to solve because of the lack of analog to digital converter (ADC) chips with more than 24 bits in the market. "

Thanks. Additional search on the web gives another interesting links.
https://converterpassion.wordpress.com/category/data-converter/adc/adc-survey/page/2/
https://converterpassion.wordpress.com/2012/08/21/adc-performance-evolution-walden-figure-of-merit-fom/

-branadic-

Echo88 · « **Reply #59 on:** August 06, 2018, 09:48:21 pm »

Hmm, checked my working LTZ1000-reference and my 5440B-calibrator against my 3458A. Both deliver reasonable ~1.5µVpp, as expected. I build a LTC1043-divider to get about 5V from a LTZ1000, to use it as a reference for my ADCs. But the relative large pp-noise bothers me.
Configuration as follows (Andreas buffer-design, battery-powered, in a cookie-box): LTZ 7.2V -> LTC2057-buffer -> LTC1043 / 3 -> LTC1043 * 2 -> LTC2057-buffer delivers 4.8V ~8µVpp.

Any advice how the noise might be improved, maybe a buffer-stage between the /3 and *2 -block?

Andreas · « **Reply #60 on:** August 07, 2018, 04:48:35 am »

Hello,

how do you measure the 1,5uVpp/8uVpp?
(Scope with 1/f LNA or with the DMM at some NPLC?)
I never built my cirquit idea (except in LTSpice).

Did you synchronize all LTC1043 clocks?
(That is the first thing I would try, I had already some strange temperature dependant noise behaviour with a LTC1050 after a LTC1043 together with a LTC2400. Most probably resulting from interferences of the chopper frequencies and the ADC sampling window. With a LTC2057 in the same configuration I have much less temperature dependant noise. )

The next step would be a buffer between the LTC1043s.
But you have to take care that the buffer can handle capacitive loads for the step up configuration.

with best regards

Andreas

Edit: lowest noise (without buffer) should be when the connections of the second stage are built in a way where all input output capacitors are connected in parallel during the same clock phase. (Eventually you have to swap input and output connections of the switch on the 2nd stage)

Echo88 · « **Reply #61 on:** August 07, 2018, 06:28:34 pm »

The clock synchronisation was a very good idea Andreas.

With 600Hz at the clk-input of both LTC1043 i now get ~1µVpp at 4.8V. No buffer between the two LTC1043 was used in this case and since the result is already very good i wont build one in.

I measure the noise with my 3458A at 100NPLC and look for stable regions without thermal drift which are reasonable long, lets say 5min for example. Of course thats not as good as a LNA, but i dont own one.

Kleinstein · « **Reply #62 on:** August 07, 2018, 06:50:01 pm »

Using the 100 PLC mode makes it look at very low frequency noise, more like an 0.25 Hz upper limit. To get a value more comparable to the usual 0.1-10 Hz data, it would be looking at windows of about 100 readings at something like 1 or 2 PLC.

Andreas · « **Reply #63 on:** August 07, 2018, 07:22:56 pm »

Quote from: Echo88 on August 07, 2018, 06:28:34 pm

I measure the noise with my 3458A at 100NPLC

Its always good to know how something is measured to estimate what is really going on.

By the way: is it the attached cirquit?

In this case you would need only one LTC1043 (so it is automatically synchronized).
(but the first buffer needs to be stable against capacitive loads).

with best regards

Andreas

Edit: explanation see here:

https://www.eevblog.com/forum/projects/ppmgeek!-5-5-digit-dvm-volt-ref-cal-(for-arduino-or-any-uc-w-spi)/msg296127/#msg296127

So with a real LTC1043 the 20nF are in reality only 10 nF.

Echo88 · « **Reply #64 on:** August 07, 2018, 08:13:04 pm »

No, the circuit uses two separate LTC1043, since i didnt found your circuit suggestion during my searches before building. Might be interesting to also test it and see if it delivers the same results and behaves as well as the circuit with the two LTC1043.
Im now using this 4.8V-LTZ-based-reference as the reference for my AD7177-Eval-Board to see how many bits i get out of it, with my low noise-Fluke 5440B as the signal source for the ADC. But maybe that belongs in another coming thread.

Gerhard_dk4xp · « **Reply #65 on:** August 30, 2018, 11:01:22 am »

I just stumbled across this thread. Interesting.

I have made a small stamp-sized board for an LT2500-32 ( home-etched :-) )
that has the LT2500, 2 * LT3042 for analog & digital VCC, LT6655 reference
the differential driver and its neg. regulator, all in 1" * 1".

A xilinx Coolrunner on a second stamp generates the sampling clock / other timing.

But currently I'm stuck with the Beaglebone Black / Debian Stretch firmware.
Any access to the SPI interface results in a bus error.

I want to use the PRU of the BBB to collect the data and forward it to the Linux
ARM CPU via toggling buffers in 1Ksample blocks.

There is already a working LAN link that can read/write GPIB-like commands
by opening port 5000-sth. on 192.168.178.111 .

I have done some VERY low noise preamplifiers including choppers, but the
1/f noise of my Agilent 89441A that I use as an FFT analyzer is limiting me.
I do not want yet another boat anchor.
Integrating the ADC would make a very nice low noise digitizer with isolated
network access.

PS
Why can't this !§$%&!! board software reduce a cell phone photo itself
to the size it likes?

iMo · « **Reply #66 on:** August 30, 2018, 10:16:02 pm »

Is there a chance to see the schematics of the 2500-32 pcb?

Gerhard_dk4xp · « **Reply #67 on:** August 30, 2018, 10:42:35 pm »

Please

That's not intended for publication, just a fast screen dump.
There are some comments what should be changed, and the
large row of pins is the BBB 16 bit multiplexed bus that is
obviously not on the stamp. More like a lab note with everything
important on one screen.

cheers, Gerhard

coppercone2 · « **Reply #68 on:** August 30, 2018, 11:49:40 pm »

Quote from: Gerhard_dk4xp on August 30, 2018, 11:01:22 am

I just stumbled across this thread. Interesting.

I have made a small stamp-sized board for an LT2500-32 ( home-etched :-) )
that has the LT2500, 2 * LT3042 for analog & digital VCC, LT6655 reference
the differential driver and its neg. regulator, all in 1" * 1".

A xilinx Coolrunner on a second stamp generates the sampling clock / other timing.

But currently I'm stuck with the Beaglebone Black / Debian Stretch firmware.
Any access to the SPI interface results in a bus error.
I want to use the PRU of the BBB to collect the data and forward it to the Linux
ARM CPU via toggling buffers in 1Ksample blocks.

There is already a working LAN link that can read/write GPIB-like commands
by opening port 5000-sth. on 192.168.178.111 .

I have done some VERY low noise preamplifiers including choppers, but the
1/f noise of my Agilent 89441A that I use as an FFT analyzer is limiting me.
I do not want yet another boat anchor.
Integrating the ADC would make a very nice low noise digitizer with isolated
network access.

PS
Why can't this !§$%&!! board software reduce a cell phone photo itself
to the size it likes?

why not just use a PIC?

This is exactly why I hate digital design.

Man I am thinking of migrating to the Z80. Is there a high speed easy to use processor that you can run off your own EEPROM with you own peripherals? I hate all that BS that I don't use on MCUs.

Gerhard_dk4xp · « **Reply #69 on:** August 31, 2018, 01:01:55 am »

> why not just use a PIC?

Because I cannot ssh into a PIC, because I cannot edit & compile my sources there
because there is no gcc, no yacc/bison, gdb right on the target, because there is no Debian
Linux and file system on the target, because there is no way to run fftw on a substantial time
series with a NEON floating point accelerator, no el cheapo web server to show results?

>This is exactly why I hate digital design.
> Man I am thinking of migrating to the Z80.

I came from there 30 years ago. There has been some progress in the mean time.
A Z80 could not read out the ADC at 1 Msample/s @ 32 Bits, much less storing the data
in its 64 Kbytes.

> Is there a high speed easy to use processor that you can run off your own EEPROM
> with you own peripherals? I hate all that BS that I don't use on MCUs.

So what. The BBB has its own flash eprom, lots of it, a 1 GHz CPU and you don't even
need a programmer. There are 2 aux. risc processors @ 200 MHz to do I/O, and DMAs,
timers, PWM units, what do you want?

Plug in the network cable, log in and start working.

regards, Gerhard

guenthert · « **Reply #70 on:** August 31, 2018, 07:45:40 am »

Quote from: coppercone2 on August 30, 2018, 11:49:40 pm

Man I am thinking of migrating to the Z80. Is there a high speed easy to use processor that you can run off your own EEPROM with you own peripherals? I hate all that BS that I don't use on MCUs.

Ok, this is getting seriously off-topic and I wouldn't recommend one for professional use and I'm not in the retro-computing Z80 fan camp (gosh I'm glad those times are over -- remember CP/M? *shudder*), but if you're looking for an easy MCU w/o EEPROM, then there is Parallax' Propeller (not affiliated, just a satisfied customer). They target chiefly the educational sector. It's popular with some hobbyist and artists, but there have been some measurement devices made with it too. Its eight core ("cogs") architecture is a bit peculiar in that it doesn't support interrupts (or even stacks and subroutine calls), but that allows for accurate bit-banging. The manual is only 400 pages and there is a small, but friendly community. 32KiB internal RAM assures that software doesn't get unwieldy large.

iMo · « **Reply #71 on:** August 31, 2018, 05:53:23 pm »

The 2500-32 returns "24bits result from SAR core at full speed" and "7bits of Common Mode result" and "1 bit overrange indication".
How to combine all those outputs actually?

coppercone2 · « **Reply #72 on:** August 31, 2018, 07:41:15 pm »

Quote from: Gerhard_dk4xp on August 31, 2018, 01:01:55 am

> why not just use a PIC?

Because I cannot ssh into a PIC, because I cannot edit & compile my sources there
because there is no gcc, no yacc/bison, gdb right on the target, because there is no Debian
Linux and file system on the target, because there is no way to run fftw on a substantial time
series with a NEON floating point accelerator, no el cheapo web server to show results?

>This is exactly why I hate digital design.
> Man I am thinking of migrating to the Z80.

I came from there 30 years ago. There has been some progress in the mean time.
A Z80 could not read out the ADC at 1 Msample/s @ 32 Bits, much less storing the data
in its 64 Kbytes.

> Is there a high speed easy to use processor that you can run off your own EEPROM
> with you own peripherals? I hate all that BS that I don't use on MCUs.

So what. The BBB has its own flash eprom, lots of it, a 1 GHz CPU and you don't even
need a programmer. There are 2 aux. risc processors @ 200 MHz to do I/O, and DMAs,
timers, PWM units, what do you want?

Plug in the network cable, log in and start working.

regards, Gerhard

Sounds like all those features the BB has are not working in your favor for some reason, since you cant seem to get any data.

I use the simplest PIC with the least peripherals and enough speed to spit out processed ADC data to serial cables logging to putty lol. Kinda wondering what will happen when I want to use faster ADCs.

My appeal behind the use of a MPU (never did) is that you can probe everything with a logic analyzer and do dumps to see whats going on without some kind of marginally useful JTAG device. It seems that it would cut down on the bullshit that comes with all the linux/other overhead.

I just had this gut feeling that using some kind of dedicated LINUX based PCB thing to program a MCU would result in hurt so I just stuck with programming PIC's with a pickit and using sockets. Even that had its fare share of misbehavior (particularly with the directives used by the compiler to configure the PIC. Would prefer separate parts rather then configurable ones, I don't care for most of the features I just like using them as ADC to PC or LCD converters).

I have less issues with MCU's when it comes to control systems though, then they are handy and their multi use is tolerable and they save time*. For home made equipment anyway it seems like using a PIC to do the control features like range switching, buttons, etc on its own LCD is a good idea to keep it away from the data processing system and having to deal with interrupts and everything... its all so cheap now\

I guess you need to slave it if you want to make some kind of self calibration but if you keep the control system separated from the measurement system it might make it alot eaiser. Just seems so much cleaner.

Seems to get alot uglier when you are trying to make some kind of source measure unit/digilog DSP feedback thing.. then you need shared memory and a feedback interrupt to make it follow some kind of known stable inaccurate looop that won't fuck up the process or completely halt while the memory is being updated

Yea the physical size of my projects would drive most people nuts LOL

*save time if you have the damn Microchip/ATMEL product family memorized otherwise you can get totally sidetracked trying to find the MCU of the correct form factor, peripherals location, etc.

Kleinstein · « **Reply #73 on:** August 31, 2018, 07:59:05 pm »

Quote from: imo on August 31, 2018, 05:53:23 pm

The 2500-32 returns "24bits result from SAR core at full speed" and "7bits of Common Mode result" and "1 bit overrange indication".
How to combine all those outputs actually?

There is usually not need to combine the results. The main result it the differential input. This could be either raw data or internally filtered at a lower speed.

The common mode and over-range indication are more for debug or error checking. The common mode voltage indicates how well the signal is balanced to the inputs.

iMo · « **Reply #74 on:** August 31, 2018, 08:26:46 pm »

Ok, it means, for example, when they provide the phone number of a board member in an additional 32bit part of the "result" their marketing will call it 2500-64, a 64bit ADC


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Author Topic: LTC2508 32 bit ADC (Read 32801 times)

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