Thanks again Andreas. You are a walking Volt Lexicon .
I am looking through all the links I have collected, and I found a new one that imo deserves presentation. There is a real DIY prototype (breadboard and copper clad), noise measurements and another version of the test code I have linked to in an earlier post in this thread. The chip is LTC2440 - a differential input version of the LTC2400. Imo interesting and useful for the real breadboarders and Manhattan style prototypers. And another proof that carbon film resistors are not precision stuff (the builder of the prototype knows very well):
Source: http://dangerousprototypes.com/forum/viewtopic.php?t=4247p=42053
The 2440 is a great little chip, widely used (in comparison, 2400 is rarely seen) and a lot easier to manage than the 2400 (it being differential).
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Yes- ironically that jbeale post on the dangerous prototypes board is what initially led me to the LTC 2400 (mentioned in one of the posts in that thread) for my project. His board using the 2440 is a work of art! and likely beyond my skill set to reproduce.....
- Both the source and the meter are unshielded.
- The USB cable for serial data to PC shifted the readings. Different ground levels and ground noise (?). The only solution is opto couplers and isolation transformers (?).
Anyway - the voltage source is the TI REF5050 based board from voltagestandard.com. The reference chip is proclaimed to have noise 3ppm / V. That should be 15 uV. But when the datasheet does not state whether it is max or typ, then there is imo no reason to believe it. The series plot is 20 uV between the upper and lower lines. The plot is raw ADC output - no averaging. Imo very good performance of the LTC6655 and LTC2400. And the cheap but in my case very stable REF5050 has a nicely distributed noise of around 25-30 uV that easily is averaged away.
the noise is "typical" 15 uVpp
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USB-cables are terrible (Standard RS232 lines are better but not good). I use opto couplers and battery supply for the references and for the ADC cirquit. Transformers also have large capacities between primary and secondary side and will create some shifts to the output.
Even the 0.25pF of the photo couplers create a few uV shift in the output depending on the placement of the USB-cables.
If you look closer to the REF5050 datasheet you will find that the noise is "typical" 15 uVpp within 0.1 .. 10 Hz.
Usually the typical noise values are reached by about 90% of the parts. Except for some "defective" parts which have much larger noise.
And the LTC2400 also has typical around 10uVpp noise. But if your pcb layout is not perfect you will easyly have a lot more noise than the 10uVpp. I would try to measure the own reference voltage or zero input (with short and long cables) to find out what the ADC noise is.
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So the noise from the ADC will be masked as long as the measured source has a noise level significantly greater. So what we see in the plot will mainly be determined by the p-p performance of the REF5050
If capacitive coupling through an opto coupler causes issues, you could use an audio fibre optic connection.
I've done this before, it's easy to do and cheap, however can be a bit power hungry if you want speed.
And I also must rethink the whole less-than-1-ppm 'philosophy'
Alternatively - since I very seldom need any precision measurements, I could take away the USB cable completely. When I want to collect measurements that will be sent to PC, I can store them in a software declared array, in I2C EEPROM or in SD card, and transfer them after the measurements are finished. I also have bought EEPROM chips, but never used them.
QuoteLTC2400
That would be challenging for anyone to use.
Sorry. I do not understand your post.
Great info in this thread!
Alternatively - since I very seldom need any precision measurements, I could take away the USB cable completely. When I want to collect measurements that will be sent to PC, I can store them in a software declared array, in I2C EEPROM or in SD card, and transfer them after the measurements are finished. I also have bought EEPROM chips, but never used them.
While not super cheap. couldn't one use an XBee set up to wirelessly stream the data to a PC?
I am obsessed with Arduino shields and am always looking for an excuse to buy a new one or two . It is just that I haven't tried anything before, because the need to get rid of USB related problems was never there. One learns to walk by crawling ...
- use optocouplers with the two-wire Rx/Tx of RS232. If the ADC+uC is battery powered and have good ground design (star ground as in the LT schematic in a post over), and the 4-wire USB cable stops at a USB-to-RS232 breakout board at the Arduino, then one can galvanically isolate the ADC+uC from the PC by using 2 optocouplers (Rx/Tx).
USB-cables are terrible (Standard RS232 lines are better but not good). I use opto couplers and battery supply for the references and for the ADC cirquit. Transformers also have large capacities between primary and secondary side and will create some shifts to the output.
Even the 0.25pF of the photo couplers create a few uV shift in the output depending on the placement of the USB-cables.
I have not yet clearly understood if there are any important differences between isolating the SPI serial bus or isolating the USB with optocouplers in RS232 Tx/Rx.
I found a good article by Jim Williams showing opto isolation on SPI.
I have not yet clearly understood if there are any important differences between isolating the SPI serial bus or isolating the USB with optocouplers in RS232 Tx/Rx. In the first case the ADC and reference will not share common ground with the microcontroller.
The first opto-isolator is connected in the wrong direction.
I have not yet clearly understood if there are any important differences between isolating the SPI serial bus or isolating the USB with optocouplers in RS232 Tx/Rx. In the first case the ADC and reference will not share common ground with the microcontroller.