two ADC on different ground potential - Page 1
Forum section: electronics / general

author: leniwiec
author_rank: Contributor
timestamp: April 20, 2011, 09:16:37 pm
content: Hello,I wonder is there any other method than opto-isolation, when I need to connect 2 ADC to one mcu, and each of them have different ground potential ?Ps. English is not my native language so if I make some terrible language disaster, I'm sorry

author: neoone
author_rank: Regular Contributor
timestamp: April 20, 2011, 09:26:59 pm
content: if they have different ground potential then how do you power them ? What kind of interface they use? Do You need them to be isolated or it's just a problem of different levels ?

author: bilko
author_rank: Frequent Contributor
timestamp: April 20, 2011, 09:28:40 pm
content: Transformer or opto isolation, depends on the signal you are capturing. Analog Device make isolated amplifiers

author: leniwiec
author_rank: Contributor
timestamp: April 20, 2011, 09:33:17 pm
content: So, situation is look like this: I need to measure output of two DC symmetric output power supply, and each of them have different ground level.So, the problem is different ground potential. I can use opt-isolation and power each of adc from measured power supply, but I think it's not so elegant solution...

author: bilko
author_rank: Frequent Contributor
timestamp: April 20, 2011, 09:42:36 pm
content: Easy way would be to use voltage to frequency converters via optocouplers, use MCU to measure frequency. Various chips do this, VCO or VtoFCheck out this chip, and application on page 9 of the data sheethttp://www.analog.com/static/imported-files/data_sheets/AD7740.pdf

author: leniwiec
author_rank: Contributor
timestamp: April 20, 2011, 09:51:28 pm
content: Quote from: yachtronics on April 20, 2011, 09:42:36 pmThx for suggestion yachtronics.I think about that solution, but chips for this is very expensive and building vco is unacceptable in my design. If I don't find better/cheaper solution, I will back to this idea...

author: bilko
author_rank: Frequent Contributor
timestamp: April 20, 2011, 09:52:38 pm
content: Chip is less than $1

author: leniwiec
author_rank: Contributor
timestamp: April 20, 2011, 09:57:09 pm
content: Quote from: yachtronics on April 20, 2011, 09:52:38 pmNot in my country, I must pay about 3EUR (~5usd) for 1 pcs. 5-10 needed.

author: am2pgs
author_rank: Contributor
timestamp: April 20, 2011, 11:06:21 pm
content: Another way is to use an ADC with differential (proper or pseudo-differential) inputs. Then both your ADCs could use the same ground as your MCU.

author: leniwiec
author_rank: Contributor
timestamp: April 21, 2011, 11:32:38 am
content: Quote from: am2pgs on April 20, 2011, 11:06:21 pmThx, for advise, but I'm afraid that adc with differential inputs it's not solution in my case. There are still need analog and digital ground on the same potential as I read in datasheets.

author: tjw
author_rank: Contributor
timestamp: April 21, 2011, 02:15:24 pm
content: Hi,have you considered using a differental amplifier ?It sounds like you have selected an ADC that does not provide a differential input so you must add an external differential amplifier.You said that the ADC and MCU have different ground potentials - I am guessing this means that the signals you are measuring are floating with respect to the MCU ground ? If this is the case I think you need to either reference the input signals to the MCU ground or reference the ADC output signal to the MCU ground - both require the use of a differential amplifier. The point here is that you must pick a ground and then reference everything else to this one ground.You can make a differential amplifier however you need precision resistors for good performance, or you can buy one in a dip package ready to go.More specific information about your circuit may help to provide a more detailed solution.Regards, tjw.

author: leniwiec
author_rank: Contributor
timestamp: April 21, 2011, 02:37:51 pm
content: Thx tjw for answer.The product which I design is current in 'scratches'. And the situation is like a write above.I have two DC power supply with symetric output, this power supplies can be connected in parallel or serial mode. So, now I have ground from 1 power supply and ground from 2 power supply and one more ground on MCU, and each of this grounds is on different potentials.Possible solution is to opt-isolate ADC or use U/f converters and opt-isolate their outputs... but I think this solutions is not so good, it complicate design and may make some further troubles.I also thinks, that perfect solution is to use something like adc with full separated analog and digital ground - and here is my question: Is there any ADC and DAC chips like that on market ? Or maybe there is better solution to measure several signals, each of which is on different ground potentials... ?

author: am2pgs
author_rank: Contributor
timestamp: April 21, 2011, 03:36:57 pm
content: I don't know of any ADC that has isolated digital / analogue grounds. Usually a differential input stage (either internal or external) is what you want. One input of the diff amp is connected to your "other" ground. the other input of the diff amp is connected to the voltage you are trying to measure. The ADC will measure the difference of the two multiplied by whatever gain you choose. The ADC itself with its digital/analogue ground is connected to your MCU ground.One thing to be aware of is that all differential amplifiers have a limited common mode inpu voltage range. Common mode voltage range would in your case be related the potential difference between your two grounds. If they grounds are truely floating this could be problematic.The potential difference between the two grounds needs to be defined and bound.Also differential amplifiers do not perfectly reject common mode voltages, so you need to calculate the common mode rejection ratio of the differential amplifier. Instrumentation amplifiers are best for this, but using an opamp with percision resistors to make a diff amp may be good enough for you.

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: April 21, 2011, 05:00:00 pm
content: What are you trying to achieve? Can you knock up a quick diagram?Why do you reject differential input ADCs? They are perfect for your task.Optoisolation makes sense when you have completely uncontrolled ground potential like two units plugged into different power loops without proper ground (think laptops), etc

author: leniwiec
author_rank: Contributor
timestamp: April 21, 2011, 06:08:59 pm
content: Thx for replay Leo Bodnar.In attachment I add quick diagram of my case. As you can see differential inputs adc still need analog ground refered to input signal, and this analog ground is connected with digital ground, It's not a problem if I only have possible situation like on 'Situation #1' - I will connect all gnd together, but on 'Situation #2' I need to measure potential between power supply #1 `+` and GND #1 and power supply #2 `+` and GND #2... So, in this case I have two GND on different potentials (and this difference may by very large).Any ideas ?Regards

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: April 21, 2011, 06:30:29 pm
content: What are your voltages? If they are below 60V you can useINA168to bring differential input to reference ground level.You don't need to measure current, just use a resistor divider to get reasonable input range.If your voltage is higher, use something likeAD629e.g. Fig.31. Again use just a voltage divider for input. This is good up to +-270V common voltage range.

author: leniwiec
author_rank: Contributor
timestamp: April 21, 2011, 06:49:57 pm
content: Thx for replay,AD629 looks good for me. One disadvantage it need symmetric supply, and I don't have it on my MCU board - but it is not a problem...And this price... huh!Regards!

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: April 21, 2011, 06:57:38 pm
content: Quote from: leniwiec on April 21, 2011, 06:49:57 pmTry AD628 but it has lower CV range.

author: leniwiec
author_rank: Contributor
timestamp: April 21, 2011, 07:05:45 pm
content: Thx Leo,I see in datasheet that INA168 have common mode voltage up to 60V so it fit in my needs. And INA168 is much cheaper than AD solution...