Use of external ADC to reduce noise and increase speed

Thread Starter

artmaster547

Joined Jan 6, 2016
409
Hi All
I had a quick query about the use of external ADC's connected to a uC via SPI/I2C vs using on board ADC's on a uC. Would using external ones located right near the analogue signal provide a cleaner ADC conversion, also would this effect the speed or reading in anyway, or would having a long track that goes to an ADC pin on a uC be slower and more prone to noise?
I also had a question about sampling a voltage from a current sensor that has a bandwidth of 400kHz to filter this what kind of cut off frequency should I use to remove noise from this signal (voltage is about 5V max)

kind regards

Art
 

cmartinez

Joined Jan 17, 2007
7,745
Hi All
I had a quick query about the use of external ADC's connected to a uC via SPI/I2C vs using on board ADC's on a uC. Would using external ones located right near the analogue signal provide a cleaner ADC conversion, also would this effect the speed or reading in anyway, or would having a long track that goes to an ADC pin on a uC be slower and more prone to noise?
I also had a question about sampling a voltage from a current sensor that has a bandwidth of 400kHz to filter this what kind of cut off frequency should I use to remove noise from this signal (voltage is about 5V max)

kind regards

Art
Hello, Art. What kind/brand of MCU are you using?
 

MrChips

Joined Oct 2, 2009
26,478
Too many questions with too little information.

What is the amplitude and frequency of the signal you wish to digitize?
What is the amplitude and frequency of the noise?
What is the resolution you wish to achieve?
Which ADC and MCU are you considering?
What would be the sampling frequency?
 

Thread Starter

artmaster547

Joined Jan 6, 2016
409
Too many questions with too little information.

What is the amplitude and frequency of the signal you wish to digitize?
What is the amplitude and frequency of the noise?
What is the resolution you wish to achieve?
Which ADC and MCU are you considering?
What would be the sampling frequency?
A lot of these things aren't really concrete I mainly just wanted a brief overview if there is much improvement when using an external one , and whether it's worth digging into these details
 

dendad

Joined Feb 20, 2016
4,167
With proper design, an external ADC can give better performance, For example, more bits for higher resolution. And a better voltage reference can be used. Often though, the conversion speed may suffer. Having an external ADC can allow it to be opto-isolated too and that could be an advantage.
Multiple ADCs for more channels than the internal ones can be an option as well. There are many reasons why you may go external
But it all depends on what you need from your ADC.
 

Thread Starter

artmaster547

Joined Jan 6, 2016
409
With proper design, an external ADC can give better performance, For example, more bits for higher resolution. And a better voltage reference can be used. Often though, the conversion speed may suffer. Having an external ADC can allow it to be opto-isolated too and that could be an advantage.
Multiple ADCs for more channels than the internal ones can be an option as well. There are many reasons why you may go external
But it all depends on what you need from your ADC.
at this point it is more about speed reassessing the system, so using the internal ones for speed is the better option?
 

dendad

Joined Feb 20, 2016
4,167
at this point it is more about speed reassessing the system, so using the internal ones for speed is the better option?
It does depend a lot on your system. But the external ADCs will have the added overhead on the serial interface to cope with. Even so, that can be very fast.
 

ebeowulf17

Joined Aug 12, 2014
3,291
I also had a question about sampling a voltage from a current sensor that has a bandwidth of 400kHz to filter this what kind of cut off frequency should I use to remove noise from this signal (voltage is about 5V max)
That depends on what you're trying to see with the current sensor. If you're looking at average power use on appliances over time, you probably don't need anything above the 50/60Hz fundamental and can filter aggressively. If you're looking at subtleties of wave shapes and pulse characteristics, you'll need a lot more bandwidth. One person's noise is another person's signal!
 

Thread Starter

artmaster547

Joined Jan 6, 2016
409
That depends on what you're trying to see with the current sensor. If you're looking at average power use on appliances over time, you probably don't need anything above the 50/60Hz fundamental and can filter aggressively. If you're looking at subtleties of wave shapes and pulse characteristics, you'll need a lot more bandwidth. One person's noise is another person's signal!
Ok great thank you for clarifying
 

Thread Starter

artmaster547

Joined Jan 6, 2016
409
Nice chip.

The internal ADC will be hard to beat for raw speed without some really nice external hardware on the 50MHz SPI.
Ok great thanks for clarifying, I do have one question about filtering the current sensor I am using (Sensitec CDS4000) has a Upper cut-off frequency of 400kHz, so what kind of filtering circuit should I use to get a really clean signal, amount of circuitry and cost isn't an issue at this moment
 

OBW0549

Joined Mar 2, 2015
3,566
Ok great thanks for clarifying, I do have one question about filtering the current sensor I am using (Sensitec CDS4000) has a Upper cut-off frequency of 400kHz, so what kind of filtering circuit should I use to get a really clean signal, amount of circuitry and cost isn't an issue at this moment
There is no way for us to advise you on that without knowing what your bandwidth of interest is-- that is, what is the highest frequency in your signal that is important in your application. (What IS your application, anyway? It would help if we could know that.) The bandwidth of the sensor itself is unimportant, provided it is sufficient.
 
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