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# How to capture 8 bipolar analog inputs simultaneously using STM32MP157D device?

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#### mre22

##### Newbie
Since STM32MP157D requires positive analog input, I have added a fully differential amplifier to capture the negative inputs. STM32MP157D devices embed two analog-to-digital converters.

I have attached a diagram that explains my problem. I need to capture the 8 bipolar analog inputs simultaneously to get accurate phase information.

What is the best and optimal way of achieving this?

#### Attachments

• analog.png
153.5 KB · Views: 205

Hi,

If you want to know about phase then the DC offset is not of interest ... as long as the signal dose not clip.

DC shift = offset = a DC parameter
Phase = AC parameter.

Just for measuring phase I'd use a series capacitance with DC biasing on the ADC side.

Klaus

Question : Capture to what resolution and sample rate and phase error ?

Classically one uses sample/hold to effect simultaneous sampling.

Here is one for example, but not knowing the above questions .....

Regards, Dana
--- Updated ---

Since STM32MP157D requires positive analog input, I have added a fully differential amplifier to capture the negative inputs. STM32MP157D devices embed two analog-to-digital converters.

I have attached a diagram that explains my problem. I need to capture the 8 bipolar analog inputs simultaneously to get accurate phase information.

What is the best and optimal way of achieving this?

A simple way to level shift is to use a R divider to a positive reference. Of course the
R's have to be precise (same order as your A/D). And not being buffered other effects
you have to consider, like divider Z impacts on A/D front end performance..

Attached is a calculator.

Regards, Dana.

#### Attachments

• TI Voltage Divider.zip
302.3 KB · Views: 197
Last edited:
Hi,

If the problem is "simultaneous samplig"
Then - to get useful phase values- all input signal need ti have same signal frequency.
If so, you still may use sequential sampling, with known fixed sampling timing.
..because you know the phase shift (timing) from one channel to the other. So a simple mathematical "subtract" function can replace high effort in hardware.

But first please explain the problem exactly. I don't know what to focus on.

Maybe a "lock in amplifier" or a "controlled rectifier" gives the more precise result, while it makes the ADC measurement less critical.

Missing information: "precision" or "accuracy"? Signal frequency and waveform? Sampling freqyency?
Expected precision/accuracy? In value with unit.

Klaus

One additional comment on using R network to shift negative voltages into
ADC input, you can always relax their precision by using an array of analog
switches to feed a know Vref thru the S/H signal path to cal out the errors
due to R ratios.

Sequential sampling, if you are not seeking cycle to cycle data, is perfectly
acceptable. But if your requirements are instantaneous values within a cycle, at
a specific phase value, then that method not useful.

So you have to clarify what you are seeking.

Regards, Dana.

Thank you all. Like you said, a few microsec delay won't have a significant effect on the phase.
I was thinking of starting from the sample design I attached and then go for other solutions like external ADC etc like you suggested.
I am using the differential amplifier to increase the input signal capture range.
Is there any problem with my design? If so, how can I make it better?
Any other simple and cost effective way to do this? Thank you again.

#### Attachments

• design1.png
191 KB · Views: 176

Hi,

I may have missed some posts.
* I can´t see where someone talked about "microseconds delay"

Klaus

Hi,

I may have missed some posts.
* I can´t see where someone talked about "microseconds delay"

Klaus
I mentioned the microsec delay because that much phase shift doesn't affect the signals I'm trying to capture.
I was just trying to indicate my starting point using a multiplexer and then go for any other solution like Sample and hold, external ADC etc. I hope you would direct me in the right direction. The signals have the same frequency as I tried to indicate in the attached file. Sorry if I created a confusion.

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