Question on ADC

[FreshmanNewbie]

I want to measure negative voltages also using this part.

My positive reference is +3.3V.
I want my negative reference to be -3.3V.

I need to give +3.3V and -3.3V to +Vref and -Vref respectively right?

But from this table,

it is mentioned as Reference voltage range (Vref+ - Vref-) should not exceed Avdd (which is 3.3V in my case).

So, how to calculate (Vref+ - Vref-) in my case?

What is the maximum reference I can give to both the external reference voltage pins?

Can someone give clarity?

 

[nitishn5]

You need a True Bipolar ADC
Something like this https://docs.rs-online.com/47ff/0900766b80e41b26.pdf

Or shift/translate your input to the levels of the ADC that you have using some bipolar opamps and such.

 

[danadakk]

Page 30 seems to clarify what you can do with this.

https://ww1.microchip.com/downloads/aemDocuments/documents/APID/ProductDocuments/DataSheets/MCP3561_2_4R-Data-Sheet-DS200006391C.pdf

Your analog inputs are constrained to this :

and this :

If you need large negative common mode than you have to offset the input. Think you have a
desired CM range of -3.3 to + 3.3, thats a symmetrical window about ground. So you have to
shift that entire range above the AVdd ground V. In turn since you have a desired total range >
Avdd allowed you have to scale that to meet the total dynamic range allowed on the pin.


Regards, Dana.

 

[KlausST]

Hi,

you say "this" ADC. But provide a datasheet for a couple of ADCs with different silicon chip and different packages.
--> Please tell us which ADC exactly you want to use: full ordering code.

You say you want to measure negative voltages.
--> please tell your exact input voltage range. Single ended, differential?

--> Why don´t you want to use internal VREF?

I want my negative reference to be -3.3V.

Not possible.
-VRef should be close to AGND.

I need to give +3.3V and -3.3V to +Vref and -Vref respectively right?

No.
Usually AGND to -VRef
and 3.3V to +VRef

Btw: I don´t recommend to use +VRef = 3.3V while AVDD is 3.3V, too. I recommend some headroom. AVDD > +VRef

****

So, how to calculate (Vref+ - Vref-) in my case?

The problem is that you are using a fully differential input ADC but you think as "single ended input ADC".

for this ADC: No ADC input is recommended to be negative!
but for differential inputs the difference may be negative.

Example: CH0 = 2.0V; CH1 = 3.0V (each single input is positive)
But when you perform a differential (CH0 -CH1) conversion, the result is -1V (negative)

What is the maximum reference I can give to both the external reference voltage pins?

We can only give the same values that are given in the datasheet.

Klaus

 

[FreshmanNewbie]

Hi,

you say "this" ADC. But provide a datasheet for a couple of ADCs with different silicon chip and different packages.
--> Please tell us which ADC exactly you want to use: full ordering code.

You say you want to measure negative voltages.
--> please tell your exact input voltage range. Single ended, differential?

--> Why don´t you want to use internal VREF?


Not possible.
-VRef should be close to AGND.


No.
Usually AGND to -VRef
and 3.3V to +VRef

Btw: I don´t recommend to use +VRef = 3.3V while AVDD is 3.3V, too. I recommend some headroom. AVDD > +VRef

****

The problem is that you are using a fully differential input ADC but you think as "single ended input ADC".

for this ADC: No ADC input is recommended to be negative!
but for differential inputs the difference may be negative.

Example: CH0 = 2.0V; CH1 = 3.0V (each single input is positive)
But when you perform a differential (CH0 -CH1) conversion, the result is -1V (negative)


We can only give the same values that are given in the datasheet.

Klaus

Can you tell why should the -Vref be close to GND?

 

[danadakk]

Keep in mind translating inputs like this, and keeping accuracy unaffected,
is not trivial. An IA front end maybe makes sense.

This tool may be of help :

ADI-DiffAmpCalc | Analog Devices

ADI's new ADI DiffAmpCalc™ is a free, downloadable calculator for designing differential amplifier circuits. The tool is easy to use and features an interactive user interface to quickly get you up and running.

www.analog.com

Regards, Dana.

 

[FreshmanNewbie]

Hi,

you say "this" ADC. But provide a datasheet for a couple of ADCs with different silicon chip and different packages.
--> Please tell us which ADC exactly you want to use: full ordering code.

You say you want to measure negative voltages.
--> please tell your exact input voltage range. Single ended, differential?

--> Why don´t you want to use internal VREF?


Not possible.
-VRef should be close to AGND.


No.
Usually AGND to -VRef
and 3.3V to +VRef

Btw: I don´t recommend to use +VRef = 3.3V while AVDD is 3.3V, too. I recommend some headroom. AVDD > +VRef

****

The problem is that you are using a fully differential input ADC but you think as "single ended input ADC".

for this ADC: No ADC input is recommended to be negative!
but for differential inputs the difference may be negative.

Example: CH0 = 2.0V; CH1 = 3.0V (each single input is positive)
But when you perform a differential (CH0 -CH1) conversion, the result is -1V (negative)


We can only give the same values that are given in the datasheet.

Klaus

Internal reference voltage is only limited to 2.4V, right? Hence, I want to use the reference till 3.3V. Hence, I am using external reference.

--- Updated Jan 11, 2024 ---

Keep in mind translating inputs like this, and keeping accuracy unaffected,
is not trivial. An IA front end maybe makes sense.

This tool may be of help :

ADI-DiffAmpCalc | Analog Devices

ADI's new ADI DiffAmpCalc™ is a free, downloadable calculator for designing differential amplifier circuits. The tool is easy to use and features an interactive user interface to quickly get you up and running.

www.analog.com

Regards, Dana.

So, I cannot give negative voltages to any input ADC pin on this IC, right?

In that case, I understand that accepting differental input is not the same as giving negative inputs.

So, what's the point of giving differential input to the ADC? What should be the differential input range? How different is it from single ended ADC?

 

[KlausST]

Hi,

Can you tell why should the -Vref be close to GND?

just look at the specifications you provided in post#1:
the minimum voltage for -VRfef is AGND. This means zero. No negative value is recommended.
So you could use a positive value ... but this just limits your analog signal input voltage range. So why?

or: read the section 3.1

or: look at Figure 7-1.

Klaus

--- Updated Jan 11, 2024 ---

Internal reference voltage is only limited to 2.4V, right? Hence, I want to use the reference till 3.3V. Hence, I am using external reference.

Pleae understand that I respond only after you answered my first two questions in post#4.

In that case, I understand that accepting differental input is not the same as giving negative inputs.

These are two different things.
* differentail input vs single ended input
* negative input vs positive input

You theoretically may have all combinations of them.

Klaus

 

[FreshmanNewbie]

Hi,

just look at the specifications you provided in post#1:
the minimum voltage for -VRfef is AGND. This means zero. No negative value is recommended.
So you could use a positive value ... but this just limits your analog signal input voltage range. So why?

or: read the section 3.1

or: look at Figure 7-1.

Klaus

--- Updated Jan 11, 2024 ---

Pleae understand that I respond only after you answered my first two questions in post#4.


These are two different things.
* differentail input vs single ended input
* negative input vs positive input

You theoretically may have all combinations of them.

Klaus

Sure, the device is MCP3564R.

And the input voltage range I want to measure is +3.3V to -3.3V.

--- Updated Jan 11, 2024 ---

Sure, the device is MCP3564R.

And the input voltage range I want to measure is +3.3V to -3.3V.

Hi,

just look at the specifications you provided in post#1:
the minimum voltage for -VRfef is AGND. This means zero. No negative value is recommended.
So you could use a positive value ... but this just limits your analog signal input voltage range. So why?

or: read the section 3.1

or: look at Figure 7-1.

Klaus

--- Updated Jan 11, 2024 ---

Pleae understand that I respond only after you answered my first two questions in post#4.


These are two different things.
* differentail input vs single ended input
* negative input vs positive input

You theoretically may have all combinations of them.

Klaus

Can you tell me since the internal reference is only 2.4V. I don't want to use. That's why I am using ext reference. Why is the internal reference not equal to Vdd?

 

[danadakk]

Roughly speaking you want to attenuate the 3.3 -_(-3.3) = 6.6V range to 3.3V,
and offset it to 0 - 3.3. And do this with precision if you want to maintain
the 24 bit performance. This is over simplification as you have noise considerations,
gain considerations (over T and V)......

IA Handbook :

https://www.analog.com/media/en/training-seminars/design-handbooks/designers-guide-instrument-amps-complete.pdf

Regards, Dana.

 

[FreshmanNewbie]

Roughly speaking you want to attenuate the 3.3 -_(-3.3) = 6.6V range to 3.3V,
and offset it to 0 - 3.3. And do this with precision if you want to maintain
the 24 bit performance. This is over simplification as you have noise considerations,
gain considerations (over T and V)......

IA Handbook :

https://www.analog.com/media/en/training-seminars/design-handbooks/designers-guide-instrument-amps-complete.pdf

Regards, Dana.

Can you tel me how to scale the 6.6V range to 3.3V range?

 

[KlausST]

Roughly speaking you want to attenuate the 3.3 -_(-3.3) = 6.6V range to 3.3V,
and offset it to 0 - 3.3.

correct .. as long as he wants single ended measurement.

But not if he wants differential measurement. I don´t want to ask again.

Klaus

 

[danadakk]

Do you want to do a single ended measurement translating an
input ( -3.3 to + 3.3 ) to ( 0 to +3.3 ), or a differential measurement
because you have common mode to get rid of ?

Bipolar to unipolar, single ended :

Summing Amplifier Calculator – Mastering Electronics Design

masteringelectronicsdesign.com

Of course if you want to maintain absolute accuracy those R's have to be very high precision,
and the OpAmp you pick very high G at the BW you want to measure.

Great basic ap note on doing high precision work :

https://www.analog.com/media/en/technical-documentation/application-notes/294542582256114777959693992461771205AN280.pdf

Regards, Dana.