Not possible.I want my negative reference to be -3.3V.
No.I need to give +3.3V and -3.3V to +Vref and -Vref respectively right?
The problem is that you are using a fully differential input ADC but you think as "single ended input ADC".So, how to calculate (Vref+ - Vref-) in my case?
We can only give the same values that are given in the datasheet.What is the maximum reference I can give to both the external reference voltage pins?
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.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
So, I cannot give negative voltages to any input ADC pin on this IC, right?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.
just look at the specifications you provided in post#1:Can you tell why should the -Vref be close to GND?
Pleae understand that I respond only after you answered my first two questions in post#4.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.
These are two different things.In that case, I understand that accepting differental input is not the same as giving negative inputs.
Sure, the device is MCP3564R.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 ---
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.
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?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 ---
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
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 :
Regards, Dana.
correct .. as long as he wants single ended measurement.Roughly speaking you want to attenuate the 3.3 -_(-3.3) = 6.6V range to 3.3V,
and offset it to 0 - 3.3.
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