how to set external REF voltage for ADC & REF voltage min and max values

I want to use ADC of Netduino Plus2 board. I want to use 500mV as REFERENCE VOLTAGE in order to increase the resolution. Does Netduino Plus2 board let 500mV as a reference voltage? What is the min&max values? How can I set VREF? 

thanks in advance,

Mirac

double value = analogInput.Read() / 3.3 * 0.5;

If you want better resolution in the 0-500mV range and you are sure you are never going to get a reading above 500mV then have you thought about using an op amp to boost the signal into the 0-3.3V range?

Regards,

Mark

Dear Mark and Chris, many thanks for your interest and replies..

I want to change the Vref for the measurement. Yes, I know and consider to use an op amp to boost the signal. However, it is much more easy for the hardware design to set the Vref and get reading between 0 and 500mV.

Yes, I am sure I will never going to get a reading above 500mV. The limit is actually 100mV. However, there is no Voltage Reference component that provides 100mV micro low noise.

As a conclusion, I want to repeat my question; is it possible to use an external voltage reference, how and what are the limits?

many thanks,

Mirac

Hello again, I've found some information.

According to STM32F405RG datasheet page 388 (table.65), It says

''The higher/positive reference voltage for the ADC, 1.8 V ? VREF+ ? VDDA''.

Then, 1.8V is the lowest to set.

How can I set reference voltage value on the Netduino side?

many thanks,

Mirac

As a conclusion, I want to repeat my question; is it possible to use an external voltage reference, how and what are the limits?

As Chris mentioned, the board uses 3.3V as Vref.

Regards,

Mark

According to STM32F405RG datasheet page 388 (table.65), It says

...

How can I set reference voltage value on the Netduino side?

The LQFP64 package does not have V_REF pins exposed, you'd need a bigger package (e.g. LQFP100).

In addition to the above comments, I do support the Mark (Nevyn) suggestion. Even in case were possible to take the reference as low as 500mV, probably its precision would much lower than expected. A *real* ADC involves many sections which produce errors.

As is, the Netduino 2 ADC has a 12-bit resolution, but a practical resolution of no more than 10 bits. Using the Vref=3.3V and your full-scale signal (100mV), you'll lose other 5 bits (3.3/0.1=33, which is roughly 5 bits).

A good op-amp stage is maybe tedious, but it avoids a so high loss of precision. Most of the times the Vref is taken by a separate (dedicate) supply, calibrated and often thermo-compensated.

Please, this consideration is for a (semi) static signal. If you signal is somewhat repetitive and/or "noised" (e.g. Gaussian), you may oversample it and gain some bit back over the said loss.

Good luck!

Mario, thank you very much for your reply. But, I will not use Netduino for ADC reading. 

Instead of using op-amp (which means a lot of noise or a lot sampling for me & I need 3 opamps, because I need 3 different readings) I'm planning to use x3 ADS7822 with 400mV vref supplied by LT6650. I'll have 400mV/4096 resolution and noise reduction with relatively less cost. 

ciao from Trieste!

many thanks, 

Mirac

Mario, thank you very much for your reply. But, I will not use Netduino for ADC reading. 

 

Instead of using op-amp (which means a lot of noise or a lot sampling for me & I need 3 opamps, because I need 3 different readings) I'm planning to use x3 ADS7822 with 400mV vref supplied by LT6650. I'll have 400mV/4096 resolution and noise reduction with relatively less cost. 

 

ciao from Trieste!

 

many thanks, 

Mirac

That's a good decision!

Trieste? Will you join the incoming mini-Maker Faire, next Saturday?

http://www.makerfair...t/index_en.html

I believe to be there on the afternoon!

Ciao

No, I did not know about it. I'll check and see..

ciao