STM32 controller -0.4V to 3.6V ADC converter

[Benjamin Brammer]

Hello,

for an upcoming project I am searching for a low-power microcontroller with a rich peripheral set and the possibility to sample low frequency signals in the range from -0.4V to 3.6V .

As far as I have looked it up I will have no chance realising this project with a STM32L4x but maybe somebody has a good solution suggestion for me on that matter.

I preferably want to avoid an extra ADC chip.

best regards

Benjamin

[Uwe Bonnes]

If the source you want to measure, you could use well choosen resistors to divide and offset -0.4 .. 3.6 V to 0.. Vref. If you have 5 Volt accessible and are willing to add two OPs, the attentiuate/shift can also be done with a high impednace source.

[Benjamin Brammer]

but even if i shift the voltage I don't have a full scae range of 4V with the STM32. Off course i would use an opamp for buffering and later a filter

[Uwe Bonnes]

The range was the reason why I proposed some attentuation...

[Benjamin Brammer]

ah ok. sorry then I misunderstood you. So if I would say I have this given voltage range and boost it up to be 5V instead of 4V and I want to scale 5V to a 3.6V range, I could use a division faktor of 1.4 and offset the opamp input with the most negative voltage possible. So in this case -0.4V/1.4 = -0.286V --> offset of 0.3V. Off course I need to take care every possible tolerance and accuracy to have a good headroom, right?

[Dvorak.Peter]

This is too simple.

" signals in the range from -0.4V to 3.6V"

A 3.6V powered controller only needs 2 external resistors.

With the ADC full scale = 3.6V you need to attenuate the signal to 3.6/4

If your signal source is low impedance then use 1 kohm resistor from input signal to ADC pin.

Add a 9kohm resistor from the ADC input to the Vcc to pull up the -0.4V to 0V at ADC pin. ( 3.6/0.4 *1k = 9kohm)

 If the controller supply is lower, you will need a third resistor for ADC pin pulldown.

[Piranha]

I'm wondering from where does the -0.4V to 3.6V range come. The thing is... it really looks like a misunderstood Absolute Maximum rating for some 3,3V chip.

[S.Ma]

use an op-amp to scale and shift the signals within operating range of the ADC.

The op-amp will also act like a follower by lowering the impedence required by the ADC.

Ideally get a calibrated reference onto another ADC channel.

[Benjamin Brammer · Accepted solution]

hello everyone,

thanks for all the possible solution suggestions. So far I will stick to a version using two different opamps. There is an excellent application note which describes correct gain and offset attenuation for this task by TI . I will scale from -0.4 to 3.6 --> 0 to 5V and then back --> 0 to 3.6V. Maybe I will leave it at 0 to 5V and use an external ADC, I don't know yet.

My specified range is no error it is correct. That has to do with the special project requirements.

best regards

Benjamin