DC Level Shifter from 0,5-4,5V to 0-3,3V?

[Javert]

Hello,
I have sensor with output level 500mV - 4500mV and ADC 12bit 0-3,3V + Amplifier with +5V voltage for power and DAC 12bit 0-3,3V.

Any simple idea how to shift the sensor output from 0 ?

 

[KlausST]

Hi,

just use a resistive voltage divider 4.5V --> 3.3V

And don´t care about the 0.5V at the analog side.
Just subtract it on the digital side with software.

Loosing 0.3V of analog input range won´t hurt.

Klaus

 

[Javert]

So I have it now, an amplifier like a buffer and a resistive divider on the output.
I was wondering if there was a cheap and easy way not to lose the 500mV range

I apologize for the duplicate post, internet outage, or I see service not available.

 

[KlausST]

Hi,

one time you have a input referred resolution of 4.5V / 4096 = 1099uV
the other time (4.5 -0.5)V / 4096 = 977uV
You really care about loosing 122uV of resolution? (this is what you really gain with an OPAMP solution)

The simple resistor diver solution is stable with not much added error.
An OPAMP adds new errors like
* OPAMP noise
* OPAMP offset and offset drift
* Power supply introduced (PSRR)
* ...

So what OPAMP do you decide to use? It should be a good one not to "overcompensate" the improved resolution.

And for sure you need an ADC that does not cause higher errors that the gained 122uV.
A microcontroller_included one often is worse: Check: offset drift, INL, gain drift (including Ref).

I don´t want to say an Opamp stage is nonsense. It has it´s benefits. But be sure it´s worth the effort.

***
Almost forgotten: You asked about a cheap and easy way.
Don´t know what´s cheap enough or easy enough for you...But if you do this: Use the ADC_VRef to generate it. A voltage divider as some input to the inverting side of the OPAMP.
OPAMP circuit: I often recommend the difference amplifier circuit with 2 x 2 equal resistors. It is easy to calculate gain and to add/subtract offset.
I´ve discussed this several times before here in this forum. (Try a forum search)

Klaus

 

[BradtheRad]

Since desired voltage drop is small it can be done with a series diode (cheap and easy).

Comparison with vs without.



The potentiometer was adjusted so output is less than 3.3 V.

It's possible to scale readings so they occupy your desired range (0-3.3) -- However response is not necessarily linear down close to 0V.

 

[carpenter]

Thank you all for your suggestions.

1. Use diode.
As an interesting idea, the temperature coefficient of a silicon diode is 2mV / C and it would then not be measured, but divination from the coffee grounds.

2. Use differential amplifier.
I'll think about it, I have 1/2 buffer MCP6022 there and I'll consider how many errors would its connection insert as differential amplifier.

500mV is approximately 11% of range. I have to consider how big the error inserts 4pcs 1% resistors in the amplifier + leaning the reference voltage on the lousy amplifier in STM32F303.

What is really going on?

I have 150PSI Air pressure sensor 500mV - 4,5V, for calibration have BME280 and calibration point on atmospheric pressure.
Yes, it's not worth solving.

 

[danadakk]

An obvious solution is to use an IA (Instrumentation Amplifier) that has
factory trimmed G and offsets, etc..

Analog Devices has extensive line of them. Here is a ref handbook -

https://www.analog.com/media/en/tra.../designers-guide-instrument-amps-complete.pdf

Here is a collection of their tools for working with diff amps as well as other calculators -

https://www.analog.com/en/design-center/design-tools-and-calculators.html


Another approach is to use a thin film R array for G setting and use your
own OpAmp. There are some standard off the shelf arrays for specific gains.

For future reference there are UP's out there with delsig converters in them,
to 20 bits, that you could use to manage the G (they have front ends with G)
and use a software controlled algorithm to do a cal cycle on a R ratio generated
offset to pull out errors. They have DACs to gen the offset, even some have on
board precision Vref.

Regards, Dana.

 

[schmitt trigger]

I also recommend a simple resistor divider and subtracting the offset in software. Even with precision (0.1% tolerance) resistors, it will be low cost, very accurate and occupy essentially no board area.

Any opamp based implementation would have to incorporate a very low Vos device, which are not cheap.

But this is your circuit, and your implementation decision.