Relatively simple questions about ADCs

Thread Starter

MindDrinker

Joined Apr 20, 2016
1
Hi Everyone,

I have a couple questions about ADCs and I couldn't find any straight forward answers to them during my googling.

1) If I have a 12 bit ADC whose reference voltage is 3.3v, is there a way to accurately measure a voltage higher than 3.3v without using a voltage divider at the input?

2) If there is not other way, is there a way to design a voltage divider to work with variable voltages and still get the same result (eg. 5v to 3.3v AND 8v to 3.3v with the same divider)?

3) Do you have any good online resources besides AAC to help me and/or expand my knowledge?

Thank you!
 

BobTPH

Joined Jun 5, 2013
8,813
1. Yes, you could scale it with an opamp.

2. Yes, design for the highest voltage it needs to handle, but you will lose accuracy when the range is lower than you designed for.

Bob
 

danadak

Joined Mar 10, 2018
4,057
1) If I have a 12 bit ADC whose reference voltage is 3.3v, is there a way to accurately measure a voltage higher than 3.3v without using a voltage divider at the input?
No, somehow you must attenuate.

2) If there is not other way, is there a way to design a voltage divider to work with variable voltages and still get the same result (eg. 5v to 3.3v AND 8v to 3.3v with the same divider)?
Yes a multitap divider, but that implies an analog mux on the input side of A/D.

Or a divider with switched R's to ground that alter the divide ratio. Like open drain
outputs on a UP to do the switching. This scheme implies you have a single input
whose V takes on either 3.3 or the higher V. If you did this you would add a clamp
to A/D input so that when on low range and the V goes high you do not destroy the
A/D. Note using open drain outputs adds error tot he measurement due to their T
sensitive Rdson affecting divide ratio.

There are many UPs out there with analog muxes.

Whats the issue you have using general dividers ?

Another approach entierly is use a series R LED combo connected to Vx
and use a photocell to measure light output. Use a cal routine to establish
the ratio between Iled and Vx. Possibly an analog photo coupler. Magnetic
approaches also could be used. Many possibilities.

Regards, Dana.
 

danadak

Joined Mar 10, 2018
4,057
The divider chain will generate < 5V for each input, so yes it will work. The
divider design controls this.

Regards, Dana.
 

DECELL

Joined Apr 23, 2018
96
Scale with an opamp and switch for Fscale 5V 0r 8V. If you buffer the input to the divider its even better.
Use precision components for best results. The switch could be an analog one as long as Ron is small.

upload_2018-4-30_11-15-18.png
 

Attachments

danadak

Joined Mar 10, 2018
4,057
Keep in mind your ADC LSB ~= .8 mV. So -

1) Divider accuracy
2) OpAmp offset
3) OpAmp Bias current thru divider Rs
4) OpAmp drift
5) OpAmp PSRR

All contribute to inaccuracy, always do a signal path end-to-end
error analysis to make sure you have met your goals for accuracy
and error.

Regards, Dana.
 
Last edited:
Top