How exactly Digital Muilti-meter automatically set the range?!

[arvinfx]

Hi, I am designing a digital multi-meter and amp meter with PIC micro-controller ... I have to set +Vref in different levels (5V and 0.8V ) I wonder how I have to do it in hardware method not internal "reference voltage regulator" . I Think my answer is in method that have designed auto range DMMs digital multi-meters .

 

[Nigel Goodwin]

Why do you imagine you need to alter Vref?.

 

[arvinfx]

Why do you imagine you need to alter Vref?.

hi buddy
Maybe wrong I don't have any idea.But the problem is measuring small quantity of ampere with 10-bits A/D , Needs to reduce Vref to make it more accurate.
Please correct me if I wrong.

 

[Nigel Goodwin]

hi buddy
Maybe wrong I don't have any idea.But the problem is measuring small quantity of ampere with 10-bits A/D , Needs to reduce Vref to make it more accurate.
Please correct me if I wrong.

Use an opamp to increase the signal if needed - there are usually strict limits on how far you can alter Vref anyway, and even if it would go low enough you still wouldn't change it to change ranges, you'd set it low and simply alter the attenuators feeding from the input.

 

[Tony Stewart]

Normally variable gain by decades start at max and then reduce by detecting overflow from ADC until in range

 

[Nigel Goodwin]

I recall an auto-ranging chip many years ago (for some reason I was tasked with researching it, can't remember why?), and that worked by feeding the input directly in to the chip via a high value series resistor. The circuitry inside the chip then switched various resistor values to ground, making then the second part of the input attenuator, along with the input series resistor.

 

[arvinfx]

Thanks guys I got the way