Building a Voltmeter, need IC datasheet help

[kjnhu]

I am building an autoranging digital voltmeter as the subject line suggests and am having a problem with the ICL7107 chip that I am using. Looking at figure 15 on: http://www.intersil.com/data/fn/fn3082.pdf
specifically RefHI and RefLO, with a 2V max input difference between the pos and neg inputs. I do not necessarily have the negative input at 0 volts, and therefore would like assistance on how to use the Vref inputs. Do I need to set Vref=1V greater than the low input, or should it be 1V with repect to ground? Also why does the figure show a potentiometer rather than simply using resistors? Also should I just short the COM pin to my low input? I am unsure of how it actually functions after reading the cryptic explanation on the data sheet. Thank you for your assistance.
Kevin Hurley

 

[berkeman]

This is a duplicate post. It's been answered in the EE forum:

https://www.physicsforums.com/showthread.php?t=114954

 

[piercas]

Hello Kevin,

Thank you for reaching out for assistance with your project. Building a voltmeter can be a challenging task, and it's great to see you taking on this project.

Firstly, in order to properly use the ICL7107 chip, it is important to understand the function of the RefHI and RefLO inputs. These inputs are used to set the reference voltage for the analog-to-digital converter (ADC) within the chip. The ADC converts the analog input voltage into a digital value that can be read by the microcontroller or display.

In order to use the RefHI and RefLO inputs, you will need to set the reference voltage to be within the range of the input voltage. This means that if you have a 2V max input difference between the positive and negative inputs, your reference voltage should be set to 2V as well.

To answer your question about setting the reference voltage, it should be set with respect to ground. This means that the reference voltage should be 1V greater than the low input voltage. This will ensure that the ADC has a full range of 2V to work with.

Now, regarding the use of a potentiometer in figure 15 of the datasheet, this is used to adjust the reference voltage. This allows for more precise control over the reference voltage, which can be important for accurate measurements. However, if you are comfortable with using fixed resistors, you can certainly use them instead.

Lastly, the COM pin should not be shorted to the low input. This pin is used to connect the common (COM) ground of the input signals to the chip. This ensures that the input voltage is measured with respect to the same ground reference as the chip.

I hope this explanation helps you in understanding the function of the ICL7107 chip and how to properly use the RefHI and RefLO inputs. If you have any further questions, please don't hesitate to ask. Good luck with your project!

Best regards,