IC7107 Confusion

[bborisov567]

I am trying to understand how the 7107 works. The main confusion comes from the GND (21) and COMMON (32) pins. What is the difference between theese pins? Also i see dual supply on some schematics and single on others. Is the dual supply only required when i want to have negative reading? For exapmle if i want to measure only positive voltage with the same reference as the 7107's power supply should i connect V-, GND, COMMON, VREF-, VIN- all to ground, VREF+ ot V+ an VIN+ will be my input? Then the full scale will be my supply voltage, rigth? I tried to find some materials other than the datasheet but could find anything explaining it. Link to the datasheet, i read it probably like 10 times but still can't see the whole picture:

https://www.renesas.com/us/en/document/dst/icl7106-icl7107-icl7107s-datasheet

[Kim Christensen]

Is the dual supply only required when i want to have negative reading?

Looks like the dual supply is for ones with a LED display and LCD uses a single supply as depicted on page 4. Note the different function of pin 21 on the LCD (7106) vs LED (7107) versions.

The main confusion comes from the GND (21) and COMMON (32) pins. What is the difference between theese pins?

Pages 6&7 of the datasheet describe this quite well. I've attached the datasheet since your link seemed to hang for me. In short, the common & inLO pins allow for reading of positive and negative voltages applied between the inHI and inLO pins because they are floating between the positive and negative supply rails.

[bborisov567]

What about the dual supply? As far as i uderstand from the datasheet it is need for the internal reference and i can use single supply but i will need to provide external voltage reference? Also i came across this schematic using 7905 for generating negative supply. The way 7905 is connected seems a little bit sketchy to me. What do you think about?

[Kim Christensen]

As far as i uderstand from the datasheet it is need for the internal reference and i can use single supply but i will need to provide external voltage reference?

Yes, if you use a single supply with the 7107, then you'll need to provide an external reference.

The way 7905 is connected seems a little bit sketchy to me. What do you think about?

Yup... Looks like they have the input polarity on the 7905 backwards. (Pin 2 should be more negative than pin 1 for proper operation) Doesn't look like that will work.

The datasheet on page 10 shows how to use a CD4009, some diodes, and capacitors to generate the -5v rail (Well, -3.3V which is good enough). Something like a 74HC04 could possibly work in place of the CD4009 as well.
How will you be powering this project anyway? Mains? Battery?

[bborisov567]

The project is a digital thermometer. I intend to use LM335 as a sensor, so  it will be pretty much a voltmeter. It will be powered from mains voltage, probably 5volt adapter. I saw the inverter circuit but i think it will be easy to provide external reference. Decide to try the following configuration. I used Fig.12 and Fig.16 as a guide.   I know that the divider is not a stable reference but it is just for the test. Unfortunately it just shows zeros...

Also i decided to try the LM335 but i am getting around 0.7 volts at room temperature. I am usign 4.7k resistor and 5v supply.

[Kleinstein]

AFAIR there is a limited common mode range for the inputs like the reference and input voltage. So in connected to V- may not work.

[bborisov567]

What configuration should i use if i want to build a voltmeter, powered by a single supply whose input is referenced to ground?

[Kim Christensen]

If I was running it off mains power, and didn't want to add the little -5v circuit, I would simply use an AC adapter. Then I'd rectify, filter, & regulate both positive and negative supplies.