IC7107 Confusion - Page 1
Forum section: electronics / beginners

author: bborisov567
author_rank: Regular Contributor
timestamp: February 09, 2023, 07:32:53 pm
content: 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

author: Kim Christensen
author_rank: Super Contributor
timestamp: February 09, 2023, 09:21:03 pm
content: QuoteLooks 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.QuotePages 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.

author: bborisov567
author_rank: Regular Contributor
timestamp: February 10, 2023, 03:58:51 pm
content: 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?

author: Kim Christensen
author_rank: Super Contributor
timestamp: February 10, 2023, 05:27:44 pm
content: QuoteYes, if you use a single supply with the 7107, then you'll need to provide an external reference.QuoteYup... 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?

author: bborisov567
author_rank: Regular Contributor
timestamp: February 10, 2023, 05:51:24 pm
content: 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.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 10, 2023, 07:06:49 pm
content: 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.

author: bborisov567
author_rank: Regular Contributor
timestamp: February 10, 2023, 07:34:25 pm
content: What configuration should i use if i want to build a voltmeter, powered by a single supply whose input is referenced to ground?

author: Kim Christensen
author_rank: Super Contributor
timestamp: February 10, 2023, 07:42:49 pm
content: 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.