measure ohm with 7107

[tamba]

Hi all
I have some extra ICL7107s so I decided to use one of them to measure resistance.I also have precision resistors with a tolerance of .1%.
Since the 7107 is very similar to the 7106, I got some ideas from a number of 7106-based multimeter diagrams, but none of them work properly on the 7107.
The range I need is from 200 ohms to 20 megaohms.
Does anyone have a practical circuit for the 7107 for this purpose?

[coromonadalix]

you have many meters schematics based  on 7106 07 variants on the web

http://improwis.com/projects/reveng_multimeters/schema-7106/

https://www.hobby-hour.com/electronics/icl7106.php

some models here


you need the ohm meter section  aka  current generator      since  theses 3.5 digit ic's are  "volt readers"

[Kleinstein]

The 7106/7 ADCs have a fully differential reference input. They can uses the DUT and reference resistor in series (and with some additional protection in series) and than get the ADC input from the DUT and the reference voltage from the voltage over the ref. resistor.  For the 20 M range this can be a bit tricky and require low leakage capacitors at the reference.

So no constant current source required, but more measuring the current via the ref. resistor.

The common mode voltage may need some care and can be a bit different between the 7106 and 7107.
There may well be a virtual ground for the common input terminal - this may be confusing.


Chances are it would be easier to get a ready made low end DMM.

[magic]

Since the 7107 is very similar to the 7106, I got some ideas from a number of 7106-based multimeter diagrams, but none of them work properly on the 7107.

Not sure what you tried and how it failed.

Start with basics, read the whole datasheet, make sure power supplies are OK, check if it can measure small voltage with REF- and IN- both at ground. IIRC, the standard circuit used in multimeters was simply a series chain: V+ --- some current limiting resistance --- precision reference resistor --- external DUT --- GND. The REF pins went across the reference resistor and IN pins across the DUT.

Make sure you have enough voltage headroom for the integrator to work (it may need positive, negative or both supplies), correct integration resistor and capacitor values. See the datasheet for details, I no longer remember that stuff and can't tell more right now.

[tamba]

thank for the answers
i have seen all circuits in improwis - my reference circuit was this one

http://improwis.com/projects/reveng_multimeters/schema-7106/ms8222g-.gif

but did not work properly on 7107

also this circuit did not work on 7107

http://improwis.com/projects/reveng_multimeters/schema-7106/vc890d-.gif

the only 7107 base multimeter that i found is here

https://downloads.elv.com/downloads/journal/elvjournal_1980_08.pdf

but the problem is that circuit have 1 M ohm input resistor and is configured on 2 v full scale

my config is 200mv full scale and 10M ohm input impedance

has any one another circuit based on 7107???

[Kleinstein]

The ELV meter is using seprate current sources in an not not good way. OK for only 3.5 digits, but still quite some effort.

The VC890 circuit looks sensible with the ohms part, but odd with the power supply.
The type of circuit should also work with the 7107.

With all the switches the drawings are hard to read and one may have to redraw the basic / simplified ciruit.
It is easy to make a mistake here when transfering to the own circuit.
The main difference from the 7106 to 7107 is in the supply and display.

What supply is used with the 7107 ?

[coromonadalix]

no offense   but  you have very cheap low cost voltmeters who will do what you need at better resolutions and portability ? 

i know  you want to do your own things, but parts, time and design ...   it will not be an economical solution


7106  7107   are way outdated, and now mostly used in volt or amps  panel meters,  even now, more modern mcu's will do better

7107 needed a +5vdc and a -5 vdc supply, or even used some oscillator tricks to get negative voltage from a single 5vdc, or used some 7660 ic to generate it

the 10 Meg input impedance is mostly done  by the voltage divider resistor network,  you add all of them and you have your 10 megs

the basic designs of a 710x   is 1 meg with some added capacitance on input +       you will never get the 10 megs of impedance ....     

you add the decimal point selector and the range divider for 200mv or 2v scale that's all,

you add other stuff / functions with other parts, alone the 710x  are not ohms meters ... they are 1999 count  voltmeters

[tamba]

Hi- thanks again for the answer
i have used  +5v and -5v for analog section -INL is connected to gnd
another +5v for digital section (segments)
also i have used lm385-1.2 for 100mv vref
DC Voltage accuracy is quite good.

[tamba]

I am very curious to know what secret 7107 has in its heart.
Although the 7106 and 7107 are presented together in the datasheet, why is there no practical design for the 7107 multimeter?
I have read the 7107 datasheets provided by different companies and none of them mention the weakness of the 7107 compared to the 7106.
Of course, there is one weakness, and that is the 7107's confusion due to the heat generated by the high current passing through it.
The datasheet says that this problem is completely solved if an external reference voltage is used.
The 7107 and 7106 are both from 50 years ago, but multimeters based on the 7106 are already on the market, but a 7107 multimeter has never been seen - why???

[magic]

Battery powered DMMs use 7106 because LCDs use much less power than LEDs.

You shouldn't need any voltage reference for ohms measurement. If the circuit doesn't work chances are that it's completely wrong or maybe at least subtly wrong - you need to pay attention to how the integrator will behave with selected values of integrating resistor and capacitor and whatever common mode input and reference voltages you use. Bad combination may result in saturation to the rail and wrong results. Make sure to use a film capacitor for integration, not ceramic.