Measuring 0.1mv with ADC

[binojkumar]

Hi friends, I am working on a project to measure the contact resistance of a relay.The idea is with a constant current source (10ma) we can measure the voltage drop of the contacts, but it is a negligibly small value(0.1mv or 0.2mv). So i need to amplify it (possibly to 10mv or 20 mv )with an instrumentation amplifier. I am very poor in analog design please help me to design it.
I found OP07 op amp is capable of amplifying very low voltage to a desired value.
Please help.

 

[amayilsamy]

You can simply do with non-inverting op-amp with gain factor 10...

You can use LM358 Op-amp..

https://www.electronics-tutorials.ws/opamp/opamp_3.html

We can changing the gain and output voltage using R1,Rf resistor.

For your application R1=1K,Rf=10K.

 

[crutschow]

.................................
You can use LM358 Op-amp..
.................

The LM358 is not a good choice for this application since its input offset can be several mV, much higher than the signal he is trying to amplify. Better would be an op amp with a low offset, such as the OP07.

To the OP: Why are you using such a low measurement current? A higher current would give a higher voltage and make it easier to measure the contact resistance.

 

[godfreyl]

It would be easier to use AC current and measure the AC voltage across the contacts. Then you don't have to worry about DC offset.

 

[amayilsamy]

The LM358 is not a good choice for this application since its input offset can be several mV, much higher than the signal he is trying to amplify. Better would be an op amp with a low offset, such as the OP07.

To the OP: Why are you using such a low measurement current? A higher current would give a higher voltage and make it easier to measure the contact resistance.

Thanks Mr. crutschow

I agree with you..

 

[dick_freebird]

A few other questions / observations:

- Are you planning to measure the relay in-situ, or by itself?
In situ would add the difficulties of the active supplies /
signals, a common-mode offset of volts from which to
extract microvolts. But...

- How much do you care about milliohms, are you trying too
hard to measure a "good" contact-set when all you care
about is the "bad enough" endpoint? That is to say, if all
you want is to know when it needs replacing, the criterion
may be much higher, hence the signal of interest as well,
and your job, simpler (or more forgiving).

 

[D.A.(Tony)Stewart]

There are lots of unstated assumptions.
What range do you need out? 100mV? => Av=1000
Can you choose a dual supply or Rail-rail input and output OA?
Choose one with very low input offset voltage or a trimmer is required.

LM358 needs input offset adjustment pot +/- to divider for up to 7mV or 70x your signal.. not so good.

For DC resistance test, use contacts as a low side load switch w.r.t. Gnd. then sample and hold result before opening to get +0.1mV boosted to +100mV with gain of 1000. ( or more)

If relay is not gold plated or sealed, then oxidation will affect contact resistance greatly over time unless loaded by a low ESR cap to arc and "wet" contacts, which is why relays are called "dry" or "wet" type, " <2A or >2A" ( gold vs silver alloy plating)

 

[binojkumar]

There are lots of unstated assumptions.
What range do you need out? 100mV? => Av=1000
Can you choose a dual supply or Rail-rail input and output OA?
Choose one with very low input offset voltage or a trimmer is required.

LM358 needs input offset adjustment pot +/- to divider for up to 7mV or 70x your signal.. not so good.

For DC resistance test, use contacts as a low side load switch w.r.t. Gnd. then sample and hold result before opening to get +0.1mV boosted to +100mV with gain of 1000. ( or more)

If relay is not gold plated or sealed, then oxidation will affect contact resistance greatly over time unless loaded by a low ESR cap to arc and "wet" contacts, which is why relays are called "dry" or "wet" type, " <2A or >2A" ( gold vs silver alloy plating)

Thanks for the reply. For the contact resistance measurement i should use 6v ,10ma power supply. This is the maximum i can use. if i use higher current i would get a higher voltage drop, but that is not recommended.
I can use only 6v,10MA only. OP07 instrumentation amplifier can use to amplify the voltage drop.

The ADC i use is MCP3208. with a reference voltage 2 or 2.7. (for 2 v the minimum voltage i can measure is 0.4mv, for 2.7v the minimum voltage can measure is 0.6mv ), what is the minimum reference voltage we can give to MCP3208?
How will be the instrumentation amplifier design? please help.

 

[schmitt trigger]

If you plan to reliably amplify 0.1 mV, (I assume it is full-scale) you are talking chopper-stabilized opamps, like the TLC2652.

Google the term, chopper stabilized amplifiers.