millivolt measuring readymade IC

[mutthunaveen]

hello Board

im trying to make a milli Volt alarm (a band (55mV - 350 mV) where beep comes and outband buzzer will not blow) for my application.
i have made ADC based voltage measurement using 16F877A PIC.

when i measure at test points of my circuit the Multimeter reading shows 68mV exactly without any oscillations
but
when i measure same voltage via ADC the voltage fluctuvates from 40-80mV which is highly unstable for my project......

i used different filters at raw power and at ADC pin too.... i dont understand where the mistake is...

finally i need some kind of sensor (readymade IC same like multimeter stable reading) which can measure and give me mV via I2C to PIC...

please anyone suggest me if any readymade ICs available in market so that it will be helpful in finishing my project...


Thanks

 

[Tahmid]

Is the input voltage stable? Where do you get this input from?

Did you decouple the microcontroller power lines and ADC reference lines (if external)? What filters did you use?

Take a look at LTC2990.

https://www.linear.com/product/LTC2990

Hope this helps.
Tahmid.

 

[doraemon]

Hello!

Complement to Tahmid's mail: without knowing where this signal comes from, it's difficult
to reply. Can you check this voltage with an oscilloscope?
From there, you can elaborate a strategy to get a stable reading.
By the way, you will probably find a circuit that does exactly what you are asking.
Depending on the impednace, I would start with an op-amp to convert your 55 ~ 350
mV range to the same range as the ADC you will use. And depending on the frequency
of the signal, you can also add a few RC filters to get a stable DC value.

Dora.

 

[mutthunaveen]

Thanks for ur reply,....

i used a SMPS powersupply unit which will give +12V & +5V as output voltage with 2A max current...
but multimeter shows stable voltage when measured.....
and the PIC is connected to same supply... with 2200uF & 0.1uf as supply filter (+5V only used)
and at microcontroller input i also used 0.1uF parallel to ground. but the reading was unstable......

finally
are there any working circuits for mV measurement same as multimeter measures....
what may be the cost of LTC2990??? in INDIA

- - - Updated - - -

ok i will come to whole ckt

im doing a milli ohm meter using PIC (to measure milli Ohm of some shunt metals... )
my details are here
1. power supply - external Hard disk sata to usb converter board's--------230 to +12V/5V 2A AC-DC adopter
2. LM317 as Constant current source with 1.2ohm 5W for 1AMP continious 1A current
3. and 2 parallel wires from the shunt to multimeter for actual mV measurement...

at those shunt output when i measure with multimeter the meter is stably showing the reading..... like 250mV so 250mOhms....
when i connect the same to ADC circuit the readings are fluctuting...

can u suggest how actually to design and develop a noise filter to exclude the ripple and measure acutal mili vtg
sorry... i dont have Oscilloscopeee......