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Author Topic: MAX139 Voltmeter problem: Last 2 digit fluctuating !  (Read 1657 times)
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MAXPAYNE
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« on: September 22, 2013, 09:10:25 09:10 »

I have built a voltmeter using MAX139 (http://www.maximintegrated.com/datasheet/index.mvp/id/1292) for using with my lab power supply project. b4 using it finally I hv tested the prototype. I powered the meter from a linear lab power supply and what i found is the last 2 digits are constantly fluctuating all time, even I grnd the input(i.e. 0V input). I measured a 4.0V battery voltage with it. reading was fluctuating from 3.1V to 4.4V. I used 200mV scale.

I went through all datasheet/appli note and didn't find any wrong correction/component value.  When I powered the meter frm a battery source. the fluctuation reduced to only 1/2 count !! Is the supply line noise disturbing the meter reading? if so what can be the remedy as I wl hv to use it in my lab power supply, which will b powered frm mains as well.
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Catcatcat
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« Reply #1 on: September 22, 2013, 09:49:02 09:49 »

Two directions of solutions may be, it is to improve the quality of Vdd and increase the time constant of the input. put a 1.0 uF
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MAXPAYNE
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« Reply #2 on: September 22, 2013, 06:25:02 18:25 »

Two directions of solutions may be, it is to improve the quality of Vdd and increase the time constant of the input. put a 1.0 uF

if I input zero volt, i.e. shorting the input pins, the fluctuation still exists !!
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« Reply #3 on: September 22, 2013, 06:40:09 18:40 »

check the reference voltage setting circuit, the presence of pulses.
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Gallymimu
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« Reply #4 on: September 22, 2013, 10:23:56 22:23 »

Show us a picture of your setup.  There could be all kinds of things wrong such as dirty VDD, bad grounding, ground loops, improper decoupling, misalignment of the stars etc.
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Vineyards
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« Reply #5 on: September 22, 2013, 11:34:14 23:34 »

Unless you have a major problem with your power supply it can not affect the read out a great deal because the IC has a power supply rejection rate of 50uV per volt. Its common mode rejection rate is also excellent (120db).

Try rebuilding the whole circuit from scratch.
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Gallymimu
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« Reply #6 on: September 23, 2013, 12:27:44 00:27 »

Unless you have a major problem with your power supply it can not affect the read out a great deal because the IC has a power supply rejection rate of 50uV per volt. Its common mode rejection rate is also excellent (120db).

Unless power supply noise is coupling into the input due to layout issues.
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« Reply #7 on: September 23, 2013, 01:13:56 01:13 »

It's got to be power supply problem, he had input to ground, and was lower when fed with a battery.
I would go with layout and/or power supply vdd, needs a few caps near the ic and also along the track.

at work we has a pcb that the supply started ringing when different parts of the circuit required extra current.
a cap fitted 1/2 way down a track solved the fault, until pcb was reworked.
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Ichan
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« Reply #8 on: September 23, 2013, 06:09:38 18:09 »

I measured a 4.0V battery voltage with it. reading was fluctuating from 3.1V to 4.4V. I used 200mV scale.

My blind guess, you try to measure 4.0V battery with a 200mV full scale voltmeter...  Shocked

-ichan
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MAXPAYNE
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« Reply #9 on: September 24, 2013, 05:34:49 05:34 »

My blind guess, you try to measure 4.0V battery with a 200mV full scale voltmeter...  Shocked

-ichan

he he he. nope. there is a voltage divider section at the input like any other voltmeter. Cheesy




Posted on: September 24, 2013, 10:31:44 am - Automerged

I think I hv found d problem. it was a power supply issue. working on it... get beck to u soon ... Smiley
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« Reply #10 on: October 29, 2013, 03:40:37 03:40 »

MAXPAYNE  DC power supplies build in later years are all SMPS type and some of them have besides high noise low frequency attenuated switching signal raiding on top of DC voltage. One can try to filter out these but best is to use a better power supply. Power supply to your ADC PCB connection also needs to be short as possible with good solid contact (long wires pick noise and power line signals from surrounding air). Your PCB should have a solid design with good ground plane and appropriate capacitive filtering (a bead on the wires close to PCB also helps). You can look at the application notes of ADC's in general and see pointers for good PCB practices.  The IC you are using is integrating type so it integrates also input noise but still it is a good practice paying attention to input connections and shielding as much as possible (coax wires, high freq filtering, grounding etc). There are many details to build the PCB so to make ADC perform better but based on your progress we could further comment. Good luck.
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MAXPAYNE
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« Reply #11 on: October 29, 2013, 05:01:54 05:01 »

MAXPAYNE  DC power supplies build in later years are all SMPS type and some of them have besides high noise low frequency attenuated switching signal raiding on top of DC voltage. One can try to filter out these but best is to use a better power supply. Power supply to your ADC PCB connection also needs to be short as possible with good solid contact (long wires pick noise and power line signals from surrounding air). Your PCB should have a solid design with good ground plane and appropriate capacitive filtering (a bead on the wires close to PCB also helps). You can look at the application notes of ADC's in general and see pointers for good PCB practices.  The IC you are using is integrating type so it integrates also input noise but still it is a good practice paying attention to input connections and shielding as much as possible (coax wires, high freq filtering, grounding etc). There are many details to build the PCB so to make ADC perform better but based on your progress we could further comment. Good luck.


The problem is solved by using 7805 regulator Smiley
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