Millivolt amplifier for ADC input !!! AD7796 connection

[mindstorm88]

Hi guys ,
i need to amplify a shunt that give 1 mV per 1 Amp . I will measure from 0 to 50 Amp so 0 to 50mV. i want to amplify it to be 0 to 2.0 V for my ADC input . Up to now i tried an instrument amplifier made of 3 OP270 . My problem right now is the low value 0 to 5 mV , i can't get precise stable measurement !! any idea ???

Thanks

 

[dick_freebird]

Millivolt amplifier for ADC input !!!

Consider how you are going to get a setup with only 1mV of
input noise. I've never seen a bench setup that quiet,
other than noise measurement jigs that are battery powered.

I think you will either have to autorange, or sacrifice resolution
for range.

You might put a 'scope to the thing and see if you can pick
out 60/120Hz as the noise actor. If so then you might be
wanting to apply a line-synchronous sampling technique
or something, to get more consistent (if not more accurate)
readings. If you can determine a null phase then this might
work even better.

 

[mindstorm88]

Millivolt amplifier for ADC input !!!

Forgot to write that i'm measuring DC current of short circuit of battery , the circuit is simply a battery in series with a shunt and a contactor !!! i measure the current till it goes down to zero !!!

 

[mikersia]

Millivolt amplifier for ADC input !!!

The easiest way is to use high precision ADC, designated for current sensing.
Such devices are producing by AD (AD77xx), AMS (AS8500), ... They have a microvolt input offset for shunt type sensors and use sigma-delta ADC with resolution >16 bit. AS850x series
is designated for battery monitoring. See also IHM-A-1500 from Isabelenhutte,- it is about the same from shunt manufacturer.

 

[synpur]

Re: Millivolt amplifier for ADC input !!!

Actually there are many IC dedicated to battery monitor applications. You may find one that suits your needs.



For example:

**broken link removed**
--------------------------------------------------
http://www.linkedin.com/groups?gid=2582754&trk=myg_ugrp_ovr

 

[mindstorm88]

Re: Millivolt amplifier for ADC input !!!

Hi guys , i did a setup with an AD7796 , now i'm having a problem with connection to the shunt , my shunt is a 200Amp at 1mV/Amp , it is connected between gnd and negative post of the unit !! i tried to connect the diferential in(-) to gnd and in(+) to the other side of the shunt , but it seems tthat the AD7796 does not want to measure from 0 !!!

any idea

 

[karesz]

Millivolt amplifier for ADC input !!!

Hi,
Did you configured for bipolar pls?
""BIPOLAR/UNIPOLAR CONFIGURATION
The analog input to the AD7796/AD7797 can accept either unipolar or bipolar input voltage ranges. A bipolar input range does not imply that the part can tolerate negative voltages with respect to system GND. Unipolar and bipolar signals on the AIN(+) input are referenced to the voltage on the AIN(–) input. For example, if AIN(−) is 2.5 V and the ADC is configured for unipolar mode, the input voltage range on the AIN(+) pin is 2.5 V to 2.02 V.
If the ADC is configured for bipolar mode, the analog input range on the AIN(+) input is 2.48 V to 2.52 V. The bipolar/ unipolar option is chosen by programming the U/B bit in the configuration register.""
K.

 

[mindstorm88]

i did try both UNI and BI , my main problem is the shunt having 1 side to gnd !!! so AIN(-) is at GND and AIN(+) on the other side of the shunt for the measure !!

it seems that the AD7796 does not like to measure with AIN(-) to GND !!!

 

[karesz]

HI, You can read it over their;I copied from AD7796 datasheet, in unipolar modus it has a signal range ta become between 2.02 & 2.5V....
Thes isnt clear me, but I dont read yet some datasheet over this IC.
K.

 

[FvM]

it seems that the AD7796 does not like to measure with AIN(-) to GND !!!

Yes. That's pretty clear from the common mode range specification in the data sheet. AD7796 is definitely not the right choice
for a GND referenced shunt measurement. You can of course use a level-shifting circuit or an instrumentation amplifier, but
you loose part of the accuracy.

You may want to use a -0.7/+3.3V supply and apply level shift on the digital interface. A dual-supply ADC would be the
straight-forward solution, however.

 

[mindstorm88]

ok , you're right , just place an order from Microchip for MCP3553 , gonna try these !!!

 

[mindstorm88]

Finally received the MCP3553 !! wow what a great result, now i can easily measure my shunt down to 0.1mV

 

[sasith_21]

Can you give us schematics.
MCP3553
we need to use it volt meter ICL 7107. or ohm meter.
we are great full if you do it.
http://www.freeinfosociety.com/electronics/schemview.php?id=2077
Thanks

 

[karesz]

Hali,
I think; Microcip`s home side hase all infos what you will need: MCP3551 Delta-Sigma ADC Demo Board
www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1406&dDocName=en024782
Than some Apll. Notes as AN1007, 1030 (on the same side )...
K.