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 Post subject: Instrumentation Amplifier/Load Cell Project
PostPosted: Fri Jun 29, 2012 6:26 am 
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Joined: Sun Jan 03, 2010 4:01 pm
Posts: 134
Location: Eau Claire, WI
Greetings all,

I am hoping that there are some Electrical Engineering/Techs/Electronics Gurus that can share some insight into my circuit. I have included the schematic below:
Attachment:
Loadcell V1.png
Loadcell V1.png [ 8.51 KiB | Viewed 5787 times ]


Here are my questions:
1) My Fluke 189 is reporting 1.545 volts on Vout when I have vRef set for 2.0VDC
2) I am still dealing with "noise" on the Vout of the INA126P (about 1mV) even adding filtering on the input (from EETimes Article)

Background information:
My no-load voltage from the load cell is .185mV (or .000185v)
The IA gain is set to 1,000 (Rg is set to 80ohms)
The design calls for a single +5volt supply
Vref is being set by a pot...maybe that is my issue with the noise in Vout?

So again if anyone can explain the lower output voltage (based on datasheets Vout=Vref+(Vin*Gain) so Vout for my case should be 2.185=2.0V+(.000180*1,0000) and how to remove the "noise" on Vout I can get my project done!

Has anyone tried the X2Y Capacitors; do they really work?

References:
INA126P Datasheet: http://www.ti.com/lit/ds/symlink/ina126.pdf
Load Cell Info: http://www.phidgets.com/products.php?product_id=3133_0
EE Times Article: http://www.eetimes.com/design/analog-de ... ttenuation
X2Y Capacitors: http://johansondielectrics.com/x2y-products.html


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 Post subject: Re: Instrumentation Amplifier/Load Cell Project
PostPosted: Tue Jul 03, 2012 1:15 am 
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Joined: Thu Mar 01, 2001 7:00 pm
Posts: 1316
Location: Temecula, CA
Looks like you are missing some values for resistors. Most importantly the voltage dividers at the begining of your circuit.

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 Post subject: Re: Instrumentation Amplifier/Load Cell Project
PostPosted: Tue Jul 03, 2012 3:04 am 
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Master

Joined: Sun Jan 03, 2010 4:01 pm
Posts: 134
Location: Eau Claire, WI
Sorry,

The resistors on the input of the IA is actually a Wheatstone Bridge (typical with Load Cells). I did this schematic using EagleCad (which is the first time I have used the product) and I couldn't find a symbol for a Wheatstone Bridge so I "cheated"...should of brought that point out in my original post.

Over the weekend I did make some progress which are outlined below:

1) When the USB Cable is connected to the Nano USB Programmer about 50mV of noise is added to the +5VDC

2) It is a no-no to connect Vref of a IA (Instrumentation Amplifier) a simple voltage divider without buffering it first (voltage references with a low thermal drift are preferred)

3) Its looking like my input capacitors (CS1, CS2, and Catt are way too large; I'll experiment and update later this week).

I have a 3 channel 24-Bit ADC with built-in IA/low drift voltage reference that has been designed for low voltage sensors used in EKGs and Scales on a "sample" order; I may try to give that after the 4th and will share the results for anyone who is interested.

Still would like some input from any electronic folks out there.

Thanks-Brian


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