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Do I need an In-amp?

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Do I need an In-amp?

Postby silhas » Mon Sep 30, 2013 9:09 am

Hi,

I'm currently trying to use a ADC-24 with a terminalboard to meassure force, temperature and pressure.
In the application and the wiring diagram, a engineer prior to my involvement wanted to use a INA125 Instrumentation amplifier. Since it's not a particularly noisy environment in form of electrical interference, I was wondering if it's a necessity? Will the different meassurement signals interfer with eachother?

Can't i just pick suitable sensors, apply correct resistans to get a appropriate voltage and then plug it into the computer?
silhas
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Re: Do I need an In-amp?

Postby PeterF » Mon Sep 30, 2013 2:29 pm

Hi,
The first engineer probably wanted to use an instrumentation amplifier in order to amplify and level shift the output of a "Bridge" load cell. This is not necessary with the ADC-24.
The ADC-24 is, in effect a single channel device with a multiplexer to give 16 single-ended or 8 differential channels. When you use more than one channel, each must be referred to the same analog ground reference. Each voltage measured must be within the common mode voltage range of the ADC-24 which is:-
Common-mode range
Channel input to Analogue Ground (AG):- ±1.9 V (±39 mV to ±1250 mV ranges)
Channel input to Analogue Ground (AG):- ±3.0 V (±2500 mV range)
Analogue Ground (AG) to earth ground:- ±30 V

Please see page 11 of the manual:-
http://www.picotech.com/document/pdf/adc20.en-3.pdf

I assume that your Load cell has a differential output and a required energisation voltage of 10V. The cell should have four terminals, a "+" & "-" energisation input and "+" & "-" signal outputs.
Connect the +10V power lead (from an external power supply) to the "+" energising input of the cell, connect the 0V lead to the "-" energising input of the cell.
Connect the "+" output of the cell to the Ch 1 input of the terminal board. Connect the "-" output of the cell to the Ch 2 input of the terminal board.
Connect the Analogue Ground (AG) connection of the ADC-24 terminal board to the "-" output of the cell, either at the cell or a channel 1 input of the terminal board. This is to give a stable analogue reference at the same voltage as the cell output.
When setting-up the channel on the ADC-24, choose "Differential input enable", +/-39mV range (or higher if necessary) and the sample rate required.
Your load cell will probably be rated for sensitivity in it's specification with a figure similar to "10mV/Volt at full scale". This means that for each volt of energisation applied, the differential output of the cell will give 10mV output at full scale load. In this case, 10V energisation will give 10 x 10mV = 100mV output at full scale. Depending on the exact output voltage of your Load Cell, you may want to increase or decrease the selected voltage range on the ADC-24 to suit. Use the most sensitive range that does not saturate on full load. Using the "Scaling" option, the correct engineering units can be displayed on the PicoLog screen and data file.
Please contact again if you still have difficulty.
Regards,
PeterF.
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Re: Do I need an In-amp?

Postby silhas » Thu Oct 10, 2013 9:58 am

Thank you for the respons.

Was wondering about the connection between AG of the terminalboard to channel 1, did you ment to say channel 2 instead of channel 1 since channel 2 has the connection to "-" signal output of the cell (according to you previous description).

Since I use the powersupply in several measurements, it'd facilitate if I'd use a 24V supply. Can I voltage divide the supply voltage to the my load cell, or do i need to use a voltage regulator?
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Re: Do I need an In-amp?

Postby PeterF » Tue Oct 15, 2013 1:45 pm

Hi,
You are right, the "-Ve" input is actually channel 2 but it does not really matter which one you use to connect to the AG since both the "+Ve" & "-Ve" outputs of the load cell are at mid-voltage (about +5V) which is all that is required as a reference ground.
For stability, it would be better to use a 9v or 10V regulator (but it is possible to use a series resistor to drop the voltage from +24V to +10V).
Regards,
PeterF.
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