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NOOB trying to use ADC-24 and terminal board

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NOOB trying to use ADC-24 and terminal board

Postby kassu » Wed Nov 28, 2012 11:05 am

hello,
I have a ADC-24 logger and i'm struggling with reading pressure sensor voltages and setting up the terminal board.
i'm using the terminal board supplied with the logger.
since the +5 volts low current power supply is only up to 2 milliamperes, i need to use an external power source for the sensors since they use more current thats supplied by the logger. i can barely use one sensor with the internal supply, since the voltage drops to 3,2 volts once a single sensor is connected.

i'm using Caterpillar pressure sensors (factory installed in the machines and engines) which use 5 volts for feed and the range of outpout is from zero to 5 volts. hence the ADC-24 max channel voltage is 2500mV i need to use a voltage divider which lowers the signal voltage by half. i'm using 22 ohm 0,5 watt resistors to lower the voltage by using the principle shown in the terminal board user guide (appendix 2.1.3 page 4).

the problem i'm facing now is: once i use an external 5 volt source for the sensors, what is the correct way to connect the ground to the channels? for example: i have a sensor signal connected to the terminal board channel 1, sensor feed and ground are connected to the power source, there is no readable voltage to the channel, probably since the analogue ground is missing. then i connect a jumper from the power source ground to the terminal board analog ground, and i get signal from the sensors.

but is this the correct way to connect the analog ground when i'm usig multiple sensors and a voltage divider?
should i route a jumper for each analogue ground from the power source?
also would it be enough to use only one 22 ohm resistor for the analogue ground (since it is shared by all channels on the terminal boad), and individual resistors for the channel inputs (inline, one per channel)?

please help...
kassu
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Re: NOOB trying to use ADC-24 and terminal board

Postby PeterF » Wed Nov 28, 2012 2:30 pm

Hi Kassu,
To start with, your resistor values are far too low, you are loading the sensor outputs too much. Instead of using 22 ohm resistors, use values of 10,000 ohms for each. That will still give you 2:1 attenuation without drawing a lot of current from the sensor output.
The multiple AG (Analogue Ground) connections on the terminal board are all connected to one point in the ADC-24. Only one of them needs to be connected with a link to the -ve terminal of the power supply to provide a reference. You do require two resistors per channel, soldered to the terminal board, to correctly attenuate the +5V signal down to +2.5V.

Another way of connecting is to not have any attenuating resistors on each channel but to connect the +5V terminal of the power supply to the +2.5V reference of the terminal board. Do not connect the AG connections of the terminal board to anything. Connect the signal outputs of your sensors direct to the channel inputs 1, 2, 3 etc. This offsets the input range of the ADC-24 by -2.5V so that the 0v to +5V output of the sensors is seen as -2.5V to +2.5V by the logger. You will need to use the "Scaling" function within PicoLog to convert from +/-2.5V to the pressure units of your sensors but you would have to do similar scaling with the first way of wiring-up. See page 35 of the PicoLog manual:-
http://www.picotech.com/document/pdf/plw.en-4.pdf
Regards,
PeterF.
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