why would i be getting 3.2volts from the analogue output ground terminal on my mydaq unit

[Dave Bartley]

i am getting approx 3.2 volts on the ground terminal of my mydaq unit. when i am looking to get a 0-10 volt signal i cannot go below 3.2 volts but i can get up to 10 volts. any ideas?

Dave

[johnsold]

Dave,

3.2 volts with respect to what?  A voltage measurement is between two points.  What is the "other" point to which your measuring instrument is connected?  What kind of instrument are you using to make this measurement?

Lynn

[Dave Bartley]

lynn,

i am getting 3.2 volts between the analogue channel (1 or 2 whichever in use)  terminal and the ground terminal. if i have a slider to go between 0-10 volts i read from approx 3.2 to 10 output fine but cannot get a voltage output reading below 3.2 volts. i.e 0-3.2 volts just gives 3.2.

i am using this to drive a 0-10 volt signal on a vsd, but to rule out the vsd i have been measuring at the DAQ terminals with the vsd disconnected.

dave.

[Dave Bartley]

just to note from above i have also been using 0-10 volts for analogue input which is working fine. Measurements are taken using a digital multimeter.

dave

[johnsold]

Dave,

What is a vsd?

If you connect a voltmeter (battery operated or analog type, without connection through the power line ground) to the output of the DAC with the computer powered off, then power up the computer with no DAQ software running, does the output go to 3.2 V immediately?  Or does it stay at 0 until some software talks to the DAQ card?  What kind of DAQ card are you using?  What happens if you repeat the procedure above but load the output with a 1000 ohm resistor?

Did it ever work the way you think it should or has something changed?

Lynn

[Dave Bartley]

 A VSD is a Variable speed drive for a Motor and so a 0-10 volt siganl input to this will control the speed just like opening and closing a modulating valve.

I am getting approx 3.2 volts on the analogue channels out regardles of any programme running. i am using a MYDAQ unit. i have not tried loading with a resistor. i am not sure if 0-10 voltts out ever worked properly as this is a new unit.

dave

[johnsold]

Dave,

One thing to watch is grounds.  The "ground" reference of the VSD may not be the same as the ground of your DAQ or the computer.  If they differ, the magic smoke may escape.  It is called magic because it always escapes from the most expensive or difficult to replace part of the system.  It is not always easy to verify how so-called grounds are connected.

If you have had the DAQ unit connected to the VSD at any time, you may have damaged the DAQ unit. 

Lynn

[Dave Bartley]

think you could be right. i have got around it by adjusting the input range on the VSD and it seems to work.

thanks for your assistance.

dave

[djs_at_eaton]

The fact that there's 3.2 volts of unexpected difference between the VSD input and the computer tells me:

1. There's a wiring error  OR
2. The person who designed the circuit didn't think about grounding for signal integrity  OR
3. Both.

In the situation you have now, you've adjusted the difference out, but there's no guarantee that the reference voltages will stay 3.2 volts apart.  If the difference changes, you will probably see unexplained motor speed variation.  That could be frustrating, expensive, or dangerous depending on your application.

If you can't resolve the problem with proper grounding techniques, you should consider using an isolation amplifier between the VSD and the DAQ card.  It's not always the cheapest solution, but it could save really expensive problems down the road.

[johnsold]

Good advice.

Lynn