F2424 DAC output

[cch1955]

I have the #1 DAC output connected to the isolated analog input on a VFD. It works well down to an output of about .7volts At that DAC value and below the DAC output voltage goes back up to around 2 volts when connected to the VFD. I am guessing the DAC output is being overdriven by the power requirements of the VFD?

I have done this in the past with the T100 MD without difficulty.

I have disconnected the DAC output from the VFD and verified that the DAC output does change voltage correctly when the VFD is not connected. The DAC is connected to the VFD input with shielded wire and the shield is grounded on the VFD end only.

So I assume I need som sort of amplifier between the two, any easy way to do this?


Thanks

[support]

You mean when DAC output falls below 0.7V without VFD load, it is pulled up to 2V by the VFD's analog input? That is a bit unusual and could suggest some problem with the VFD's analog circuit?

If you add a 10K ohm resistor between the DAC analog output and the VFD analog input, you will then be able to measure the voltage at the F2424's analog output and the VFD's analog input and see if there is a difference.  You can also calculate how much current the analog input on the VFD draws from the DAC output on the PLC at various SETDAC values.

[cch1955]

I only see the votlage go back up when the DAC output is connected to the VFD.

If the DAC is disconnected from the VFD, the measured voltage on the DAC output is as expected and follows the DAC as it should.

The resistor is a good idea, I will give that a try. I am also going to look at the signal on a scope to make sure it isnt being overpowered by VFD noise in some way.

[cch1955]

I have been able to resolve this issue by not connecting either end of the shield to ground on the analog cable to the VFD. All works well then for all analaog output voltages.

Prior to removing the ground I made sure everything was grounded and that the DC power supply voltage looked clean. I tried some ferrite beads on the inputs. The noise from the inverter remained on the analog signal with enough strength to overpower the low level analog signal.

My thought is that the ground for the machine has noise on it and when you connect the shield to the ground it is induced in the analog output cabling.  

I know it's not you problem, but it is surely a common one. I would appreciate your thoughts.

[support]

Did you check if there is a potential between the "ground" that the shield was connected to and the 0V of the PLC's power supply terminal? How about connecting one end of the shield to the PLC's 0V instead of to the chasis ground? Would that have any impact?