How to output analog current more than 2mA

[cli21]

Hi johnsold,

The device is made by myself, it can be considered as a resistor. Actually, I tried using a resistor(100Ohm), then the voltage of the resistor can't exceed 0.23V.

Thanks.

CJL

[johnsold]

0.23 V/100 ohms = 2.3 mA = limit for AO.

Lynn

[cli21]

Yes. So it seems protection diode is working. I have suspected that,too. But I think there should be a way to solve this problem. 

CJL

[Henrik_Volkers]

Did you connect the powersupply GND ?

[cli21]

Hi Henrik,

My power supply is a DC power supply which doesn't have GND. So I designed a circuit to split the voltage and construct a virtual ground. But I need some compenents such as copacitors which I don't have right now. I plan to get these components and try your method tomorrow. 

Thanks,

CJL

[johnsold]

Even two equal value resistors to create a 'pseudo'ground might be enough to eliminate the diode issue. Try two 1000 ohm resistors in series from +15 V to -15 V and connect the DAC ground to the junction of the two resistors.

Both systems, the DAQ and the controlled device, MUST use the same common reference or ground. If your device is floating, the impedance of the voltage divider I described above may not matter.

Lynn

[cli21]

Hi Henrik,

I tried your method. It works! You are right, it's the ground issue. I connected DAQ, power supply and my device all to the virtual ground, then I could get current more than 2mA. Thanks for your help. 

CJL

[cli21]

Hi johnsold,

You are right. I didn't what you said, it works pretty well. I could get enough current value now. Appreciate for your help. 

Thanks.

CJL

[johnsold]

Glad you got it working.

Please mark Henrik's post as the solution. That way others who look at this thread can go directly to the solution.

Lynn

[cli21]

Hi Henrik,

I got another problem. Below is my circuit where I use a load resistor to replace my device for simplification. First I tried RL=390ohm, the voltage of the load resistor will follow my setting analog output value when the value is less than 6.15V. The current can reach 15mA which solved my earlier problem. But I found that once I set the analog output voltage bigger than 6.15V, the voltage of the load resistor won't follow my set value, just keeping at 6.15V. And then I tried RL=100ohm, same thing happened, only the transition voltage value became around 2.2V. Do you know the reason?

CJL