device capacitance and analog output

[cooltools]

A device engineer recently sent this email in regards to a pressure controller he sells ...the QB.

"I did not see the load capacitance listed [for the DAQ cards].

Our QB has a 0.01uF capacitor between the command input and DC common.

Several controllers on the market with op-amp outputs do not like that much capacitance. You should check this with National."

Would you please tell me if this capacitance is a problem with basic cards like PCI 6014?

Thank you,

tom

[Jeremy_L]

Hi Tom,

Our boards have a failsafe where the analog outputs have a short-circuit to ground, so you are generally protected from frying the outputs. Also, the 6014 has an output rating of +/-5 mA. So, depending on your output being AC or DC, you may need to watch the value of your load. If it is AC, then you need to make sure that your load is not producing a current that exceeds this rating. If DC, most likely you are not going to exceed the rating, so no worries.

Hope this helps!

Jeremy L.
National Instruments

[cooltools]

Hi Jeremy:

I just spoke with the device engineer. He is still not clear on whether or not the PCI-6014 will be able to control his device. I passed along your answer above and here is what he said:

"Controllers with Op-Amp outputs can "lock up" or oscillate when there is too much capacitance on the output (load) side. Some op-amps can drive voltage into a couple of microfarads. However, some op-amps will oscillate with only a hundredth of a microfarad.

The question is, what op-amp is national using as their output ?

(I'm assuming an op-amp since there is a 5mA output limit, transistor outputs are usually much more and are not effected by capacitance)."

The device has a 0.01uF capacitor between the command input and DC common

Tha
nk you Jeremy,

tom

[cooltools]

Hi Jeremy:

Can you answer the previous question or concern I posted yesterday ... our project is under a tight timeline?

thanks a lot,

tom

[Jeremy_L]

Hi Tom,

My apologies for the delay. I found out that we have circuit compensation on the output that will keep it from oscillating with your load capacitance. The circuit basically consists of a 50 ohm resistor between the opamp output and the load with 100pF directly between the opamp output and inverting input. The opamp is MC33078.

Jeremy L.
National Instruments