Looking for Help! PCI 6723 output a wrong voltage

[YoungHang]

I am using a PCI-6723 card for analog output.And I set the output voltage to 1V with a 50Ω load, but the voltage of the load is just around 0.38V.I tried to increase the output voltage of the card to 10V, while the voltagte of the load was always 0.38V. I think it may because the current drive of the PCI-6723 is 5mA max. But I also used the PCI-6733,which also with a current drive 5mA, with 1V output voltage and 50Ω  load,and the voltage of the load was correct,1V. So ,I am wondering why? Can anubody give me some adviec?Thank you!

[crossrulz]

0.38V/50 Ohm = 7.6mA.  Yeah, you are going over current.  You need to use an op-amp or some other buffer to increase the current capability.

[YoungHang]

But the other important quetion is that why PCI-6733 output a correct voltage ,while its current drive is also 5mA?

[crossrulz]

My guess is that you didn't really have a 50 Ohm load.  Or somebody was smart enough to put in a buffer.  But that card should not have been putting out 1V on a 50 Ohm load.  That would require 20mA.

[YoungHang · Accepted solution]

You are right !But not all! The load was actually 50Ω，because I used a 50Ω resistor as the load. But the output of PCI-6733 was actually wrong too. The 1V output of PCI-6733 was just a coincidence, if I set the voltage of other value ,then it will not work.

[YoungHang]

Another question,can you give me some advice about what kind of  op-amp I can use?Or , how to choose a correct op-amp?

[crossrulz]

Which op amp you want depends on a lot of factors: skew rate, frequency response, max and min voltages, available supplies.  I am not an op-amp expert, but there are some guys on here that are excellent at signal conditioning.  I'll try to flag one to this thread.

[YoungHang]

Thank you!

[johnsold]

crossrulz asked me to help with this thread.

The values you see with both devices are typical of the behavior of devices loaded beyond their maximum rated current. The current does not limit at exactly the rated value (1 mA or 5 mA). Usually the manufacturers build some margin into the devices so that they may actually work to 10% or 20% above the rated value. They just do not guarantee that they will work correctly above the rated value. Then, above the actual limit the current may increase slightly with voltage, possibly non-linearly. 

As crossrulz pointed out it is necessary to know the range of voltages and frequencies you want to drive. Is the load always a resistor or will there be significant reactance or non-linearity in the load?

Driving a 50 ohm load at 10 volts is not trivial. The current requirement is 0.2 amperes. Very few op amps can drive that much current. 

Give us some good specifications on your actual requirements and someone will probably be able to recommend something for you.

Lynn

[YoungHang]

The load is almost always a 50Ω resistor.

I want an unit-gain op-amp .And I just want to use the op-amp to improve the current drive of PCI-6723, from initially 5mA to at least 30mA.And the output voltage range is -6 t0 6V. And the settling time, the faster the better, but not very strict.