Re : Re: scale daqmx

[chrisger]

well, than whats the gain of the amplifier? If possible take a look at the raw signals with an oscilloscope and find the correct voltage range to acquire the whole signal. 

Do you amplify the current too?

[stage84]

well, actually I'm using a patch-clamp amplifier, so I set the voltage to 50mV and I want to record the current. So this is the current which is amplified. As the current is proportionnal to the voltage, I'm recording the voltage with Labview because when I tried to change the DAQmx read from voltage to current, it read nothing. But the values recorded are good, I can confirm with the value from the amplifier. the only problem is the range, because I need to go above 10nA (or 10nV) and it doesn't work... I will have a look at the amplifier and see if there is a problem with its scaling.

thanks a lot for your help!

Sophie

[chrisger]

slowly, let's get this straight:

- What type is the signal at your PCI DAQ-device, AFTER the amp: Voltage or current?

- What is the expected range of the signal at your PCI DAQ-device, AFTER the amp in V/A?

- If you need to measure current, you have to set the virtual channel type to "AI current"!

[stage84]

the signal after the amp is a current in the range  +/-100nA. I already tried th AI current, but it doesn't work! I obtain no values... I tried to do it with Labview signal express, and when I wanted to calibrate the card, with AI current, I read nothing but when I tried with the AI voltage, I read the same values as my amplifier....

[chrisger]

OK, first take a look at the datasheet of the device to see what's possible:

http://sine.ni.com/nips/cds/view/p/lang/en/nid/14685

http://sine.ni.com/ds/app/doc/p/id/ds-25/lang/en#header0

As far as i has seen the device is NOT capable of "AI current" (sorry, ma fault). That would explain why you don't get any values.

So you have to transform the current back to a proportional voltage using a resistor. With a maximum current of 100nA and a resistor of 1 MOhm you get a voltage of 100mV (but i'm not sureif that will work...)

Maybe you need a higher gain or an additional amplifier to amplify voltage.

[stage84]

well, so I think I have to change the card because I'm measuring the current in order to calculate the resistance... and maybe it could explain a lot of my other problems...

thanks for everything, it opens my eyes and I have to change the protocol of my experiement.

[chrisger]

You may transform the signal type from current to voltage. For doing so pass the current through a resistor and measure the voltage at the ends of the resistor. Use V = R x I to calculate the current from the measured voltage. Take a look at the data sheet to find the devices internal resistor value. This value has to be >> than that of the external resistor.