maximum analog input?

[suethechemist]

I am currently using a PCI 6035E card which allows analog input ranging from -10 to +10V. I intend to stabilize a voltage source using a PID controller in LabVIEW, but this voltage varies from -20 to +20V. Any suggestions as to what is the simplest solution? I don't really want to buy another card (do National Instruments supply a card that meets my requirements?) Can I attenuate (before Analog Input) and amplify (after Anlog Output) the voltage accurately and reliably using an external circuit? If so, how should this be done? I have very little knowledge of electronics :(!

[Dr._Imad]

A voltage divider circuit should do the job

connect two resistors (R1 and R2) in series (each 500 k ohm or larger, so as not to create large current).
one end (resistor R1) --- connect to the measurement signal, and
the other end to ground

connect the point between ressitors to the daq channel

you then should be able to read a range of -10 V to +10 V.


Note, you can change the values of R1 and R2 to get different voltage range.

Vout/V_original= R2/ (R1 + R2)

R1+R2 should be way above the output impedence of the measurement device. That is why they should be large. values of 1 k or 500 k ohm to use is a matter of not loading your measurement device

[suethechemist]

Thanks Dr Imad, what about amplifying the stabilized analog output? The 'voltage source' I mentioned is the input to a diode laser (which determines the output wavelength of the laser) - I need to select a particular voltage (corresponding to our required wavelength), redirect this to the LabVIEW PID regulator, stabilize it, and then send it back to the diode. If I can't amplify the stabilized signal PRECISELY, my purpose is defeated.

[Dr._Imad]

Yes, you can take the output signal of the daq card (-10 to +10V) and amplify it to any level of interest (I assume -20 to +20 V) using proper electronic circuitry.

What is you input reference to the PID,

is it voltage ? inn this case the PID output stablizies when it finds a voltage match (input voltage setting = readout voltage)

or

wavelength ? Do you have a calibration curve for voltage vs. wavelength ? in this case convert the read out voltage --calibration-curve----wavelength, and input to the PID a setting in wavelength

[suethechemist]

I essentially have wavelength-voltage calibration, I input a voltage to the PID which represents the wavelength of interest. So I guess the validity of my idea just depends on how accurately I can read this voltage, and how well I can reproduce it using amplification. The stabilization will be achieved by PID control. Thanks for your help,

Suzanne.