LabView Analog Input and Analog Output synchronization

[eleceng77]

Hello,

I am developing a VI to test the capacitor of a transimpedance amplifier so what I am trying to do is generate current and measure respective output voltage and calculate gain.
The problem I am having is the read block is giving same voltage no matter what current input is, I think the problem is regarding the synchronization.

[GerdW]

Hi eleceng,

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@eleceng77 wrote:

The problem I am having is the read block is giving same voltage no matter what current input is, I think the problem is regarding the synchronization.

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Which synchronization are you talking about?

• All you do is enforcing dataflow by using error wires, but just parts of the code.
• You start the AI task before writing the AO/DO tasks…
• You start the DO/AO task in parallel (NO dataflow dependency!) to preparing/starting the AI task…

Did you examine the example VIs coming with LabVIEW/DAQmx? There are example VIs explaining how to sync AO and AI tasks!

Btw. do you really need 3 error in wires and 3 error out wires, even though you already chain the DAQmx stop functions using the error wire?

[Henrik_Volkers]

A TIA works with a feedback resistor  and a charge amplifier has a feedback capacitor. 

(OK, both usually have a R and a C in the feedback, however one is the effective one in the frequency range of interest 😉 )

Both build a virtual ground at the input. (Well, nearly an input impedance of 0 Ohm, for real world charge amplifier I measured values from 10 to 1k Ohm (real part) and since the real world is complex ( 😉 ) also some C or L ... )

If you want to characterise the transferfunction of the amplifier, you usually use a voltage source and a known (high value)   resistor to create a known current  or a known capacitor to create a known charge , both proportional the input voltage.  You can use a capacitor with a TIA (or a resistor for a charge amp) and use the dU/dt but you have to make shure that your voltage source (DAC) produce known dU/dt , what often is a step function (stair way instead of a ramp)  😉

I would recomment to read two voltages (simultanious! if you have a multiplexed ADC DAQ there will be a phase shift). 

If you want to read the output of your DAC, it is not a good idea to use the same clock edge. Due to small jitter in the DAC and ADC you might sometimes read the old or the new DAC value. Since the clock is known, the delay can be corrected.