Read and Output Voltage using LabVIEW and USB 6343

[ashifulk]

Hello, I'm running into a problem with my application. I'm using a NI USB-6343 to read an analog voltage and output one using the same VI. I keep getting an error with the output voltage DAQ Assistant (I'm using a DAQ Assistant for both the input and output voltage). The error says "Generation cannot be started, because the selected buffer size is too small. Increase the buffer size." I tried to input and output analog voltages using separate VI's and it works. I have changed all the parameters and it still doesn't work. Attached is the VI I'm using.

I'm also using a Multisim circuit for this application, which is also included. The Multisim circuit is a simple voltage divider. What I want to do is be able to have a physical resistor connected outside of the USB-6343 and have the other one sit inside a Multisim circuit and create a voltage divider circuit. Even though I have a physical resistor sitting out the DAQ box I don't see a voltage drop when reading the voltage. For example when I supply 3V to the external resistor (which is 1k Ohm) and if my "virtual" resistor is 1k Ohm, I assume the Vout would be Vin/2, a voltage divider but in LabVIEW I'm seeing the 3V from my power supply; there seems to be no voltage drop in the external resistor. Is type of application possible? Currently I'm more worried about the LabVIEW input and output voltage application but I'm also looking into this because my end application will depend on weather the concept of having a physical circuit interact with a virtual one using Multisim and LabVIEW Co-Simulation works or not.

Our end application will require us to output a voltage from a Multisim circuit to an external sensor and read voltages from the sensorout and use some signal conditioning using another Multisim circuit. We need to output about 7mA to 20mA of current from the Multisim circuit using LabVIEW to the sensor.

[crossrulz]

You are not going to see a voltage drop across your external resistor.  There is nowhere for your current to go.

[ashifulk]

Thank you for the reply. I had a feeling that was the issue. I have one question about the USB 6343. How does the accquiring current signals through the analog input work?

[crossrulz]

A/D Converter.  I'm not sure what you are asking here.

[ashifulk]

I'm sorry I asked that question the wrong way. I guess I was trying to understand the USB 6343 better. I was thinking if I know what the current is I can translate that into voltage in the Multisim circuit. Sorry but this might sound like a stupid question... How do I hookup the circuit to acquire a current signal? I thought the same concept would apply here as when reading a current value using a Multimeter but that didn't work. So I tried to read the current by looking at the node that connects the input voltage to the resistor (so like this Vin ----> [Probe] Resistor ---> GND).

[crossrulz]

The DAQ can only measure voltage.  So with your circuit, measuring the voltage drop across the resistor between power and ground would really just be measuring the power supply voltage.

I guess I don't understand what you are trying to do.  Does your power supply vary?

[ashifulk]

Right now I'm just using a DC supply and it's pretty constant but in the future I would like to use an AC signal (most likely sine but could also use a square wave). I was asking how the circuit should be setup to measure the current in the circuit using the USB device. The USB device (6343) has a "acquire current signal" function and I'm wondering what type of circuit I need to utilize that function.

[crossrulz]

What I've always had to do was measure the voltage across a resistor.  Use the differential configuration for this (A0 will be the + and A8 will be the -).  You can than apply a scale to get the current (I = V/R).  So with a 1k resistor, the scale would be 1/1000 = 0.001.

[ashifulk]

Sorry for the late reply but thank you for your help. It worked.