Why do I measure an output level (1.38V) on an analog input?

[RenéM]

I am making a curve trace appliation using the NI USB6008.
On the analog input terminals I measure a level of 1.38 V.

My circuit is a analog output port driving a 20k resistor in series with a diode.

When I attempt to measure the voltage over the diode the measument is affected by the voltage from the analog input.

[hoffiz]

Hi ReneM:

Regarding your issue I tried it with a USB-6009, the way I understood it, was connecting the analog output straight to the analog input; so AI0 hardwire to AO0 and AI4 to GND (next to AO1). I was able to generate a DC value between 0 and 5 Volts and measure it. You have to consider that the analog output of the USB-6008 can only handle 0 to 5 Volts. USB-6008/6009 User Guide and Specifications, page 16. In the specification you can find the circuit diagram for input and output of the card.  You can also find how to make the connection for differential measurements when setting up an Analog Input.

Since you are trying to drive a resistor and a diode, I was able to test the card instead of a 20K ohm resistor with a 66.5K Ohm nor does the resistance matter since the Analog input have its own resistance of 127Kohm, in any point of my testingy the measure was affected by the analog input.  The examples I use for this test were:  Continuous Software Timed Analog Output for the USB-6008 and USB-6009 for generating the voltage and this shipped example “Cont Acq&Graph Voltage-Int Clk.vi”. (Help-> find examples). I have attached both just in case.

[RenéM]

Hello Jaime,

I have attached the schematic of my test setup and a graph of what I measure.

Note that the first point of the graph is not at 0V/0mA, were I would expect it to be when the output starts at 0V.

When Ao0 is set to 0V :

• I measure -0.3V on Ai0.
• Using a multimeter I measure -0.3V between pin 2 and 3.
• Using a multimeter I measure 0V between pin 14 and 15.
  

If I disconnect pin 3 from the circuit :

• Using a multimeter I measure -0.15V between pin 2 and 5.
• Using a multimeter I measure -1.37V between pin 2 and 3.

If I connect Ao0 (pin 14/16) directly to Ai0 (pin 2/3)  then I read the correct value.

There seems to be a bias on the analog input port which affects my circuit.

[hoffiz]

Hi ReneM.

If no signal is connected you will read that 1.4 Volts bias.  The default value for the analog input is RSE make sure you are reading in “differential mode”.  Also take a look at this other Knowledge base: USB-6008/6009 Analog Input Reads Approximately -1.4V Offset with RSE Connections,  What might be happening is that even if you are doing differential measurement the analog input is providing a voltage to the diode and that is the drop you are reading in the multimeter. What I will suggest is following these instructions: Using a Unity Gain Buffer (Voltage Follower) with a DAQ Device to isolate the analog output from the analog input.

Let me thank you for all the information you have gave me, it really helped me to give you a better answer and hope this post helps you finish your "project".