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6008 Analog Input termination voltage

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I am using the NI USB-6008 Analog Inputs.  The spec says they have an input impedance of 144k-ohms.  But it does not say what the termination voltage is.  If you leave the AI's disconnected and measure voltage you will get 1.4 volts.  So I'm assuming that this is the termination voltage, but it is not spec'd.

 

Does anyone agree that these AI's are terminatied by 144k-ohms to 1.4V?  Is this in the documentation somewhere?

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impedance is always relative to ground


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I characterized the Analog Input on the USB-6008 using a SMU.  They are 144k-ohm input impedance and they are terminated to 1.38V.  If you measure the input with the 6008 is disconnected from the usb it will measure 144k-ohms to ground.  But once you connect the usb it is pulled to 1.38V.  Then sweep voltage or current on a SMU and you will get the 144k-ohm impedance.  This is measured single ended.  Differential input will get you 288k-ohm input impedance.

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Accepted by topic author jeff.kittle

Figure 7 on page 16 of the NI USB-6008/6009 User Guide and Specifications shows the strange input circuitry of this device.

 

Lynn

 

USB-6008 Input.png

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