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How to change input impedance settings for analog input on DAQami



When measuring voltage with analog input ACH0 and ACH1 using USB-2627 and DAQami, the measured voltage will not match the actual voltage.

Is it possible to change the input impedance of analog input on DAQami?

If possible, could you please tell me the command? Or can I do that using UL and InstaCal?

I think this phenomenon is caused by installing voltage dividing resistors of 300kΩ and 100kΩ to reduce the voltage to a quarter before inputting it. By inputting 1V into this, I can see the following:

  • Voltage at the connector end of the channel containing the voltage dividing resistor: Measured about 0.7V
  • Measure voltage of channel without voltage dividing resistor: 1V

From this phenomenon, I inferred that the input impedance of the analog input port was low.

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The USB-2627 input impedance is 1 G ohm and changing it is not possible. Instead buffer the signal. Use a voltage-follower op amp configuration to change your high impedance signal to low impedance. 

The board also uses analog multiplexers to expand the number of channels. High impedance signals will cause cross talk between channels and this could explain what you're seeing. A trick some use is to turn on an empty channel (connected to ground) after each high impedance connection. 


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