NCB-68A Impedance

[alessandro.rossetta]

Dear NI Communiy,

I can't find tech specs for DIO impedance of DAQ ncb-68A. Can anyone help me?

Thanks in advance,

Alessandro

[alessandro.rossetta]

Gentile NI community,

Vi scrivo poichè nel mauale utente della DAQ NCB-68A non sono riuscito a trovare il valore di impedenza delle line (DIO) digitali di output. Potreste cortesemente aiutarmi nell'individuare questa specifica?

Grazie

Alessandro Rossetta

[crossrulz]

Since I am not finding a NCB-68A, I am going to assume you meant the SCB-68A.  This is just a connector block.  What DAQ is on the other side of the connector block?

[alessandro.rossetta]

Yes you are right! is the SCB-68A, my bad!

On the other side the DAQ model is NI USB-7846R.

Thank you,

Alessandro

[crossrulz]

According to the specification (look on page 10), digital inputs have an impedance of 50kOhms (typical pull-down) and digital outputs have an impedance of 50 Ohms.

[alessandro.rossetta]

Dear Crossrulz,

If I set my digital signal output voltage at 3.3V and see this signal with an oscilloscope I measure 3.3V on the oscilloscope only if I set the oscilloscope impedance to be 1MOhm.

If instead I set 50Ohm on the oscilloscope, the 3.3V becomes 1.2V.

Any clue why this might happens? I thought the internal impedance may not be 50Ohm as in the datasheet.

Thanks,

Alessandro

[crossrulz]

You are current limited on the DIO line.  The data sheet claims 4mA, but doing some math puts is more around 24mA OR we actually have an output impedance of 300 Ohms.  It could also be that the output impedance changes as part of the current limiting circuit (more current draw causes the output impedance to increase).