Electrical – Why we put 10Kohm resistor in parallel with 144Kohm that exist inside USB-6008?


When using the LM35 Temperature Sensor and USB-6008, a 10K resistor is placed between GND and AI0 which connect on the LM35 to GND and Vout, respectively.

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Referring to the Analog Input circuitry in the USB-6008 datasheet,
you'll find the equivalent internal resistance is 144K between GND and AIO, and AIO is at 1.4V with no signal. (They use two in parallel and one in series, and the combination is 144 Kohm: (30.9*39.2/30.9+39.2)+127=144.27 Kohm)

Why do we need a 10K resistor in parallel with the 144K inside the USB-6008?

Datasheet for LM35
Datasheet for USB-6008

Best Answer

The LM35 has very low current sinking capability (1uA). The USB-6008 AIO pins are internally terminated to 1.4V with 144K. If you don't add something to sink the current from the AIO termination, the LM35 output would just sit near 1.4V (no lower than 1.4V - 144K * 1uA) and does nothing.