NI USB 622x input impedance clarification

[mikomatik]

Hi,

I guess I missunderstand the NI USB 622x serie's datasheet (pdf# 2007-9286-301-101-D). It states :

...

Input impedance
Device on
AI+ to AI GND............................. >10 GΩ in parallel with 100 pF
AI– to AI GND ............................ >10 GΩ in parallel with 100 pF
Device off
AI+ to AI GND............................. 820 Ω
AI– to AI GND ............................ 820 Ω

...

Will the analog input channel impedances change as the device is switch on or off?

How should I understand these specs?

Michel

[johnsold]

Michel,

The low input impedance with the power off is probably due to parasitic diode paths through the input circuits.  When the power is on, those diodes are reverse biased so that the input impedance is high.  This can occur with many operational amplifiers and instrumentation amplifiers.

The consequence is that any external signals which could be applied to the inputs while the device power is off should be kept to low voltages to avoid the possibility of damge to the input circuits. Again, by comparison to standard op amps, currents of a few milliamperes or more applied to an unpowered input may cause damage.  With some op amps the damage may not be a catastrophic failure but a degradation of the input characteristics: increased offsets and noise.

Lynn