question about input impedance

[JudeLi]

Hi everyone,

I was trying to measure a voltage source with 10kohm internal impedance, and I connect the source directly with NI-USB 6009. When the voltage source is 5.043V, the data recorded by labview is around 4.8V. I checked the manual of USB-6009 and found that the input circuit (see the attachment). So I guess when the device was taking data, it was actually taking the data on the crossing node and then converted it back with impedance figures shown in the graph. That's why I got data lower than it should have been.

I was wondering is there any way to deal with it, other than calculating back and converting it with correct impedance?

Regards,

Jude

[GerdW · Accepted solution]

Hi Jude,

you could use an OpAmp to amplify your signal (1:1) an so lower the impedance seen by the USB6009. You could also use some other DAQ hardware with higher input impedance.

In the end the conclusion is: you have to choose your DAQ hardware according to specifications of signal source. You have chosen the wrong hardware...

[JudeLi]

Hi GerdW,

Thanks for replying! It helps a lot!

Regards,

Jude

[SnowMule]

---

@GerdW wrote:

you have to choose your DAQ hardware according to specifications of signal source. You have chosen the wrong hardware...

---

While I normally agree with this...

... The cost difference between a $1500 DAQ with a reasonable Z-in and a 600x with an op-amp ... is a pretty big difference.

I don't mind doing building signal conditioning circuits if it allows me to lower the cost of a system by $1k.  And when you multiply that system by 10+ units, that adds up to a -lot- of money.