AC signal input to PXI 4462 with 1Mohm resistor in series. The reading is bad.

[jytang]

I input a 300mV, 20KHz AC signal from PXI 4461 AO to 4462 AI with 100Kohm resistor in series between. This is a test as part of a special measurement. However, the reading from 4462 is not 300mV, instead it read much lower voltage (~60mV). With lower frequency (50Hz), I can get ~270mV. And this is clearly depend on the resistance series since without the resistor I can get 300mV at all frequency. What's wrong with my setup?

[dsb@NI]

What is your terminal configuration on AO? AI?

[jytang]

Here are the vi for AO and AI

[dsb@NI]

AO terminal configuration is set to differential in DAQ Assistant in AO0 out.vi. How did you insert your resistor? Can you try changing your output terminal configuration to pseudodifferential?

[jytang]

The resistor is connected between the inner core of AO and AI. The outers are directly connected with BNC cable. Change the mode does not affect the result.

[Henrik_Volkers]

Please make a sketch of your setup including type and length of cables ...

With impedances in the 1Meg or 100k range and 'hidden' capacities you quickly build a RC lowpass.

100pF in 1m BNC cable with 1Meg source impedance is a lowpass with (about) fc=1.6kHz 😉

[Larry_Morgan]

For the error in your 50Hz reading:

The 446x devices have a fixed input impedance of 1Mohm. The 100K of series resistance in the signal path is forming a voltage divider with an attenuation of ~0.91, which gives the 270mV you are reading when 

generating a 300mV signal.

For the error in your 20kHz reading:

In addition to the capacitance Henrik mentioned that is present in cables, the 446x series has input capacitance of its own inherent to the device, which cannot be avoided. This capacitance alone in combination with the

series 100K of resistance you are putting in the signal path can form a low pass filter as low as ~7.3kHz. Such a response would give readings very close to what you are reading. 

Essentially, all of discrepancies you are seeing are due to effects caused by the series 100k resistor in your signal path. 

I hope this helps you solve and understand some of your concerns!