common mode rejection ratio

[chipie]

I am using the PXI-4461 and the sound and vibration toolkit. I need to test the common mode rejection ratio of Amplifier.

Can sound and vibration toolkit do that?  I can find  the same phase input???

[gwd]

I'm assuming you want to test the CMRR of an external amplifier (not inside the PXI-4461).

I don't see why you want to use the sound and vibration toolkit. This can be done perfectly well in just plain Labview. In fact if you only need a single frequency you could even do it in measurement and automation explorer.

Testing CMRR of a GOOD amplifier can be tricky to set up, as there will be lots of ways to get small spurious signals at the output.

[chipie]

Yes, I want to test the CMRR of an external Amplifier.

Because I  use sound and vibration toolkit to test performence of the Amplifier all the while. I am not clearly how to use DAQmx  to Read and Write directly. I think it 's perfect that  the sound and vibration have the same phase balanced output, then I can simply changed the  output setting of the ao0 or ao1.

Can you give me a examply how to  realize testing the CMRR with DAQmx? Thanks..

[gwd]

CMRR is defined as the magnitude of the ratio of the gain of a differential mode signal to the gain for a common mode signal. .The common mode gain is measured by connecting an IDENTICAL signal to the two inputs and measuring the signal at the output.

If you use two different analog outputs there will be some difference between them (due to instrument errors). So you would measure a ratio that is partly determined by the characteristics of the amplifier and partly determined by the characteristics of the analog sources.

Better is to connect both inputs to ONE source and measure the gain to the output.

[chipie]

In sound and vibration toolkit, output  AO0 or AO1 can select the follows setting: differentital, pseudodifferential, RSE. The setting indicates one output. That means AO0 can set to differentital, pseudodifferential..... and AO1 can set to differentital...... too.  I want to test the CMRR of the Amplifier, and I certainly only use one of the output to test.

When the output signal is set to differentital, voltage of AO(+) and AO(-) are reverse phase. and when I want to test CMRR, I must set voltage of the AO(+) and AO(-) to same phase. The AO(+) and AO(-) is the two voltage of the same output.

The trouble of  test CMRR is I don't know how to set AO(+) and AO(-) to same phase.  

Thanks for your help!  

[gwd]

I don't understand the problem. You want both inputs to the differential amplifier to be the same. So just connect them to the same output.

(I am not familiar with either sound and vibration toolset or the particular DAQ you have. Maybe someone who has used these things will understand the issues.)