Measuring AC signal with cDAQ using two slots, phase shift between AC measurement

[AI_CC]

Hi, 

I'm trying to measure AC signal with cDAQ 9174 using two slots. 

The frequency of AC signal is about 50 Hz so I measure the frequency with 1000 Hz sampling rate.

At first try, I just extand the physical channels as shown in figure below.

However, when I measured same voltage with slot1 and slot2, the measrued voltages have phase shift between them. 

After some search, I modified my labview code to share sample clock and starttrigger so that slot1 and slot2 measure the voltage at the sametie.

And I got error code as follows.

Could I get some help please? 

I also attached my code.

Thank you in advance.

[dsb@NI]

I am a fan of channel expansion (putting all the input channels in the same task) as it allows me to normally avoid the complexities of synchronization.

How much phase did you measure? Was it consistent from run to run? Does it correlate to the different input impedance between the 9203 and 9220?