voltage measurement error

[jeet]

Hello,

I am using a PXI system (PXIe-1073 chassis, PXIe-6363 DAQ card, PXI-2567 relay driver card) connected to my custom PCB.

My PCB has a 2.5V output precision reference IC whose output is connected to an analog input of PXIe-6363 DAQ card through the 68-pin connector and DAQ cable (SHC68-68).

When I measure the 2.5V output at the 68-pin connector (Analog input pin of the connector) using an external DMM (with High Z input impedance), I measure exactly 2.5003V, which is what I expect. However, LabVIEW reports back 2.4987V with an error of 1.6mV.

So, I disconnect my PXI system from my PCB and connect the DAQ card cable to the SCB-68 connector block and supply 2.5V external supply to the analog input and GND through the connector block. Now, both external DMM and LabVIEW report exactly 2.5V. This suggests that the DAQ card and software (voltage measurement VI) are working fine.

Then why do I have an error of 1.6mV when the PXI system is connected to my PCB ? If the problem is in my PCB, then why does the external DMM measure accurately right at the DAQ 68-pin connector ? What could be going on ?

Thanks

Jeet

[Mockingbird]

Hi Jeet,

When you hook up your PCB how are you wiring the analog input? Specifically, how are things wired between your PCB board and the DAQ card? When you see the 2.5V with the external suppy is the PXI chassis still in the system?

[jeet]

Hello,

Yes, the PXI chassis is still connected to the PCB when I measured with an external DMM.

Upon further investigation, I found that my switched mode power supply was injecting 240 mV-pp noise on the Ground. This Ground noise was getting filtered down to 60 mV-pp through a choke but it still wasn't good enough. So I used another RF bead (with 1K impedance @ 100 MHz) to further suppress the ground noise and that did the trick. Now my voltage measurements are right on.

Thanks

Jeet.