Offset on analog output chanel without signal.

[fishywisky]

Hello,
I've been working with the USB 1808X DAQ, outputting an analog signal on AOUT1. Everything is fine until I output my siangal in the pluged SMA cable, then my signal develops an unwanted offset. I waited a day and redid the test with flying cables, and I got a perfect signal with a 0V offset. However, when I plug in the SMA cable again, BOOM, the offset comes back.

The cable doesn't short circuit the analog output to ground. No voltage is applied to the non-DAQ end of the cable. I tried connecting the pins to the ground of the DAQ and the ground of a generator. I tried to unplug and replug the DAQ but the offset is still there just after the init led stops blinking. My computer in linked to the ground via it's charing cable.

Thank you in advance.

[Fausto]

Hello @fishywisky.

What is the brand and model of the SMA cable?  Please share a datasheet.

Did you modify the SMA cable?

Have you tested a standard BNC 50 Ohm cable with the USB-1808X output?

Can you share an image of your wiring connection between the SMA cable and USB-1808X?

Regards,

Fausto

[fishywisky]

This a cable that was ordered on https://fr.rs-online.com/web/p/cables-coaxiaux/7839644?srsltid=AfmBOooQtuDgQOfWAG9ac5nyBZN7bDeP1XEpyXZ23D92NFiU_-NIuusP. 

 

Informations : "

RG174 Coaxial Antenna Cable

This low-power radio solution offers users antenna cables to connect SMA connectors to wireless applications. The 150 mm RG174 is an antenna cable with a straight male SMA plug on one end and a panel-mount female SMA plug on the other.

What is a Coaxial Cable?

A coaxial cable is a specific type of cable made of copper combined with other metallic shielding materials and components to block signal interference. It includes a physical channel for carrying the surrounding signal, usually accompanied by an insulating layer.

How Do Coaxial Cables Work?

Coaxial cables work through concentric layers of electrical conductors and insulating material. This structure ensures that a signal is contained within the cable and prevents any electrical noise from disturbing the signal.

Typical Applications

Signal transmission for televisions and antenna connections

Signal transmission for surveillance camera images

Broadband signal transmission

Connecting satellite antenna equipment to homes and businesses

Interconnecting telecommunications and data equipment in industrial settings

Facilitates the transmission of radio frequency signals

Excellent for industrial purposes as well as portable devices

Commonly used for low and high-power radio frequency (RF) connections

Examples include radio tower antenna connections, CB and cell phone antenna connections, and microwave transmitter and receiver applications

Features and Benefits

RoHS compliant

Weight: 10 g

Nominal impedance: 50 Ω

Straight male SMA connector and panel-mount female SMA connector

LPRS 150 mm RG174 Antenna Cable

This LPRS 150 mm RG174 antenna cable is equipped with a male SMA plug on connector A and a female SMA plug on connector B. The total length is 167 mm and it offers a characteristic impedance of 50 Ω.

External Antennas

"

 

Did you modify the SMA cable?

Yes, I extended the ground and signal wires (see picture).

 

Have you tested a standard BNC 50 Ohm cable with the USB-1808X output?

Yes, but I still noticed the offset.

 

What’s strange is that even when I unplug the SMA the cable, wait, restart my computer, run an example code from the MCCULW library (a_out_scan.py), I still see this small offset. 

a_out_scan.py

[Fausto]

Thank you for the images.

The UL Python example attached to your post sends signals out of both analog output channels.

1 hour ago, fishywisky said:

What’s strange is that even when I unplug the SMA the cable, wait, restart my computer, run an example code from the MCCULW library (a_out_scan.py), I still see this small offset. 

If there's no cable connected to the USB-1808X, then the offset is coming from the SMA cable to your oscilloscope.

I tested a USB-1808X and a BNC (50 Ohm) to mini-grabber cable with DAQami.  The BNC end was connected to an oscilloscope and I did not see an offset on the display.

When I disconnected the mini-grabber ends from my USB-1808X, then I saw the following signal on the oscilloscope, with the BNC end still connected to the oscilloscope.

 

Was your cable modified inhouse?

Confirm the wire connections are secured at the screw terminals.

Since you do not see an offset using the flywires, take a closer look at your modified cable.