DT9844 Incorrect Measurements on Single Ended Input

[lesthaeghet]

I have a DT9844 that we have been successfully using in differential mode without any major issues.  However, we switched to single ended mode for the first time and are finding that reported voltages are not correct.  This is both with QuickDAQ and the API.  All unused channels are tied to the adjacent analog ground and TB3,12 (analog ground) is tied to TB3,13 (amplifier low).  

A few test scenarios... if I perform a differential measurement with a test voltage of +6V in on TB1,1 (analog in 0) and ground from the power supply at TB1,2 (analog in 0 di return/analog in 8 se), we see +6V perfectly as expected.  If I switch to single ended measurement in the same configuration, I see -0.64 V at Ain 0 and -6.6 V at Ain 8 with lots of noise (which I think is to be expected in this configuration, but the difference comes out to 6V).  If I short TB1,2 and the power supply ground connection to TB1,3, I see 1.23 V (in both single ended and differential). Obviously, 1.23 V is not 6 V.

I have independently checked potentials in all of these circumstances across different connection points externally with a multimeter in all of these conditions and confirm that we see 0V (relative to power supply ground) where we expect to see 0V and 6V where we expect to see 6V.  

The other relevant piece of information is that when adjusting the source voltage input during the single ended measurement, there is a limit to how large the voltage will read.  Roughly 1.5 V is the max.

Am I missing something here or is there something wrong with this board?

Thanks!

[Fausto]

Hello @lesthaeghet.

Please confirm the acquisition input mode was changed to SingleEnded in QuickDAQ.  

 

What is the set sampling frequency?

From the DT9844 User Manual, figure 7 shows a simple single-ended connection to a DT9844-32-STP box.

 

Please test the following scenario with QuickDAQ and in single-ended mode.  Disconnect all your wiring to the terminal blocks.  Afterward, connect TB3, 18 (5V Analog) to TB1, 1 (Analog In 0).  Also, connect TB3, 17 (Digital Ground) to TB1, 3 (Analog Ground) and TB3, 13 (Amplifier Low).  Reply with a screen capture of the acquisition.  The reading should be around 5V.  If it is, retest your signal source.

 

Regards,

Fausto

[lesthaeghet]

Hi @Fausto,

Thanks for the quick follow up!  Just ran the test.  It is set up as you instructed... TB3,18 connected to TB1,1; TB3,17 to TB1,3 and TB3,13 and set for single ended mode (also, I didn't mention this before, but 10k termination is also disabled).  Here's the result:

The 1.15V reading is consistent with what we've seen so far with an external power supply. 

Also, just to be sure, I did use a multimeter to measure the potential across TB3,18 and TB3,17.  It is reading 5.10 V.

Thoughts?

[Fausto]

Hello @lesthaeghet.

Thank you for testing.

Please test a few other channels with TB3, 18 (5V Analog) and in single-ended mode.  If you continue to get similar readings, then please reply via private message with the serial number from the backside label on the DT9844; an image of the label is helpful.  The serial number is used to determine if the module can be serviced.

Regards,

Fausto

[lesthaeghet]

Hi @Fausto, 

Thanks for the quick follow up!  I can confirm repeating the test procedure on all channels and getting the same result of 1.15V.  I'll send you a private message shortly.

Thanks!