How to match impedance between AT-MIO-16 and 50 Ohm output of preamp

[sonnymoon]

I am trying to count TTL signal from a preamplifier/discriminator (ARI Corp's MTS-100) using the AM9513 chip built into the NI AT-MIO-16 A/D board.
With only power to the preamp (i.e. no input signal) I get ~150,000 or so counts per second, while with a different rate meter I get normal 5-10 counts per second. These excess counts would seem to be caused by an impedance mismatch leading to ringing in the line from the preamp to the A/D board.

The TTL output from the preamp has an impedance of 50 Ohms, minimum pulse width of 10 ns. I use a coax cable with impedance of 50 Ohms that is terminated with a 50 Ohm cap just before going to the connector block for the A/D board. Bottom line, I match the imp
edance as well as I can and still get reflection. The detector,preamp,A/D board,computer,etc are all tied to the same ground. The discriminator level has little or no effect.

Could I be connecting to the connector block of the A/D board incorrectly? Is there a threshold adjustment available for the Source (i.e. Vhigh) that might be set too low?

Is this hopeless? Or are there other things to try?

Any help would be appreciated.

Thanks,

sonnymoon

[Ben]

Hi its me again.

Can you see the ringing with a scope?

What exactly are you using to get the counts from the board?
Are you sure you are re-setting the counter and have the clock configed to come from your external signal?

Ben

[Ben]

Also go back back to one of NI's canned examples for the counter. Make sure it works.

Double check the ground connection for your external clock. It may not be the same as for the AI.

Ben

[sonnymoon]

Ben on 7/25/2002 answered:
"Hi its me again.

Can you see the ringing with a scope?


What exactly are you using to get the counts from the board?
Are you sure you are re-setting the counter and have the clock configed to come from your external signal?


Ben
*******
Hi Ben,

Thanks for your help on this. I have tried using both Labview software and NI Measurement and Automation test panels. I remember trying to figure out how to address the counter correctly using Labview "Counter Start", "Counter Read", "Counter Stop" VI's. Perhaps you could confirm for me, I am putting the signal from the preamp into "Source 1" on the A/D connector board, and I use "GPCTR1" to identify the counter.

Does the "Counter Stop" VI reset the counter? I was as
suming it does. If not, how do I do that?

But, using the NI Measurement and Automation test panels, selecting "Simple event counting" and "GPCTR1" I see lots (100,000's) of counts pile up in 1 second when I should only see 5-10.

I haven't checked with a scope (we don't have one in our lab!) but I may try to borrow one to confirm it is ringing.

My basic set up is as follows:

BNC (50 Ohm) from preamp TTL out to BNC T adapter with a 50 Ohm Terminator cap.
Signal lead from BNC plug to PIN 41 (Source1) of AT-MIO-16 I/O connector. Ground lead to PIN 24 (DIG GND) which is also grounded to be the same as that of the preamp and dectector.

Thanks again for your help,

Joel

[Ben]

Hi Joel,

You did not mention a gate in any of all of this.

When you installed NI-DAQ for LV an example should have been included that will tell you exactly how to wire thigs up and how to do your measurement.

When LV fires up, go to Find Examples. Under there, navigate to

Hardware Input and Output >>> DAQ >>> Counters >>> am9513 >>> Measure Frequency Easy(9513).vi

Read the instructions on how to hook things up. You will need an external jumper to route a gate signal through.

Ben