impedance maching

[stuwilkins]

Hi,

I am using the 6602 as a counter / timer for x-ray detector pulses. I have a few questions which (sorry if they are in the manual) I need to design the board round this.

What is the input impedance of the source inputs and gates? Is this terminated (by 50 ohm) or by some other value? If so what?

What is the output impedance of the "out" pins??

Many thanks,
Stuart

[AnalogKid2DigitalMan]

Looks like for both the digital I/O and timing I/O:

Inputs: 25K-500Kohm

Outputs: 75Kohm

From the 66xx family specifications:

http://www.ni.com/pdf/manuals/372141a.pdf

[stuwilkins]

Thanks very much,

I am confused however,

The specs state:

Output impedance...........................75 kΩ (56 kΩ from an
onboard resistor and 19 Ω
from the TIO ASIC)

I assume that the last is a typo is it 75 ohms or 75 k ohms? That makes quite a difference!

Stu

[AnalogKid2DigitalMan]

Stu:

Good catch, I didn't think too much about it at the time, I needed coffee.

I looked at a spec sheet I had on file, dated Feb '04 (P/N 370990A-01) and it shows 75 ohms (56 + 19). I would think 75 ohms is correct, since it is a high speed card.

I have noted quite a bit of typos in NI's spec sheets and manual in the past.

Hopefully someone from NI will see this post and give the definitive answer.

-AK2DM

[EBL]

75 ohms is correct for the output impedance. Each line has a 56 ohm resistor between the TIO and the connector, and the TIO's output impedance is apparently around 19 ohms. So if you were using the lines as outputs you might want to match to 75 ohms.

On the other hand, the inputs (which you would be using for pulse-counting) are essentially high-impedance. The wide-ranging 25 kohm to 500 kohm input impedance listed in the spec is no doubt due to tri-state driver leakage and untrimmed pull-down resistors inside the TIO. In any case, the input impedance is high, and no impedance matching per se is possible. If your pulses are fast and you want to maintain good signal integrity, match your signal source impedance to your cable impedance; that way you'll present the 6602 input with a Thévenin equivalent of your source impedance. There will be a single reflection at the load, but in spite of the reflection the signal at the very end (at the TIO input) should be fine, providing that your pulse rise and fall times are not much less than 1 ns.

Hope this helps,
Ed L.



Message Edited by EBL on 03-28-2008 02:23 PM

[stuwilkins]

Thanks,

I was intending on terminating the detector output signal with a 50ohm resistor (terminator). The 75ohm output is more of a problem because most of the x-ray detector and counting electronics is all 50 ohm, I sometimes wonder why NI use such a strange impedance (for a counter / timer).

I have just got of the phone and NI have confirmed that it is definitely 75 ohm. The datasheet is in error.

Stu

[AnalogKid2DigitalMan]

You could use a TTL to 50 ohm line driver (ie 74S140), it will invert your signal though. Could use an inverter (ie 74S04)ahead of the 74S140 to correct for this.

Are you plannin on using 50 ohm coax cable to connect from the 6602 outputs to inputs of your equipment?

[stuwilkins]

Hi,

Yes, that is exactly what I am going to do, actually the reason I am asking these questions is I am making a breakout PCB which has LEMO-OO connectors for the inputs. I need to get out the trigger (out) and gate signals also. So, it is going

NI6602 -> 1m cable (hopefully 0.4m) -> PCB with LEMO-00 (terminated 50ohm) -> EXPT

The "out" will be via

NI6602 -> 1m cable -> PCB (inverter + line driver) ending in LEMO-00

What I am not sure about is wether there is enough current to drive a 75 ohm termination of the output (I guess not) so rebuffering is most likely necessary.

All detectors are connected with 50ohm coax, either with LEMO-00 or BNC connectors.

S

[EBL]

Hi, Stu.

>>What I am not sure about is wether there is enough current to drive a 75 ohm termination of the output (I guess not) so rebuffering is most likely necessary.<<

Probably maybe perhaps. The TIO output is just a CMOS switch to either ground or 5 V, where the resistance to ground is probably something like 15 ohms and the resistance to 5 V is probably something like 25 ohms (strictly a semi-informed guess). Except at some point the MOSFETs saturate and stop producing increasing current with increasing voltage across the MOSFET. Unfortunately, I don't know what that current limit is. In any case, it will -probably- work if loaded with 75 ohms.

However, as with the input case, I'd recommend a singly-terminated system, where the 6602 output has 75 ohms output impedance, you run to the buffer with a 75 ohm line, and your buffer input is un-terminated. You're far less likely to see loading problems with that scenario, and the single reflection should not be noticed by the buffer.

One question occurs to me: will your detector produce "TTL" level pulses when loaded with 50 ohms? That is, will it swing below 0.8 V and above 2.0 V, the required input levels for the 6602?

Cheers,
Ed

[stuwilkins]

Hi Ed,

Thanks for your comments. I took another look through the manual of the 6602, and it suggests terminating with at 68ohm resistor in series with a 150pF capacitor. I will probably do this for the OUT signals.

For the input, I personally would love a way of terminating the 50 ohm on the 6602 board but that does not seem to be possible! I will have to terminate on the PCB. I am thinking it may be good to buffer the signals before sending them to the 6602 but that seems to be starting to make something quite complicated!

Yes, most x-ray detectors use 50 ohm TTL which is either 0 to 5V into 50 ohm or 0 to 2.5V into 50 ohm depending on how good the output stage is.

Stu