Drive CO Timing Off of Non-TTL Clock?

[iBrow1000]

Hi,

I'm using a PCIe-6321. I'm trying to send a counter output for specified numbers of ticks based on an external clock coming into a PFI input.

The problem I'm having is that my external signal is not TTL, it is an AC coupled 50 ohm 1 volt signal (+/- 0.5 V about the middle). The voltage difference is too small for the DAQ to notice.

What is the best solution to this? Is there a better approach than using an external amplifier to turn the clock into something the DAQ understands (0 - 5 V signal)?

thanks,

Isaac

[iBrow1000]

Actually, to expand on my question, what are the specifications for a valid external clock signal to drive a counter? Is it basically just the DIO line TTL specifications?

Which from what I can find are:

High level, min. 2.2 V, max. 5.25 V

Low level, min 0 V, max. 0.8 V

If I go the amplifier route, I need to make sure I get something that will work and not cause damage.

thanks,

Isaac

[Tanner!]

iBrow1000, you're correct in stating that you'll need to shift your signal. There are several approaches to solving this with varying degrees of difficulty and robustness ranging from op-amp configurations to ICs made for this purpose.

As far as not causing damage, just make sure that the supply voltage on your op-amp is something safe. Your output signal can't exceed that. This device has over protection up to 20V, so if you put in 5.5 V you should be fine.

This is a pretty good article about how to select a decent instrumentation amplifier for your application as well as avoiding some common pitfalls:
http://www.edn.com/design/analog/4322833/The-right-way-to-use-instrumentation-amplifiers

[iBrow1000]

Thanks for getting back to me, and for the info about the voltage protection.

Lets say that I just amplify my +/- 0.5 V signal into a +/- 5 V signal without shifting the level. Based on the +/- 20 V protection listed in the specifications, this would not damage the board and would allow normal edge counting operation. Is that correct?

thanks,

Isaac

[Tanner!]

I would advise against it. It might be alright, but it's not a true TTL signal and the input is not expecting negative voltages. I would recommend either amplifying to +/-2.5V and then a DC offset, or you could amplify like you're intending and use a clamper circuit to only get the positive side of your signal.

[johnsold]

The proper and most robust way of converting your signal is to use a comparator. Simply amplifying the signal, besides not having the correct voltage levels, may also not have the required edge speeds.  I have not looked at the specifications of the PFI input, but many digital input circuits do not work well with slow transitions between the high and low levels. Select a comparator with an active TTL output and everything should work well. Most op amps are too slow to make good comparators for logic circuits.

Lynn