At 12:57 +0100 02/11/2004, Roman Golubovski wrote: >I would like to be able to modify the InterchannelDelay (configurable) with >the "AI Config.vi". > >Specifically, I need as much "simultaneity" as possible within E cards' 16 >channels.
You can specify the interchannel delay to be as small as the speed of the card. Thus a 100 kS/s card could have a 10 microsecond interchannel delay. BUT!!! you get decreased accuracy and channel bleed over if you push the card to it's top speed. For example the 6030 is 100 kS/s, but if you want to get the full settling of each channel to less than 1/2 LSB then you need to reduce the card to 60 kS/S (I believe, this is from memory). The automatic feature tries to set this interchannel delay such that it is as short as possible and allowing as much settling as possible. Thus the interchannel delay for the card above will be 60 khz as long as the data acq is less than 60 kHz. Above 60 kHz the delay will be the data acq speed. There are even lower limits if you change the gain of the amplifier as you switch from channel to channel. For each multiplex channel switch there is some capacitance and gain settling time. If you are measuring low impedance signals then the capacitance may not be as big a problem, if you are measuring widely different signals then the interchannel cross talk may be too much. Check the board documentation and see what you need in terms of accuracy. Pay close attention to the resistors that allow a return path for input bias currents on the amplifier. Those resistors can change the interchannel cross talk immensely. The default setting gives you as much simultaneity as possible at the full accuracy of the card. If you want to trade off more accuracy then you can specify a different interchannel delay. There are simultaneous sampling cards that have a sample-and-hold circuit on the input to get truly simultaneous measurements. There is ALWAYS a trade off between accuracy and speed. Again, the data could be re-sampled in software to give an interpolated simultaneous measurement given knowledge of the true interchannel delay. It is impossible to really correct for interchannel cross talk in software. -Scott
