Dear Roger,

On page 51 of DT800 user's manual , It enumerates the sampling speeds for each channel, when one channel is sampled as voltage(NC)and voltage(corrected), there is no much difference. But when channels sampled simultaneously becomes 5, great changes take place.

Could you explain the reason? Besides, as for voltage(VNC), the sampling speeds between 1 channel and 5 channels are almost the same. Why?

Best Wishes.

Good morning kangle2miao,

The way the AtoD works is actually quite complex. I think what you are seeing here is the way the channels are grouped to fit into the AtoD buffer frame. basically the AtoD has a binary buffer that is 2^2 up to 2^8 bytes in size. (That is 2,4,8,16,32,64,128 or 256 bytes in size) This is buffer is used to communication between the AtoD and the CPU.

If the number of fundamental samples is less than the binary buffer size the extra spaces are padded out with null readings. If you happen to fit exactly into a buffer frame you have the best sample speed.

Cheers,
Roger

Dear Roger,

When the number of channels and the type are specified , the operation mode is also specified, then sampling speed is decided, for example, for one VNC channel type, the sampling speed is 45Hz in normal mode. Am I right? May I change the sampling speed? How?

Thank you!

Good afternoon kangle2miao,

That is the fastest sampling speed for normal mode. You can always go slower by simply setting the speed you want in the schedule rate.

In normal mode it won't go any faster, that is why we have the three different modes. When you think about the 45 Hz speed it's quite impressive when you consider that each channel is samples for 20 milliseconds so has a theoretical maximum of 50 samples per second. This means the dataTaker is taking 2 milliseconds read the value from the AtoD, calculate the voltage, do the noise rejection calculations, add any scaling factors, spans, polynomials etc. Save the data to the disk and do all the required comms work.

One you remove the main rejection you are no longer restricted to sampling over the 20 millisecond time period and so the process is much faster.

Cheers,
Roger