Chapter 4 Analog InputNI USB-621x User Manual 4-16 ni.comAI Convert Clock SignalUse the AI Convert Clock (ai/ConvertClock) signal to initiate a single A/Dconversion on a single channel. A sample (controlled by the AI SampleClock) consists of one or more conversions.You can specify either an internal or external signal as the source ofAI Convert Clock. You also can specify whether the measurement samplebegins on the rising edge or falling edge of AI Convert Clock.By default, NI-DAQmx chooses the fastest conversion rate possible basedon the speed of the A/D converter and adds 10 μs of padding between eachchannel to allow for adequate settling time. This scheme enables thechannels to approximate simultaneous sampling and still allow foradequate settling time. If the AI Sample Clock rate is too fast to allowfor this 10 μs of padding, NI-DAQmx chooses the conversion rate so thatthe AI Convert Clock pulses are evenly spaced throughout the sample.To explicitly specify the conversion rate, use AI Convert Clock RateDAQmx Timing property node or function.Caution Setting the conversion rate higher than the maximum rate specified for yourdevice will result in errors.Using an Internal SourceOne of the following internal signals can drive AI Convert Clock:• AI Convert Clock Timebase (divided down)• Counter n Internal OutputA programmable internal counter divides down the AI Convert ClockTimebase to generate AI Convert Clock. Started by AI Sample Clock, thecounter counts down to zero, produces an AI Convert Clock, reloads itself,and repeats the process until the sample is finished. It then reloads itself inpreparation for the next AI Sample Clock pulse.Using an External SourceUse any input PFI line as the source of AI Convert Clock.Routing AI Convert Clock to an Output TerminalYou can route AI Convert Clock (as an active low signal) out to any outputPFI terminal.
