5.2.2.9 Analog Trigger Threshold
The analog trigger threshold can only be set by the output of D/A channel 1, with an equivalent range from -10V to +10V (full A/D converter input range). The threshold level is set at the A/D converter input (after the programmable gain amplifier), not the one at the input connector (before the PGA). The following formula is used to convert the code value (C/-2048 to 2047) written into D/A channel 1 to the trigger level set against the analog input voltage (U) for the selected A/D input channel:
U = (C/2048) * (10/G)
where U is in volts and G is the internal gain of the selected analog input channel. In the case of an expansion card, further calculation can be performed by converting the value of U obtained above with the transfer function of the expansion card. There is a fixed hysteresis of about 10 mV in the analog comparator circuit used for generating the analog trigger. The accuracy of the analog trigger level is typically 1% of the A/D input range (±10v), or within ±0.1v.
The selected analog input channel is always the one specified as the first entry of the scan list queue (the one with the first channel mark set to “1”). However, this is not the case with the pre-trigger option. After receiving the arm command, the A/D converter is already scanning the analog input channels specified in the scan list. The analog comparator generates an analog trigger any time it’s input passes through the trigger threshold. For example, if one of the input channels specified in the scan list is below the threshold while another is above it, the trigger is generated because the analog comparator input passes through the threshold in either direction during channel scanning. The following conditions must both be met to use the analog trigger with pre-trigger option selected: (1) The input channels specified in the scan list must be either all below or all above the analog trigger threshold before the trigger event occurs. (2) When the trigger event occurs, at least one of the input channels passes across the threshold in either direction and at least one input channel stays on the original side.
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