With the auxiliary output of the LMH6518 offering a second output that follows the main one (except for a slightly reduced distortion performance), the oscilloscope trigger function is implemented by tapping this output. The auxiliary common mode is set with the V_{CM_Aux} input of the LMH6518. If required, the trigger function is placed at a distance from the main signal path by taking advantage of the differential auxiliary output and rejecting any board-related common-mode interference pick-up at the receive end.

If trigger circuitry is physically close to the LMH6518, the circuit diagram shown in Figure 7-8 allows operation using only one of two auxiliary outputs. Unused outputs require proper termination using R₁, R₁₁ combination. U3 (DAC101C085) generates a 0-V to 2.5-V trigger level, with 2.4-mV resolution as in Equation 15 or 0.7%
(= 2.4-mV × 100/0.35 V_PP) of FS, which is compared to the LMH6518 +OUT AUX by using an ultra-fast comparator, U2 (LMH7220). The U2 complimentary LVDS output is terminated in the required 100-Ω load (R₁₀), for best performance, where the LVDS trigger output is available.

Equation 15.

The LMH7220 offset voltage (±9.5 mV) and offset voltage drift (±50-µV/°C) error is 5.9 LSB of the trigger DAC (U3) as in Equation 16.

Equation 16.

The offset voltage related portion of this error is nulled, if necessary, during the oscilloscope initial calibration. To do so, the LMH6518 input is terminated properly with no input applied and U3 output is adjusted around V_{CM_Aux} voltage (1.2 V ±10 mV) while looking for the U2 output transition. The U3 output, relative to V_{CM_Aux} at transition corresponds to the U2 offset error, which is factored into the trigger readings and thus eliminated, leaving only the offset voltage temperature drift component (= 2 LSB).

Figure 7-8 Single-Ended Trigger From the LMH6518 Auxiliary Output

The U2 minimum toggle rate specification of 750Mbps with ±50-mV overdrive allows the oscilloscope to trigger on repetitive waveforms much greater than the 500-MHz oscilloscope bandwidth applications, when the input signal is at least 14.3% of FS swing with Equation 17.

Equation 17.

The worst-case, single-event, minimum-discernible pulse duration is set by the LMH7220 propagation delay specification of 3.63 ns (20‑mV overdrive).

Both the main and the auxiliary outputs recover gracefully and quickly from a 50% overdrive condition, as tabulated in Section 5.5 under overdrive recovery time. However, overdrive conditions beyond 50% can result in longer recovery times due to the interaction between an internal clamp and the common-mode feedback loop that sets the output common-mode voltage. This long recovery time can have an impact on both the displayed waveform and the oscilloscope trigger. The result is a loss of trigger pulse or visual distortion of the displayed waveform. To avoid this scenario, the oscilloscope must detect an excessive overdrive and go into trigger-loss mode. Done in this way, the oscilloscope display shows the last waveform that did not violate the overdrive condition. Preferably, there is a visual indicator on the screen that alerts the user of the excessive condition, and returns the display to normal after the condition is corrected.