Figure 7-2 shows a block diagram of the LMH6518 main output signal path.

The auxiliary output (not shown) uses another but similar output amplifier that taps into the ladder attenuator output. In this data sheet, preamp gain of 30 dB is referred to as high gain (HG), and preamp gain of 10 dB as low gain (LG).

The LMH6518 2-dB/step gain resolution and 40-dB adjustment range (from −1.16 dB to 38.8 dB). These specifications allow this device to be used with the TI GSPS ADCs, which have full scale (FS) adjustment through the extended control mode (ECM) to provide near-continuous variability (8.5-mdB resolution) that covers 42.6-dB FS input range using Equation 1.

TI's GSPS ECM control allows the ADC FS to be set using the ADC SPI bus. The ADC FS voltage range is from 560 mV to 840 mV with 9 bits of FS voltage control.

The ADC ECM gain resolution is calculated with Equation 2.

However, the recommended ADC FS operating range is narrower: from 595 mV to 805 mV with 700 mV_PP as the midpoint. Raising the value of ADC FS voltage is tantamount to reducing the signal path gain to accommodate a larger input and vice versa, thus providing a method of gain fine-adjust. The ADC ECM gain adjustment is −1.21-dB, as in Equation 3.

The ADC FS fine-adjust range of 2.62 dB (= 1.41 dB + 1.21 dB) is larger than the LMH6518 2-dB/step resolution; therefore, there is always at least one LMH6518 gain setting to accommodate any FS signal from 6.8 mV_PP to 920 mV_PP at the LMH6518 input, with 0.62-dB (= 2.62-2) overlap.

Assuming a nominal 0.7-V_PP output, the LMH6518 minimum FS input swing is limited by the maximum signal path gain possible and vice versa with Equation 4.

(or 8 mV_PP with no ADC fine adjust in Equation 5)

(or 800 mV_PP with no ADC FS adjust)

To accommodate a higher FS input, an additional attenuator is required before the LMH6518. This front-end attenuator is shown in the Figure 7-1 with details shown in Figure 7-12. The highest minimum attenuation level is determined by the largest FS input signal (FS_max) in Equation 6.

Therefore, to accommodate 80 V_PP, a 40-dB minimum attenuation is required before the LMH6518.

In a typical oscilloscope application, the voltage range encountered is from 1 mV/DIV to 10 V/DIV with eight vertical divisions visible on the screen. One of the primary concerns in a digital oscilloscope is SNR that translates to display trace width to thickness. Typically, oscilloscope manufacturers require the noise level to be low enough so that the no-input visible trace width is less than 1% of FS. Experience shows that this corresponds to a minimum SNR of 52 dB.

The factors that influence SNR are:

• Scope front-end noise (Front-end attenuator + scope probe Hi-Z buffer which is discussed later in this data sheet and shown in Figure 7-1)
• LMH6518
• ADC

The LMH6518 related SNR factors are:

• Bandwidth
• Preamp used (Preamp HG or LG)
• Ladder attenuation
• Signal level

SNR increases with the inverse square root of the bandwidth. Therefore, reducing bandwidth from 450 MHz to 200 MHz for example, improves SNR by 3.5-dB, as seen in Equation 7.

The other factors listed previously, preamp and ladder attenuation, depend on the signal level and also impact SNR. The combined effect of these factors is summarized in Figure 7-3, where SNR is plotted as a function of the LMH6518 FS input voltage (assuming scope bandwidth of 200-MHz) and not including the ADC and the front-end noise.

As Figure 7-3 shows, SNR of at least 52 dB is maintained for FS inputs greater than 24 mV_PP (3 mV/DIV on a scope) assuming the LMH6518 internal 200‑MHz filter is enabled. Most oscilloscope manufacturers relax the SNR specifications to 40 dB for the highest gain (lowest scope voltage setting). From Figure 7-3, the LMH6518 minimum SNR is 43.5 dB, thereby meeting the relaxed SNR specification for the lower range of scope front panel voltages.

In Figure 7-3, the step change in SNR near Input FS of 90 mV_PP is the transition point from preamp LG to preamp HG with a subsequent 3‑dB difference due to the preamp HG to 20‑dB ladder attenuation lower output noise compared to preamp LG to 2‑dB ladder attenuation noise. Judicious choice of front-end attenuators maintains the 52‑dB SNR specification for scope FS inputs ≥ 24 mV_PP by confining the LMH6518 gain range to the lower 30.5‑dB using Equation 8 from the total range of 40‑dB (= 38.8 – (−1.16)) is possible.

For example, to cover the range of 1 mV/DIV to 10 V/DIV (80-dB range), Table 7-1 lists a configuration that affords good SNR.

In Table 7-1, the highest FS input in row 5, column 2 (80 V_PP), and the LMH6518 highest FS input allowed (0.8 V_PP) set the front-end attenuator value with Equation 9.

The 100 × attenuator allows high-SNR operation down to 30.5‑dB, as explained earlier, or 2.4 V_PP at scope input. In that same table, rows 1 to 3 with no front-end attenuation (1 ×) cover the scope FS input range from 8 mV_PP to 800 mV_PP. That leaves the scope FS input range of 0.8 V_PP to 2.4 V_PP. If the 100 × attenuator is used for the entire scope FS range of 0.8 V_PP to 80 V_PP, SNR dips below 52-dB for a portion of that range. Another attenuation level is thus required to maintain the SNR specification requirement of 52 dB.

One possible attenuation partitioning is to select the additional attenuator value to cover a 20 dB range above 0.8 V_PP FS (to 8 V_PP) with the 100 × attenuator covering the remaining 20‑dB range from 8 V_PP to 80 V_PP. Mapping 8 V_PP FS scope input to 0.8 V_PP at LMH6518 input means the additional attenuator is 10 ×, as shown in Table 7-1, row 4. The remaining scope input range of 8 V_PP to 80 V_PP is then covered by the 100 × front-end attenuator derived earlier. The entire scope input range is now covered with SNR maintained approximately 52 dB for a scope FS input ≥ 24 mV_PP, as shown in Table 7-1.