Covering the range from 1 mV/div to 10 V/div requires the following adjustment within the digital oscilloscope:

• Front-end attenuator
• LMH6518 preamp
• LMH6518 ladder attenuation
• ADC FS value (ECM)

The LMH6518 product folder contains a spreadsheet that helps calculate the front-end attenuator, LMH6518 preamp gain (HG or LG), ladder attenuation, and ADC FS setting based on the scope vertical scale (S in V/div).

The following step-by-step procedure explains the operations performed by the spreadsheet based on the scope vertical scale setting (S in V/div) and front-end attenuation A (from Table 7-1). A numerical example is also worked out for more clarification:

1. Determine the required signal path gain, K, with Equation 10:
   Equation 10.
   Assuming the full-scale signal occupies 95% of the 0.7 V_PP FS for 5% overhead that occupies eight vertical scope divisions.
   Required condition: −2.37 dB ≤ K ≤ 40.3 dB
   Example: With S = 110 mV/div, Table 7-1 shows that A = 10 V/V in Equation 11.
   Equation 11.
2. Determine the LMH6518 gain, G:
   G is the closest LMH6518 gain to the value of K where
   • G = (38.8 – 2n)dB; n = 0, 1, 2, …, 20

   For this example, the closest G to K = 17.57 dB is 16.8 dB (with n = 11). The next LMH6518 gain, 18.8 dB (with n = 10) is incorrect as 16.8 is closer. If 18.8 dB were mistakenly selected, the ADC FS setting is out of range. Therefore, G = 16.8 dB

3. Determine preamp (HG or LG) and ladder attenuation:
   • If G ≥ 18.8 dB → Preamp is HG and ladder attenuation = 38.8 – G
   • If G < 18.8 dB → Preamp is LG and ladder attenuation = 18.8 – G

   For this example, with G = 16.8 → Preamp LG and Ladder Attenuation = 2 dB (= 18.8 to 16.8).

4. Determine the required ADC FS voltage, FS_E, with Equation 12:
   Equation 12.
   The 1.05 factor is to add 5% FS overhead margin to avoid ADC overdrive with Equation 13.
   Equation 13.
   Required condition: 0.56 V ≤ FS_E ≤ 0.84 V
   Recommend condition: 0.595 V ≤ FS_E ≤ 0.805 V for optimum ADC FS
5. Determine the ADC ECM code ratio with Equation 14:
   Equation 14.

   where

   • 0.28 V = (0.84 – 0.56) V
   • 0.56 V is the lower end of the ADC FS adjustability

   For this example:

   

   Required condition: 0 ≤ ECM (ratio) ≤ 1

6. Determine the ECM binary code sent on ADC SPI bus:
   • Convert the ECM value represented by the ratio calculated previously, to binary:
   • ECM (binary) = DEC2BIN{ECM(ratio) × 511, 9}
     where DEC2BIN is a spreadsheet function that converts the decimal ECM ratio, from step 5, multiplied by 511 distinct levels, into binary 9 bits.
   Note: The web-based spreadsheet computes ECM without the use of DEC2BIN function to ease use by all spreadsheet users who do not have this function installed.

   For this example: ECM (binary) = DEC2BIN(0.283 × 511, 9) = 010010000. This number is sent to the ADC on the SPI bus to program the ADC to proper FS voltage.