LMH6518_15 Datasheet, PDF (22/41 Pages) Texas Instruments – LMH6518 900 MHz, Digitally Controlled, Variable Gain Amplifier

LMH6518
SNOSB21C – MAY 2008 – REVISED JULY 2013
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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 100x 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 10x, as shown in Table 2,
Row 4. The remaining scope input range of 8 V_PP-80 V_PP would then be covered by the 100x front-end
attenuator derived earlier. The entire scope input range is now covered with SNR maintained about 52 dB for
scope FS input ≥ 24 mV_PP, as shown in Table 2.
SETTINGS AND ADC SPI CODE (ECM)
Covering the range from 1 mV/DIV to 10 V/DIV requires the following to be adjusted within the digital
oscilloscope:
• Front-End Attenuator
• LMH6518 Preamp
• LMH6518 Ladder Attenuation
• ADC FS Value (ECM)
The LMH6518 Product Folder contains a spreadsheet which allows one to calculate the front-end attenuator,
LMH6518 Preamp gain (HG or LG) and ladder attenuation, and ADC FS setting based on the scope vertical
scale (S in V/DIV).
Here is the step by step procedure that explains the operations performed by the said spreadsheet based on the
scope vertical scale setting (S in V/div) and front-end attenuation “A” (from Table 2). A numerical example is also
worked out for more clarification:
1. Determine the required signal path gain, K:
0.95 x 700 mV_PP
K = 20 x log
= -21.6 + 20 x log
A
8 x S(V/div)
S(V/div)
A
(10)
assuming the full scale signal occupies 95% of the 0.7 V_PP FS (for 5% overhead) which occupies 8 vertical
scope divisions).
Required condition: −2.37 dB ≤ K ≤ 40.3 dB
Example: With S = 110 mV/DIV, Table 2 shows that A = 10 V/V:
o K = -21.6 + 20 x log ¹⁰ = 17.57 dB
110 mV
(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) would be incorrect as 16.8 is closer. If 18.8 dB were mistakenly chosen, the ADC FS
setting would be out of range.
– Therefore: G = 16.8 dB
3. Determine Preamp (HG or LG) & 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 (= 18.8-16.8).
4.
Determine
the
required ADC
G
FS
voltage,
FS_E:
FS_E =
Sx8
A
x 1.05 x 10²⁰
(12)
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