LMH6518_15 Datasheet, PDF (21/41 Pages) Texas Instruments – LMH6518 900 MHz, Digitally Controlled, Variable Gain Amplifier

LMH6518
www.ti.com
SNOSB21C – MAY 2008 – REVISED JULY 2013
62
22
60 ^{200 MHz Filter}
20
58
18
56
16
54
14
52
12
50
10
48
8
46
6
44
42
40
38
0.001
Preamp HG
0.01
4
2
Preamp LG ₀
-2
0.1
1
INPUT FS (V)
Figure 58. LMH6518 SNR & Ladder Attenuation used vs. Input
As can be seen from Figure 58, SNR of at least 52 dB is maintained for FS inputs above 24 mV_PP (3 mV/DIV on
a scope) assuming the LMH6518’s 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 58, LMH6518’s
minimum SNR is 43.5 dB, thereby meeting the relaxed SNR specification for the lower range of scope front panel
voltages.
In Figure 58, 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/ 20 dB ladder attenuation’s lower output
noise compared to Preamp LG/ 2 dB ladder attenuation’s noise. Judicious choice of front end attenuators can
ensure that the 52 dB SNR specification is maintained for scope FS inputs ≥ 24 mV_PP by confining the LMH6518
gain range to the lower 30.5 dB
(= 20 x log ^{0.8 VPP} )
24 mV_PP
(8)
from the total range of 40 dB (= 38.8 - (−1.16)) possible.
Here is an example:
To cover the range of 1 mV/DIV to 10 V/DIV (80 dB range), here is a configuration which affords good SNR:
Row
1
2
3
4
5
Table 2. Oscilloscope Example Including Front-End Attenuators
Scope FS Input
(V_PP)
8m-24m
24m-80m
80m-0.8
0.8-8
8-80
“S”, Scope Vertical
Scale (V/DIV)
1m-3m
3m-10m
10m-0.1
0.1-1
1-10
Preamp
HG
HG
LG
LG
LG
Ladder Attenuation
Range (dB)
0-10
10-20
0-20
0-20
0-20
“A”, Front-end
attenuation (V/V)
1
1
1
10
100
Minimum SNR (dB)
with 200 MHz filter
44
52.0
53.4
53.4
53.4
In Table 2, the highest FS input in Row 5, Column 2 (80 V_PP), and the LMH6518’s highest FS input allowed (0.8
V_PP) set the
80 V_PP
100x (= _{0.8 VPP} )
(9)
front-end attenuator value. The 100x attenuator will allow high SNR operation to 30.5 dB down, as explained
earlier, or 2.4 V_PP at scope input. In that same table, Rows 1-3 with no front-end attenuation (1x) cover the scope
FS input range from 8 mV_PP-800 mV_PP. That leaves the scope FS input range of 0.8 V_PP-2.4 V_PP. If the 100x
attenuator were used for the entire scope FS range of 0.8 V_PP-80 V_PP, SNR would dip below 52 dB for a portion
of that range. Another attenuation level is thus required to maintain the SNR specification requirement of 52 dB.
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