LMH6518SQEVK National Semiconductor, LMH6518SQEVK Datasheet - Page 19

• Manufacturer Part Number: LMH6518SQEVK
• Description: KIT EVAL FOR LMH6518 VGA
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LMH6518SQEVK

• Channels Per Ic: 1 - Single
• Amplifier Type: Variable Gain
• Output Type: Differential
• -3db Bandwidth: 900MHz
• Operating Temperature: -40°C ~ 85°C
• Current - Supply (main Ic): 210mA
• Voltage - Supply, Single/dual (±): 4.75 V ~ 5.25 V
• Board Type: Fully Populated
• Utilized Ic / Part: LMH6518
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output / Channel: -
• Slew Rate: -
• Lead Free Status / Rohs Status: Supplier Unconfirmed

In Table 1, the highest FS input in Row 5, Column 2
(80 V
V
front-end attenuator value. The 100x attenuator will allow high
SNR operation to 30.5 dB down, as explained earlier, or
2.4 V
front-end attenuation (1x) cover the scope FS input range
from 8 mV
of 0.8 V
entire scope FS range of 0.8 V
low 52 dB for a portion of that range. Another attenuation level
is thus required to maintain the SNR specification require-
ment of 52 dB.
One possible attenuation partitioning is to select the addition-
al attenuator value to cover a 20 dB range above 0.8 V
(to 8 V
20 dB range from 8 V
input to 0.8 V
tenuator is 10x, as shown in Table 1, Row 4. The remaining
scope input range of 8 V
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
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:
•
•
•
•
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). This
spreadsheet can be found at:
http://www.national.com/appinfo/amps/LMH6518_Cal.xls
Here is the step by step procedure that explains the opera-
tions performed by the said spreadsheet based on the scope
vertical scale setting (S in V/div) and front-end attenuation
“A” (from Table 1). A numerical example is also worked out
for more clarification:
1.
PP
Row
Front-end attenuator
LMH6518 Preamp
LMH6518 Ladder Attenuation
ADC FS value (ECM)
) set the
Determine the required signal path gain, K:
1
2
3
4
5
PP
PP
PP
), and the LMH6518’s highest FS input allowed (0.8
PP
at scope input. In that same table, Rows 1-3 with no
) with the 100x attenuator covering the remaining
PP
-2.4 V
Scope FS Input
-800 mV
PP
24m-80m
8m-24m
80m-0.8
PP
at LMH6518 input means the additional at-
(V
0.8-8
8-80
. If the 100x attenuator were used for the
PP
PP
)
PP
. That leaves the scope FS input range
to 80 V
PP
≥
-80 V
24 mV
TABLE 1. Oscilloscope Example Including Front-End Attenuators
“S”, Scope Vertical
PP
PP
PP
-80 V
Scale (V/DIV)
. Mapping 8 V
PP
would then be covered by
3m-10m
10m-0.1
, as shown in Table 1.
1m-3m
0.1-1
1-10
PP
, SNR would dip be-
PP
FS scope
Preamp
PP
HG
HG
LG
LG
LG
FS
19
Ladder Attenuation
2.
3.
4.
5.
Range (dB)
10-20
0-10
0-20
0-20
0-20
(assuming the full scale signal occupies 95% of the 0.7
V
scope divisions).
Required condition: −2.37 dB
Example: With S = 110 mV/DIV, Table 1 shows that
A = 10 V/V:
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
Determine Preamp (HG or LG) & Ladder Attenuation:
If G
= 38.8 - G
If G < 18.8 dB
= 18.8 - G
For this example, with G = 16.8
Attenuation = 2 dB (= 18.8-16.8).
Determine the required ADC FS voltage, FS
The “1.05” factor is to add 5% FS overhead margin to
avoid ADC overdrive.
Required condition: 0.56V
Recommend condition: 0.595V
optimum ADC FS
Determine the ADC ECM code ratio:
where:
For this example:
PP
a) 0.28V= (0.84-0.56)V
b) 0.56V is the lower end of the ADC FS adjustability
FS (for 5% overhead) which occupies 8 vertical
≥
18.8 dB
attenuation (V/V)
→
→
“A”, Front-end
Preamp is LG and Ladder Attenuation
Preamp is HG and Ladder Attenuation
100
10
1
1
1
≤
FS
≤
→
≤
E
K
Preamp LG and Ladder
≤
FS
Minimum SNR (dB)
≤
with 200 MHz filter
0.84V
40.3 dB
E
≤
0.805V for
52.0
53.4
53.4
53.4
44
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E
: