LMH6518SQEVK National Semiconductor, LMH6518SQEVK Datasheet - Page 20

• 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

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6.
INPUT/OUTPUT CONSIDERATIONS
The LMH6518’s ideal Input/Output Conditions, considered in-
dividually, are listed below:
In addition to the individual conditions listed in Table 2, the
Input/Output terminal conditions should match differentially
(i.e. +IN to −IN and +OUT to −OUT), as well, for best perfor-
mance.
The input is differential but can be driven single-ended as long
as the conditions of Table 2 are met and there is good match-
ing between the driven and the undriven inputs from DC to
the highest frequency of interest. If not, there could be a set-
tling time impact among other possible performance degra-
dations. The datasheet specifications are with single-ended
input, unless specified. Here is the recommended bench-test
schematic to drive one input and to bias the other input with
good matching in mind:
With the schematic of Figure 3, each LMH6518 input sees
25Ω to ground at the higher frequencies when the capacitors
look like shorts. This impedance increases to 125Ω at DC for
both inputs, thereby preserving the required matching at any
frequency. This configuration, using properly selected R’s
and C’s, allows four times less biasing power dissipation than
when the undriven input is biased with an effective 25Ω from
the LMH6518 input to ground.
It is possible to drive the LMH6518 input from a ground ref-
erenced 50Ω source by providing level shift circuitry on the
driven input. Figure 4 shows a circuit where ½ the input signal
reaches the LMH6518 input while the negative supply voltage
Impedance from
FIGURE 3. Recommended Single-Ended Bench-Test
each input to
Required condition: 0
Determine the ECM binary code to be sent on ADC SPI
bus:
Convert the ECM value represented by the ratio
calculated above, to binary:
ECM (binary) = DEC2BIN{ECM(ratio)* 511, 9}
ground (Ω)
≤
50
Input Drive from 50Ω Source
Common Mode
≤
1.5 to 3.1
Input (V)
ECM (ratio)
TABLE 2. LMH6518's Ideal Input/Output Conditions
≤
1
Differential Input
(V
<0.8
30068824
PP
)
20
100 (differential)/ 50
Load Impedance
(single ended)
(V
any biasing current while providing 50Ω termination to the
source. The driven input (+IN) is biased to 2.5V (V
In the schematic of Figure 4, the equivalent impedance from
each LMH6518 input to ground is around 38Ω. This
configuration’s power consumption of
higher than that of Figure 3 because of additional power dis-
sipated to perform the level shifting. Additional 50Ω attenua-
tors can be placed between J1 and R
4 in order to accommodate higher input voltages.
It is also possible to shift the LMH6518 output common mode
level using a level shift approach similar to that of Figure 4.
The circuit in Figure 5 shows an implementation where the
LMH6518’s nominal 1.2V CM output, set by a 1.2V on V
input from the Gsample/s ADC, is shifted lower for proper in-
terface to different ADC's which require V
high input impedance:
FIGURE 4. LMH6518 Driven by a Ground Referenced
EE
) ensures that the 50Ω source at J1 does not experience
where “DEC2BIN” is a spreadsheet function which
converts the decimal ECM ratio, from step 5 above,
multiplied by 511 distinct levels, into binary 9 bits.
Note: The Web based spreadsheet computes ECM without the use of
For this example: ECM (binary) = DEC2BIN(0.283*511,
9) = 010010000. This would be the number to be sent to
the ADC on the SPI bus to program the ADC to the proper
FS voltage.
(Ω)
“DEC2BIN” function to ease usage by all spreadsheet users
who may not have this function installed.
Differential Output
<0.77
Source
(V)
2
∼
/R
0.5W (in R
3
Common Mode
CM
junction in Figure
Output (V)
0.95-1.45
= 0V and have
CC
1
/2):
- R
30068825
5
) is
CM