LMH6518SQEVK National Semiconductor, LMH6518SQEVK Datasheet - Page 27

• 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

Table 8 lists other possible JFET candidates that fall in the
range of speed (f
*Noise data at
The LNA noise could degrade the scope’s SNR if it is com-
parable to the input referred noise of the LMH6518. LNA noise
is influenced by the following operating conditions:
a) JFET equivalent input noise
b) BJT Base current
Reducing either “a” or “b” above, or both, reduces noise. One
way to reduce “a” is to increase R
will reduce the noise impact of J8 but requires a JFET which
has a higher I
rent of J10 so that J10’s noise contribution is minimized.
Reducing the BJT Base current can be accomplished with in-
creasing R
β will also reduce the Base current (keep in mind that β and
f
Figure 14 shows the impact of the JFET buffer noise on SNR,
compared to SNR in Figure 2, assuming either 3 nV/
1.5 nV/
ATTENUATOR DESIGN
Figure 15 shows a front-end attenuator designed to work with
the JFET LNA of Figure 13.
Interfet
Interfet
Interfet
Interfet
Interfet
Philips
Fairchild
Vishay
Siliconix
t
at the operating Collector current is what matters).
Company
20
FIGURE 14. LNA Buffer SNR Impact
buffer noise for comparison:
at the expenses of higher rise/fall times. A higher
dss
∼
Part Number
rating in order to maintain the operating cur-
MMBF5486
t
I
) and low noise needed:
dss
2N5397/8
2N5911/2
J308/9/10
SST441
BF513
IF140
IF142
/2
V
−2.2
−2.2
−2.5
−2.5
−2.3
−3.5
8
P
−4
-3
(currently set to 0Ω). This
(V)
TABLE 8. Suitable JFET Candidates Specifications
(mA)
I
10
10
13
13
21
15
14
13
dss
30068834
gm (mS)
5.5
5.5
17
10
8
8
7
6
or
27
Input C
(pF)
2.3
2.3
5.8
3.5
5
5
5
4
FIGURE 15. Front End Attenuator for Figure 13 JFET LNA
R_LNA” and “C_LNA” are the input impedance components
of the JFET LNA. The 10:1 and 100:1 attenuators bottom re-
sistors (R
LNA’s 1 MΩ input impedance, compared to the case where a
high-input-impedance LNA is used. The two switches used
on the input and output of the attenuator block must be low
capacitance, high isolation switches in order to reduce any
speed or crosstalk impact. C
cy response (and step response) by creating “zeros” that
flatten the response for wide-band operation. For the 10:1 at-
tenuator, R
uator. The shunt capacitors C
benefit in that they roll-off the resistor thermal noise at a low
frequency (low pass response, −3 dB down at
thereby eliminating any significant noise contribution from the
attenuation resistors. Otherwise, the channel noise would be
dominated by the attenuator resistor thermal noise. C
C
pacitance regardless of attenuator used.
REFERENCE
1. Wideband amplifiers by Peter Staric and Erik Margan, pub-
lished by Springer in 2006. (Section 5.2).
6
trimmer capacitors can be adjusted to match the input ca-
2
and R
1
C
1
(nV/RtHz)
= R
noise *
4
2.5
2.5
2.5
) are adjusted higher to compensate for the
4
4
4
2
C
2
. The same applies to the 100:1 atten-
1
1
-C
-C
down (V)
4
4
Break
provide the proper frequen-
have a very important other
−20
−25
−25
−25
−25
−35
Calculated f
(MHz)
www.national.com
380
380
254
254
466
318
278
272
∼
20 kHz)
2
30068877
and
t