Datasheet

NF + 10log
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1 )
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ǒ
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2
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IN ) R
S
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2
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4 kTR
S
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20
5
0
10 100
NF − NoiseFigure − dB
25
NOISEFIGURE
vs
SOURCERESISTANCE
30
1k 100k
15
10
Source Resistance − Ω
f=10kHz
T
A
=25°C
10k
+
_
THS401x
C
LOAD
1.3kΩ
Input
Output
1.3kΩ
20 Ω
THS4011
THS4012
SLOS216E –JUNE 1999–REVISED APRIL 2010 www.ti.com
Figure 26 shows the NF graph for the THS401x.
Figure 26. Noise Figure vs Source Resistance
DRIVING A CAPACITIVE LOAD
Driving capacitive loads with high performance amplifiers is not a problem, as long as certain precautions are
taken. The first precaution is to note that the THS401x has been internally compensated to maximize its
bandwidth and slew-rate performance. When the amplifier is compensated in this manner, capacitive loading
directly on the output decreases the device phase margin leading to high-frequency ringing or oscillations.
Therefore, for capacitive loads of greater than 10 pF, it is recommended that a resistor be placed in series with
the output of the amplifier, as shown in Figure 27. A minimum value of 20 Ω should work well for most
applications. For example, in 75-Ω transmission systems, setting the series-resistor value to 75 Ω both isolates
any capacitance loading and provides the proper line-impedance matching at the source end.
Figure 27. Driving a Capacitive Load
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