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_
+
R
F
R
S
R
G
e
Rg
e
Rf
e
Rs
e
n
IN+
Noiseless
IN−
e
ni
e
no
e
ni
+
ǒ
e
n
Ǔ
2
)
ǒ
IN ) R
S
Ǔ
2
)
ǒ
IN–
ǒ
R
F
ø R
G
ǓǓ
2
) 4 kTR
s
) 4 kT
ǒ
R
F
ø R
G
Ǔ
Ǹ
Where:
k = Boltzmann’s constant = 1.380658 × 10
−23
T = Temperature in degrees Kelvin (273 +°C)
R
F
|| R
G
= Parallel resistance of R
F
and R
G
e
no
+ e
ni
A
V
+ e
ni
ǒ
1 )
R
F
R
G
Ǔ
(Noninverting Case)
DRIVING A CAPACITIVE LOAD
THS6182
SLLS544H – SEPTEMBER 2002 – REVISED JUNE 2007
APPLICATION INFORMATION (continued)
Figure 71. Noise Model
The total equivalent input noise density (e
ni
) is calculated by using the following equation:
To get the equivalent output noise of the amplifier, just multiply the equivalent input noise density (e
ni
) by the
overall amplifier gain (A
V
).
As the previous equations show, to keep noise at a minimum, small value resistors should be used. As the
closed-loop gain is increased (by reducing R
G
), the input noise is reduced considerably because of the parallel
resistance term.
Driving capacitive loads with high performance amplifiers is not a problem as long as certain precautions are
taken. The first is to realize that the THS6182 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 will decrease the device's 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 72 . A minimum value of 2 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.
18
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