THS6184_16 Datasheet, PDF (22/39 Pages) Texas Instruments – DUAL-PORT, LOW-POWER DIFFERENTIAL xDSL LINE DRIVER AMPLIFIERS

THS6184
SLLS635D – AUGUST 2005 – REVISED JANUARY 2009 ................................................................................................................................................. www.ti.com
The total equivalent input noise density (e_ni) is calculated by using the following equation:
Ǹ _{ǒ Ǔ ǒ ǒ ǓǓ ǒ Ǔ eni +}
ǒ_enǓ² _{) IN )}
2
RS ) IN–
2
RF ø RG ) 4 kTRs ) 4 kT RF ø RG
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
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).
ǒ Ǔ eno + eni AV
+
e_ni
1
)
R_F
R_G
(Noninverting Case)
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 A CAPACITIVE LOAD
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 THS6184 has been internally compensated to maximize its bandwidth and
slew rate performance at low quiescent current. When the amplifier is compensated in this manner, capacitive
loading directly on the output decreases 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 55. A minimum value of 2 Ω should work well for most
applications.
1 kΩ
Input
1 kΩ
_
THS6184
+
2Ω
Output
C_LOAD
Figure 55. Driving a Capacitive Load
GENERAL CONFIGURATIONS
A common error for the first-time CFB user is to create a unity gain buffer amplifier by shorting the output directly
to the inverting input. A CFB amplifier in this configuration oscillates and is not recommended. The THS6184,
like all CFB amplifiers, must have a feedback resistor for stable operation. Additionally, placing capacitors
directly from the output to the inverting input is not recommended. This is because, at high frequencies, a
capacitor has a very low impedance. This results in an unstable amplifier and should not be considered when
using a current-feedback amplifier. Because of this, integrators and simple low-pass filters, which are easily
implemented on a VFB amplifier, must be designed slightly differently. If filtering is required, simply place an
RC-filter at the noninverting terminal of the operational-amplifier (see Figure 56).
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