THS6204 Datasheet, PDF (29/43 Pages) Texas Instruments – Dual-Port, Differential VDSL2 Line Driver Amplifiers

THS6204
www.ti.com ................................................................................................................................................... SBOS416C – OCTOBER 2007 – REVISED APRIL 2009
DIFFERENTIAL NOISE PERFORMANCE
As the THS6204 is used as a differential driver in
xDSL applications, it is important to analyze the noise
in such a configuration. Figure 87 shows the op amp
noise model for the differential configuration.
I_N
Driver
E_N
R_S
E_RS
Ö4kTR_S
I_N
R_F
Ö4kTR_F
R_G
E_O2
Ö4kTR_G
R_F
Ö4kTR_F
I_N
R_S
E_N
I_N
E_RS
Ö4kTR_S
Figure 87. Differential Op Amp Noise Analysis
Model
As a reminder, the differential gain is expressed as:
G_D = 1 +
2 ´ R_F
R_G
(16)
The output noise can be expressed as shown below:
E_O = 2 ´ G_D2 ´ e_N² + (i_N ´ R_S)² + 4kTR_S + 2(i_IR_F)² + 2(4kTR_FG_D)
(17)
Dividing this expression by the differential noise gain
(G_D = (1 + 2R_F/R_G)) gives the equivalent input
referred spot noise voltage at the noninverting input,
as shown in Equation 18.
E_O =
2 ´ e_N² + (i_N ´ R_S)² + 4kTR_S
+2
i_IR_F
G
2
+2
4kTR_F
G
D
D
(18)
Evaluating these equations for the THS6204 ADSL
circuit and component values of Figure 82 gives a
total output spot noise voltage of 38.9nV/√Hz and a
total equivalent input spot noise voltage of 7nV/√Hz.
In order to minimize the output noise due to the
noninverting input bias current noise, it is
recommended to keep the noninverting source
impedance as low as possible.
DC ACCURACY AND OFFSET CONTROL
A current-feedback op amp such as the THS6204
provides exceptional bandwidth in high gains, giving
fast pulse settling but only moderate dc accuracy.
The Electrical Characteristics show an input offset
voltage comparable to high-speed, voltage-feedback
amplifiers; however, the two input bias currents are
somewhat higher and are unmatched. While bias
current cancellation techniques are very effective with
most voltage-feedback op amps, they do not
generally reduce the output dc offset for wideband
current-feedback op amps. Because the two input
bias currents are unrelated in both magnitude and
polarity, matching the input source impedance to
reduce error contribution to the output is ineffective.
Evaluating the configuration of Figure 81, using
worst-case +25°C input offset voltage and the two
input bias currents, gives a worst-case output offset
range equal to:
V_OFF = ± (NG × V_OS(MAX)) + (I_BN × R_S/2 × NG) ± (I_BI ×
R_F)
where NG = noninverting signal gain
= ± (10 × 5mV) + (3µA × 25Ω × 10) ± (1.24kΩ ×
40µA)
= ±50mV + 0.75mV ± 49.6mV
V_OFF = –98.85mV to +100.35mV
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