THS4520_14 Datasheet, PDF (23/37 Pages) Texas Instruments – WIDEBAND, LOW NOISE, LOW DISTORTION FULLY DIFFERENTIAL AMPLIFIER

THS4520
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Table 4 is a modification of Table 1 to add the proper
values with R_PU assuming a 50-Ω source impedance
and setting the input and output common-mode
voltage to mid-supply.
There are two drawbacks to this configuration. One
is it requires additional current from the power
supply. Using the values shown for a gain of 0 dB
requires 10 mA more current with 5-V supply, and
6.5 mA more current with 3.3-V supply.
The other drawback is this configuration also
increases the noise gain of the circuit. In the 10-dB
gain case, noise gain increases by a factor of 1.7.
Table 4. RPU Values for Various Gains
Gain
0 dB
6 dB
10 dB
14 dB
20 dB
R_F
499 Ω
499 Ω
499 Ω
499 Ω
499 Ω
R_G
487 Ω
243 Ω
150 Ω
93.1 Ω
40.2 Ω
R_IT
54.9 Ω
59 Ω
68.1 Ω
82.5 Ω
221 Ω
R_PU
511 Ω
270 Ω
178 Ω
124 Ω
80.6 Ω
RS
V Signal
RS
V S+
R PU
RG
RF
RT
V S+
V S+
R PU
RG
RT
THS4520
CM
V S-
RO
V OUT-
RO
V OUT+
RF
Figure 63. THS4520 DC Coupled Single-Supply
with R_PU Used to Set V_IC
Figure 64 shows AC coupling to the source. Using
capacitors in series with the termination resistors
allows the amplifier to self-bias both input and output
to mid-supply.
RS
C
RG
RF
V _Signal
RS
RT
C
RG
RT
V S+ = 3 V to 5 V
THS4520
CM
V S-
RO
V _OUT-
RO
V OUT+
C
C
R_F
Figure 64. THS4520 AC Coupled Single-Supply
SLOS503B – SEPTEMBER 2006 – REVISED JULY 2007
FULLY DIFFERENTIAL AMPLIFIER WITH
REDUCED PEAKING
Figure 65 shows a fully differential amplifier that
reduces peaking at low gains. The resistor R_C
compensates the THS4520 to have higher noise gain
(NG), which reduces the AC response peaking
(typically 3.8dB at G = +1 without R_C) without
changing the DC forward gain. The input signal, V_IN,
is assumed to be from a low impedance source,
such as an op amp.
When the two feedback paths are symmetrical, the
noise gain is given by the expression:
NG
+
1
)
R_F
R_G
)
2R_F
R_C
(4)
R_G
V_IN
R_F
V_S+
R_C
R_G
THS4520
CM
V_S-
-
V_OUT
+
R_F
Figure 65. THS4520 with Noise Gain
Compensation
A unity-gain buffer can be designed by selecting R_F
= 499 Ω, R_G = 499 Ω and R_C = open. The resulting
forward gain response is similar to the characteristics
plots with G = 0dB (see Figure 1), and the noise gain
equal to 2. If R_C is then made equal to 200 Ω the
noise gain increases to 7, which typically gives a
frequency response with less peaking and with less
bandwidth, and the forward gain remains equal to
unity.
The plot in Figure 66 shows the measured
small-signal AC response of a THS4520 EVM in the
default unity-gain configuration (see Figure 72).
When the termination resistors present on the EVM
(R1, R2, and R12 in Figure 72) and the source
resistance of the signal generator (R_S = 50 Ω) are
taken into account, the calculated noise gain of the
default EVM is NG = 1.97. Also included in the plot
are two curves which represent the measured
response of the same board with two values of R_C,
one with R_C = 200 Ω (NG = 6.96) and one with R_C
=487 Ω (NG = 4.02). The low-frequency roll-off of the
AC response is due to the transformer (T1 in
Figure 72). The curves illustrate the reduced peaking
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