Datasheet

www.ti.com
FULLY DIFFERENTIAL AMPLIFIER WITH
NG + 1 )
R
F
R
G
)
2R
F
R
C
(4)
CM
-
+
V
IN
R
G
R
F
V
S+
R
C
R
G
V
OUT
V
S-
R
F
THS4520
V
S+
CM
V
Signal
V
S-
R
F
R
F
R
G
R
G
R
T
R
O
R
O
V
OUT+
V
OUT-
THS4520
R
S
R
T
R
S
V
S+
V
S+
R
PU
R
PU
V
S+
=3Vto5V
CM
V
Signal
V
S-
R
F
R
F
R
G
R
G
R
T
R
O
R
O
V
OUT+
V
OUT-
THS4520
C
R
S
R
T
C
C
R
S
C
THS4520
SLOS503B – SEPTEMBER 2006 – REVISED JULY 2007
Table 4 is a modification of Table 1 to add the proper
values with R
PU
assuming a 50- Ω source impedance REDUCED PEAKING
and setting the input and output common-mode
Figure 65 shows a fully differential amplifier that
voltage to mid-supply.
reduces peaking at low gains. The resistor R
C
There are two drawbacks to this configuration. One compensates the THS4520 to have higher noise gain
is it requires additional current from the power (NG), which reduces the AC response peaking
supply. Using the values shown for a gain of 0 dB (typically 3.8dB at G = +1 without R
C
) without
requires 10 mA more current with 5-V supply, and changing the DC forward gain. The input signal, V
IN
,
6.5 mA more current with 3.3-V supply. is assumed to be from a low impedance source,
such as an op amp.
The other drawback is this configuration also
increases the noise gain of the circuit. In the 10-dB When the two feedback paths are symmetrical, the
gain case, noise gain increases by a factor of 1.7. noise gain is given by the expression:
Table 4. RPU Values for Various Gains
Gain R
F
R
G
R
IT
R
PU
0 dB 499 Ω 487 Ω 54.9 Ω 511 Ω
6 dB 499 Ω 243 Ω 59 Ω 270 Ω
10 dB 499 Ω 150 Ω 68.1 Ω 178 Ω
14 dB 499 Ω 93.1 Ω 82.5 Ω 124 Ω
20 dB 499 Ω 40.2 Ω 221 Ω 80.6 Ω
Figure 65. THS4520 with Noise Gain
Compensation
A unity-gain buffer can be designed by selecting R
F
Figure 63. THS4520 DC Coupled Single-Supply
= 499 Ω , R
G
= 499 Ω and R
C
= open. The resulting
with R
PU
Used to Set V
IC
forward gain response is similar to the characteristics
plots with G = 0dB (see Figure 1 ), and the noise gain
Figure 64 shows AC coupling to the source. Using
equal to 2. If R
C
is then made equal to 200 Ω the
capacitors in series with the termination resistors
noise gain increases to 7, which typically gives a
allows the amplifier to self-bias both input and output
frequency response with less peaking and with less
to mid-supply.
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
Figure 64. THS4520 AC Coupled Single-Supply
=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
23
Submit Documentation Feedback