Datasheet

V
N
+ V
IN–
(1–β) ) V
OUT)
β
Where:
β +
R
G
R
F
) R
G
V
P
+ V
IN)
(1–β) ) V
OUT–
β
(3)
(4)
(5)
V
OCM
R
g
R
g
R
f
R
f
+
-
-
+
V
p
V
n
V
OUT-
V
OUT+
V
IN+
V
IN-
Diagram For Input Common-Mode Range Equations
THS4502
THS4503
www.ti.com SLOS352E –APRIL 2002–REVISED OCTOBER 2011
Table 2. Negative-Rail Referenced (continued)
Gain V
IN+
V
IN-
V
IN
V
OCM
V
OD
V
NMIN
V
NMAX
(V/V) (V) (V) (V
PP
) (V) (V
PP
) (V) (V)
NOTE: This table assumes a negative-rail referenced,
single-ended input signal on a single 5-V supply as shown in
Figure 93. V
NMIN
= V
PMIN
and V
NMAX
= V
PMAX
Table 3. Midrail Referenced
NOTE
The equations denote the
Gain V
IN+
V
IN-
V
IN
V
OCM
V
OD
V
NMIN
V
NMAX
device inputs as V
N
and V
P
, (V/V) (V) (V) (V
PP
) (V) (V
PP
) (V) (V)
and the circuit inputs as V
IN+
0.5 to
1 2.5 4 2.5 4 2 3
and V
IN-
.
4.5
1.5 to
2 2.5 2 2.5 4 2.16 2.83
3.5
2.0 to
4 2.5 1 2.5 4 2.3 2.7
3.0
2.25 to
8 2.5 0.5 2.5 4 2.389 2.61
2.75
NOTE: This table assumes a midrail referenced, single-ended
input signal on a single 5-V supply. V
NMIN
= V
PMIN
and V
NMAX
=
V
PMAX
CHOOSING THE PROPER VALUE FOR THE
FEEDBACK AND GAIN RESISTORS
The selection of feedback and gain resistors impacts
Figure 95.
circuit performance in a number of ways. The values
in this section provide the optimum high frequency
The two tables below depict the input common-mode
performance (lowest distortion, flat frequency
range requirements for two different input scenarios,
response). Since the THS4500 family of amplifiers is
an input referenced around the negative rail and an
developed with a voltage feedback architecture, the
input referenced around midrail. The tables highlight
choice of resistor values does not have a dominant
the differing requirements on input common-mode
effect on bandwidth, unlike a current feedback
range, and illustrate reasoning for choosing either the
amplifier. However, resistor choices do have
THS4500/1 or the THS4502/3. For signals referenced
second-order effects. For optimal performance, the
around the negative power supply, the THS4500/1
following feedback resistor values are recommended.
should be chosen since its input common-mode
In higher gain configurations (gain greater than two),
range includes the negative supply rail. For all other
the feedback resistor values have much less effect on
situations, the THS4502/3 offers slightly improved
the high frequency performance. Example feedback
distortion and noise performance for applications with
and gain resistor values are given in the section on
input signals centered between the power supply
basic design considerations (Table 4).
rails.
Amplifier loading, noise, and the flatness of the
frequency response are three design parameters that
Table 2. Negative-Rail Referenced
should be considered when selecting feedback
Gain V
IN+
V
IN-
V
IN
V
OCM
V
OD
V
NMIN
V
NMAX
resistors. Larger resistor values contribute more noise
(V/V) (V) (V) (V
PP
) (V) (V
PP
) (V) (V)
and can induce peaking in the ac response in low
-2.0 to
gain configurations, and smaller resistor values can
1 0 4 2.5 4 0.75 1.75
2.0
load the amplifier more heavily, resulting in a
-1.0 to
reduction in distortion performance. In addition,
2 0 2 2.5 4 0.5 1.167
1.0
feedback resistor values, coupled with gain
-0.5 to
requirements, determine the value of the gain
4 0 1 2.5 4 0.3 0.7
0.5
resistors, directly impacting the input impedance of
-0.25
the entire circuit. While there are no strict rules about
8 0 0.5 2.5 4 0.167 0.389
to 0.25
resistor selection, these trends can provide qualitative
design guidance.
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