Datasheet

( )
W
--
=
-+
k50
VVV2
I
SSCM
EXT
(2)
V =3.75Vto5V
S+
CM
V
Signal
V
S–
R
F
R
F
R
G
R
G
R
IT
R
IT
R
O
R
O
V
OUT+
V
OUT-
THS4508
R
S
V
CM
R
PD
R
PD
R
S
V
S+
CM
V
S–
50kW
tointernal
CMcircuit
I
EXT
50kW
I
R
S
V
F
R
P D
R
1
6.1
2
6.11
1
-
ú
ú
ú
ú
û
ù
ê
ê
ê
ê
ë
é
-
+
=
Device Operation with Single Power Supplies
THS4508
SLAS459E – SEPTEMBER 2005 – REVISED SEPTEMBER 2008 .................................................................................................................................... www.ti.com
is required for best performance, but it is intended to
be a dc-bias input pin. Bypass capacitors are
recommended on this pin to reduce noise at the
output. The external current required to overdrive the
internal resistor divider is given by Equation 2 :
where V
CM
is the voltage applied to the CM pin, and
V
S+
ranges from 3.75 V to 5 V, and V
S –
is 0 V
(ground).
Figure 52. THS4508 DC-Coupled Single-Source
Supply Range From 3.75 V to 5 V With R
PD
Used
To Set V
IC
Note that in Figure 52 , the source is referenced to
ground as is the input termination resistor R
IT
. The
proper value of resistance to add can be calculated
from Equation 3 :
Figure 51. CM Input Circuit
(3)
Less than 5 V
where R
I
= R
G
+ R
S
|| R
IT
.
The THS4508 is optimized to work in systems using a
5-V single supply, and the characterization data
V
S+
is the power-supply voltage, R
F
is the feedback
presented in this data sheet were taken with 5-V
resistance, R
G
is the gain-setting resistance, R
S
is the
single-supply inputs. For ac-coupled systems or
signal source resistance, and R
IT
is the termination
dc-coupled systems operating with supplies less than
resistance.
5 V and greater than 3.75 V, the amplifier input
Table 3 is a modification of Table 1 to add the proper
common-mode range is maximized by adding
values with R
PD
assuming V
S+
= 3.75 V, a dc-coupled
pull-down resistors at the device inputs. The
50- Ω source impedance, and setting the output
pull-down resistors provide additional loading at the
common-mode voltage to mid-supply.
input, and lower the common-mode voltage that is fed
back into the device input through resistor R
F
.
Table 3. R
PD
Values for Various Gains,
Figure 52 shows the circuit configuration for this
V
S+
= 3.75 V, DC-Coupled Signal Source
mode of operation where R
PD
is added to the
dc-coupled circuit to avoid violating the V
ICR
of the
Gain R
F
R
G
R
IT
R
PD
operational amplifier. Note R
S
and R
IT
are added to
6 dB 348 Ω 169 Ω 64.9 Ω 86.6 Ω
the alternate input from the signal input to balance
10 dB 348 Ω 102 Ω 78.7 Ω 110 Ω
the amplifier. One resistor that is equal to the
14 dB 348 Ω 61.9 Ω 115 Ω 158 Ω
combined value R
I
= R
G
+ R
S
|| R
IT
can be placed at
20 dB 348 Ω 40.2 Ω 221 Ω 226 Ω
the alternate input.
If the signal originates from an ac-coupled 50- Ω
source (see Figure 53 ), the equivalent dc-source
resistance is an open circuit and R
I
= R
G
+ R
IT
.
Table 4 is a modification of Table 1 to add the proper
values with R
PD
assuming V
S+
= 3.75 V, an
ac-coupled 50- Ω source impedance, and setting the
output common-mode voltage to mid-supply.
20 Submit Documentation Feedback Copyright © 2005 – 2008, Texas Instruments Incorporated
Product Folder Link(s): THS4508