THS4520_14 Datasheet, PDF (22/37 Pages) Texas Instruments – WIDEBAND, LOW NOISE, LOW DISTORTION FULLY DIFFERENTIAL AMPLIFIER

THS4520
SLOS503B – SEPTEMBER 2006 – REVISED JULY 2007
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:
( ) _IEXT
=
2V_CM
- V_S+ - V_S-
50 kW
(2)
where V_CM is the voltage applied to the CM pin.
V_S+
to internal
CM circuit
50 kW
50 kW
I_EXT
CM
V _S–
Figure 61. CM Input Circuit
Powerdown Operation: Device
Enable/Disable Thresholds
The enable/disable thresholds of the THS4520 are
dependent upon the power supplies, and the
thresholds are always referenced to the lower power
supply rail. The device is enabled or disabled for the
following conditions:
• Device enabled: V_PD > V_S- + 0.8 x (V_S+ - V_S-)
• Device disabled: V_PD < V_S- + 0.2 x (V_S+ - V_S-)
If the PD pin is left open, the device will default to the
enabled state.
Table 3 shows the thresholds for some common
power supply configurations:
Table 3. Power Supply Configurations
Power Supply
(V_S+, V_S-)
Enable
Threshold
(V)
±2.5 V
1.5
±1.65 V
1
(4 V , -1 V)
3
(5 V, gnd)
4
(3.3 V, gnd)
2.64
(3 V, gnd)
2.4
Disable
Threshold
(V)
-1.5
-1
0
1
0.66
0.6
Comment
Shown in data
table
Shown in data
table
Split, unbalanced
supplies
Single-sided
supply
Single-sided
supply
Single-sided
supply
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Single-Supply Operation (3 V to 5 V)
To facilitate testing with common lab equipment, the
THS4520 EVM allows split-supply operation, and the
characterization data presented in this data sheet
was taken with split-supply power inputs. The device
can easily be used with a single-supply power input
without degrading the performance. Figure 62,
Figure 63, and Figure 64 show DC and AC-coupled
single-supply circuits with single-ended inputs. These
configurations all allow the input and output
common-mode voltage to be set to mid-supply
allowing for optimum performance. The information
presented here can also be applied to differential
input sources.
In Figure 62, the source is referenced to the same
voltage as the CM pin (V_CM). V_CM is set by the
internal circuit to mid-supply. R_T along with the input
impedance of the amplifier circuit provides input
termination, which is also referenced to V_CM.
Note R_S and R_T are added to the alternate input from
the signal input to balance the amplifier. Alternately,
one resistor can be used equal to the combined
value R_G+ R_S||R_T on this input. This is also true of
the circuits shown in Figure 63 and Figure 64.
R_S
R_G
R_F
V_Signal
V_Bias= V_CM
R_T
V_CM
R_G
R_S
R_T
V_CM V_CM
V_S+
THS4520
CM
V_S–
V_CM
R_F
R_O
V_OUT-
R_O
V_OUT+
Figure 62. THS4520 DC Coupled Single-Supply
with Input Biased to V_CM
In Figure 63 the source is referenced to ground and
so is the input termination resistor. R_PU is added to
the circuit to avoid violating the V_ICR of the op amp.
The proper value of resistor to add can be calculated
from Equation 3:
( ) _RPU
=
V_CM
ççèæ
1
R_F
V_IC - V_S+
÷÷øö
-
V_IC
ççèæ
1
R_IN
+
1
R_F
÷÷øö
(3)
V_IC is the desired input common-mode voltage,
V_CM = CM, and R_IN = R_G+ R_S||R_T. To set to
mid-supply, make the value of R_PU = R_G+ R_S||R_T.
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