THS4303 Datasheet, PDF (15/24 Pages) Texas Instruments – WIDEBAND FIXED-GAIN AMPLIFIER

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SINGLE SUPPLY OPERATION
The THS4303 is designed to operate from a single
3-V to 5-V power supply. When operating from a
single power supply, care must be taken to ensure
the input signal and amplifier are biased appropriately
to allow for the maximum output voltage swing. The
circuits shown in Figure 44 demonstrate methods to
configure an amplifier in a manner conducive for
single supply operation.
V_S+
+
FB
22 µF
50 Ω Source
R_g
*2.5 V
V_I
49.9 Ω
47 pF 0.1 µF
30.1 Ω
R_f
_
+ _THS4303
50 Ω Load
V_O
49.9 Ω
*2.5 V
FB = Ferrite Bead
* = Low Impedance
Figure 44. DC-Coupled Single Supply Operation
SAVING POWER
FUNCTIONALITY
WITH
POWER-DOWN
The THS4303 features a power-down pin (PD) which
lowers the quiescent current from 34 mA down to 1
mA, ideal for reducing system power.
The power-down pin of the amplifier defaults to the
positive supply voltage in the absence of an applied
voltage, putting the amplifier in the power-on mode of
operation. To turn off the amplifier in an effort to
conserve power, the power-down pin can be driven
towards the negative rail. The threshold voltages for
power-on and power-down are relative to the supply
rails and given in the specification tables. Above the
Enable Threshold Voltage, the device is on. Below
the Disable Threshold Voltage, the device is off.
Behavior in between these threshold voltages is not
specified.
Note that this power-down functionality is just that;
the amplifier consumes less power in power-down
mode. The power-down mode is not intended to
provide a high- impedance output. In other words, the
power-down functionality is not intended to allow use
as a 3-state bus driver. When in power-down mode,
THS4303
SLOS421B – NOVEMBER 2003 – REVISED JANUARY 2005
the impedance looking back into the output of the
amplifier is dominated by the feedback and gain
setting resistors, but the output impedance of the
device itself varies depending on the voltage applied
to the outputs.
The time delays associated with turning the device on
and off are specified as the time it takes for the
amplifier to reach 50% of the nominal quiescent
current. The time delays are on the order of
microseconds because the amplifier moves in and out
of the linear mode of operation in these transitions.
APPLICATION CIRCUITS
DRIVING AN ANALOG-TO-DIGITAL CONVERTER
WITH THE THS4303
The THS4303 amplifier can be used to drive
high-performance analog-to-digital converters. Two
example circuits are presented below.
The first circuit uses a wideband transformer to
convert a single-ended input signal into a differential
signal. The amplified signal from the output of the
THS4303 is fed through a low-pass filter, via an
isolation resistor and an ac-coupling capacitor, to the
transformer.
For applications without signal content at dc, this
method of driving ADCs is very useful. Where dc
information content is required, the THS4500 family
of fully differential amplifiers may be applicable.
V_S+
+
FB
22 µF
47 pF
50 Ω Source
R_g
*2.5 V
V_I
49.9 Ω
R_f
_
+ _THS4303
0.1 µF
30.1 Ω
*2.5 V _{RISO C}
FB = Ferrite Bead
* = Low Impedance
R
IN
ADC
R
IN CM
Figure 45. Driving an ADC Via a Transformer
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