THS4302 Datasheet, PDF (14/23 Pages) Texas Instruments – WIDEBAND FIXED-GAIN AMPLIFIER

THS4302
SLOS403G – OCTOBER 2002 – REVISED JANUARY 2005
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
_
+ _THS4302
V_O
100 Ω
*2.5 V
FB = Ferrite Bead
* = Low Impedance
Figure 45. DC-Coupled Single Supply Operation
Saving Power With Power-Down Functionality
The THS4302 features a power-down pin (PD) which
lowers the quiescent current from 37 mA down to
800 µA, 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,
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.
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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
THS4302
The THS4302 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
THS4302 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 useful. Where dc infor-
mation 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
_
+ _THS4302
0.1 µF
30.1 Ω
*2.5 V _{RISO 0.1 µF}
16.5 Ω
FB = Ferrite Bead
* = Low Impedance
24.9 Ω
IN
ADS5422
14-Bit, 63 Msps
IN CM
24.9 Ω
Figure 46. Driving an ADC Via a Transformer
The second circuit depicts single-ended ADC drive.
While not recommended for optimum performance
using converters with differential inputs, satisfactory
performance can sometimes be achieved with single-
ended input drive. An example circuit is shown here
for reference.
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