Datasheet

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APPLICATION CIRCUITS
Driving an Analog-to-Digital Converter With the
_
+
THS4302
R
f
100 Ω
49.9 Ω
V
I
+
22 µF 47 pF 0.1 µF
V
S+
50-Ω Source
R
g
V
O
30.1 Ω
FB
FB = Ferrite Bead
*2.5 V
*2.5 V
* = Low Impedance
ADS5422
14-Bit, 63 Msps
R
ISO
0.1 µF
24.9 Ω
_
+
THS4302
R
f
49.9 Ω
V
I
+
22 µF 47 pF 0.1 µF
V
S+
50-Ω Source
R
g
30.1 Ω
FB
FB = Ferrite Bead
*2.5 V
*2.5 V
* = Low Impedance
24.9 Ω
16.5 Ω
IN
IN
CM
THS4302
SLOS403H – OCTOBER 2002 – REVISED AUGUST 2006
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
information content is required, the THS4500 family
of fully differential amplifiers may be applicable.
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 μ A, 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
Figure 46. Driving an ADC Via a Transformer
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
The second circuit depicts single-ended ADC drive.
as a 3-state bus driver. When in power-down mode,
While not recommended for optimum performance
the impedance looking back into the output of the
using converters with differential inputs, satisfactory
amplifier is dominated by the feedback and gain
performance can sometimes be achieved with single-
setting resistors, but the output impedance of the
ended input drive. An example circuit is shown here
device itself varies depending on the voltage applied
for reference.
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.
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