THS4211 Datasheet, PDF (21/41 Pages) Texas Instruments – LOW-DISTORTION HIGH-SPEED VOLTAGE FEEDBACK AMPLIFIER

www.ti.com
tive rail. The threshold voltages for power-on and
power-down are relative to the supply rails, and are
given in the specification tables. Above the Enable
Threshold Voltage, the device is on. Below the
Disable Threshold Voltage, the device is off. Behavior
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.
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.
Power-Down Reference Pin Operation
In addition to the power-down pin, the THS4215 also
features a reference pin (REF) which allows the user
to control the enable or disable power-down voltage
levels applied to the PD pin. Operation of the
reference pin as it relates to the power-down pin is
described below.
In most split-supply applications, the reference pin will
be connected to ground. In some cases, the user
may want to connect it to the negative or positive
supply rail. In either case, the user needs to be aware
of the voltage level thresholds that apply to the
power-down pin. The table below illustrates the re-
lationship between the reference voltage and the
power-down thresholds.
REFERENCE
VOLTAGE
V_S– to 0.5 (V_S– + V_S+)
0.5 (V_S– + V_S+) to V_S+
POWER-DOWN PIN VOLTAGE
DEVICE
DISABLED
DEVICE
ENABLED
≤ Ref + 1.0 V
≥ Ref + 1.8 V
≤ Ref – 1.5 V
≥ Ref – 1 V
The recommended mode of operation is to tie the
reference pin to mid-rail, thus setting the threshold
levels to mid-rail +1.0 V and midrail +1.8 V.
THS4211
THS4215
SLOS400D – SEPTEMBER 2002 – REVISED NOVEMBER 2004
NO. OF CHANNELS PACKAGES
Single (8-pin)
THS4215D, THS4215DGN, and
THS4215DRB
Power Supply Decoupling Techniques and
Recommendations
Power supply decoupling is a critical aspect of any
high-performance amplifier design process. Careful
decoupling provides higher quality ac performance
(most notably improved distortion performance). The
following guidelines ensure the highest level of per-
formance.
1. Place decoupling capacitors as close to the
power supply inputs as possible, with the goal of
minimizing the inductance of the path from
ground to the power supply.
2. Placement priority should put the smallest valued
capacitors closest to the device.
3. Use of solid power and ground planes is rec-
ommended to reduce the inductance along power
supply return current paths, with the exception of
the areas underneath the input and output pins.
4. Recommended values for power supply decoup-
ling include a bulk decoupling capacitor (6.8 to 22
µF), a mid-range decoupling capacitor (0.1 µF)
and a high frequency decoupling capacitor (1000
pF) for each supply. A 100 pF capacitor can be
used across the supplies as well for extremely
high frequency return currents, but often is not
required.
APPLICATION CIRCUITS
Driving an Analog-to-Digital Converter With the
THS4211
The THS4211 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 differential signal is then amplified and
filtered by two THS4211 amplifiers. This circuit pro-
vides low intermodulation distortion, suppressed
even-order distortion, 14 dB of voltage gain, a 50-Ω
input impedance, and a single-pole filter at 100 MHz.
For applications without signal content at dc, this
method of driving ADCs can be very useful. Where dc
information content is required, the THS4500 family
of fully differential amplifiers may be applicable.
21