THS4211 Datasheet, PDF (22/50 Pages) Texas Instruments – LOW-DISTORTION HIGH-SPEED VOLTAGE FEEDBACK AMPLIFIER

THS4211
THS4215
SLOS400E – SEPTEMBER 2002 – REVISED SEPTEMBER 2009 ................................................................................................................................... www.ti.com
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
performance.
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
recommended 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
decoupling include a bulk decoupling capacitor
(6.8 µF 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 (in Figure 78) 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 provides 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.
V_CM
50 Ω (1:4 Ω)
Source 1:2
196 Ω
196 Ω
V_CM
5V
+
THS4211
_
-5 V
392 Ω
24.9 Ω
15 pF
15 pF
392 Ω
ADS5422
14-Bit, 62 Msps
24.9 Ω
_
THS4211
+
Figure 78. A Linear, Low-Noise, High-Gain
ADC Preamplifier
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
in Figure 79 for reference.
50 Ω
Source
V_I
49.9 Ω ^RT
+5 V
+
THS4211
_
R_ISO ^{0.1 µF}
16.5 Ω
68 pf
-5 V
R_f
392 Ω
R_g 392 Ω
1.82 kΩ
IN
ADS807
12-Bit,
CM 53 Msps
IN
0.1 µF
NOTE: For best performance, high-speed ADCs should be driven
differentially. See the THS4500 family of devices for more
information.
Figure 79. Driving an ADC With a
Single-Ended Input
22
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