THS3201_14 Datasheet, PDF (18/40 Pages) Texas Instruments – 1.8-GHz, LOW DISTORTION, CURRENT-FEEDBACK AMPLIFIER

THS3201
SLOS416C – JUNE 2003 – REVISED JUNE 2009 ............................................................................................................................................................. www.ti.com
V_S+
0.1 µF
R_G
R_F
V_IN 1:n
THS3201
R_T
R_G
THS3201
R_F
24.9 Ω
47pF
24.9 Ω
47pF
ADC
CM
V_S-
0.1 µF
0.1 µF
Figure 60. Differential ADC Driver Circuit 2
It is almost universally recommended to use a
resistor and capacitor between the op amp output
and the ADC input as shown in both figures.
This resistor-capacitor (RC) combination has multiple
functions:
• The capacitor is a local charge reservoir for ADC
• The resistor isolates the amplifier from the ADC
• In conjunction, they form a low-pass noise filter
During the sampling phase, current is required to
charge the ADC input sampling capacitors. By placing
external capacitors directly at the input pins, most of
the current is drawn from them. They are seen as a
very low impedance source. They can be thought of
as serving much the same purpose as a
power-supply bypass capacitor to supply transient
current, with the amplifier then providing the bulk
charge.
Typically, a low-value capacitor in the range of 10 pF
to 100 pF provides the required transient charge
reservoir.
The capacitance and the switching action of the ADC
is one of the worst loading scenarios that a
high-speed amplifier encounters. The resistor
provides a simple means of isolating the associated
phase shift from the feedback network and
maintaining the phase margin of the amplifier.
Typically, a low value resistor in the range of 10 Ω to
100 Ω provides the required isolation. Together, the
R and C form a real pole in the s-plane located at the
frequency:
f_P
+
1
2pRC
Placing this pole at about 10x the highest frequency
of interest ensures it has no impact on the signal.
Since the resistor is typically a small value, it is very
bad practice to place the pole at (or very near)
frequencies of interest. At the pole frequency, the
amplifiers sees a load with a magnitude of:
Ǹ_{2 xR}
If R is only 10 Ω, the amplifier is very heavily loaded
above the pole frequency, and generates excessive
distortion.
DAC DRIVER APPLICATION
The THS3201 can be used as a high-performance
DAC output driver in applications like radio transmitter
stages and arbitrary waveform generators. All
high-performance DACs have differential current
outputs. Two THS3201s can be used as a differential
drive amplifier in these applications, as shown in
Figure 61.
R_PU on the DAC output is used to convert the output
current to voltage. The 24.9-Ω resistor and 47-pF
capacitor between each DAC output and the op amp
input is used to reduce the images generated at
multiples of the sampling rate. The values shown
form a pole at 136 MHz. R_OUT sets the output
impedance of each amplifier.
V_S+
AV_DD
0.1 µF
R_G
R_F
R_PU
IOUT1
DAC
IOUT2
R_PU
24.9 Ω
47pF
24.9 Ω
47pF
R_G
AV_DD
THS3201
THS3201
R_F
V_S-
R_OUT
V_OUT1
R_OUT
V_OUT2
0.1 µF
Figure 61. Differential DAC Driver Circuit
18
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