THS3201-EP_14 Datasheet, PDF (20/34 Pages) Texas Instruments – 1.8-GHz LOW-DISTORTION CURRENT-FEEDBACK AMPLIFIER

THS3201-EP
SGLS283B – APRIL 2005 – REVISED JANUARY 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
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 bad
practice to place the pole at (or near) frequencies of
interest. At the pole frequency, the amplifiers see a
load with a magnitude of:
Ǹ_{2 xR}
If R is only 10 Ω, the amplifier is heavily loaded
above the pole frequency, and generates excessive
distortion.
V_S-
0.1 µF
0.1 µF
Figure 61. Differential ADC Driver Circuit 2
It is almost universally recommended to use a
resistor and capacitor between the operational
amplifier output and the ADC input as shown in
Figure 60 and Figure 61.
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
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:
DAC Driver Application
The THS3201 can be used as a high-performance
DAC output driver in applications such as 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 62.
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
operational amplifier input is used to reduce the
images generated at multiples of the sampling rate.
The values shown form a pole a 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 62. Differential DAC Driver Circuit
20
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