THS3201 Datasheet, PDF (16/29 Pages) Texas Instruments – 1.8-GHz, LOW DISTORTION, CURRENT FEEDBACK AMPLIFIER

THS3201
SLOS416A − JUNE 2003 − REVISED JANUARY 2004
VS+
0.1 µF
RG
RF
VIN 1:n
THS3201
RT
RG
THS3201
RF
24.9 Ω
47pF
24.9 Ω
47pF
ADC
CM
VS−
0.1 µF
0.1 µF
Figure 55. Differential ADC Driver Circuit 2
It is almost universally recommended to use a resistor and
capacitor between the op amp’s output and the ADC’s
input as shown in both Figures.
This resistor-capacitor (RC) combination has multiple
functions:
D The capacitor is a local charge reservoir for ADC
D The resistor isolates the amplifier from the ADC
D In conjunction, they form a low-pass noise filter
During the sampling phase, current is required to charge
the ADC’s 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
16
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Placing this pole at about 10x the highest frequency of
interest insures 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 56.
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 a 136 MHz.
R_OUT sets the output impedance of each amplifier.
VS+
AVDD
0.1 µF
RG
RF
RPU
IOUT1
DAC
IOUT2
RPU
24.9 Ω
47pF
24.9 Ω
47pF
RG
AVDD
THS3201
ROUT
VOUT1
THS3201
RF
VS−
ROUT
VOUT2
0.1 µF
Figure 56. Differential DAC Driver Circuit