Texas Instruments THS3201 Evaluation Module THS3201EVM THS3201EVM Fiche De Données
Codes de produits
THS3201EVM
f
P
+
1
2
p
RC
2
Ǹ
x R
THS3201
THS3201
47pF
CM
ADC
47pF
1:n
24.9
Ω
0.1
µ
F
R
F
R
G
V
S-
V
S+
24.9
Ω
0.1
µ
F
0.1
µ
F
V
IN
R
T
R
G
R
F
DAC DRIVER APPLICATION
IOUT2
DAC
IOUT1
THS3201
THS3201
47pF
47pF
24.9
Ω
V
S+
R
F
R
G
0.1
µ
F
R
OUT
V
OUT1
V
OUT2
R
OUT
V
S-
0.1
µ
F
R
F
R
G
24.9
Ω
AV
DD
AV
DD
R
PU
R
PU
0.1
µ
F
SLOS416C – JUNE 2003 – REVISED JUNE 2009
.............................................................................................................................................................
www.ti.com
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:
frequency:
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:
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:
If R is only 10
Ω, the amplifier is very heavily loaded
above the pole frequency, and generates excessive
distortion.
distortion.
Figure 60. Differential ADC Driver Circuit 2
The THS3201 can be used as a high-performance
DAC output driver in applications like radio transmitter
DAC output driver in applications like radio transmitter
It is almost universally recommended to use a
stages
and
arbitrary
waveform
generators.
All
resistor and capacitor between the op amp output
high-performance DACs have differential current
and the ADC input as shown in both figures.
outputs. Two THS3201s can be used as a differential
drive amplifier in these applications, as shown in
drive amplifier in these applications, as shown in
This resistor-capacitor (RC) combination has multiple
.
functions:
•
The capacitor is a local charge reservoir for ADC
R
PU
on the DAC output is used to convert the output
current to voltage. The 24.9-
Ω resistor and 47-pF
•
The resistor isolates the amplifier from the ADC
capacitor between each DAC output and the op amp
•
In conjunction, they form a low-pass noise filter
input is used to reduce the images generated at
During the sampling phase, current is required to
multiples of the sampling rate. The values shown
charge the ADC input sampling capacitors. By placing
form a pole at 136 MHz. R
OUT
sets the output
external capacitors directly at the input pins, most of
impedance of each amplifier.
the current is drawn from them. They are seen as a
very low impedance source. They can be thought of
as
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.
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.
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
is one of the worst loading scenarios that a
high-speed
amplifier
encounters.
The
resistor
provides a simple means of isolating the associated
phase
phase
shift
from
the
feedback
network
and
maintaining the phase margin of the amplifier.
Figure 61. Differential DAC Driver Circuit
18
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