Datasheet
f
P
+
1
2pRC
2
Ǹ
xR
DAC Driver Application
THS3201
THS3201
47pF
CM
ADC
47pF
1:n
24.9 Ω
0.1 µF
R
F
R
G
V
S-
V
S+
24.9 Ω
0.1 µF
0.1 µF
V
IN
R
T
R
G
R
F
IOUT2
DAC
IOUT1
THS3201
THS3201
47pF
47pF
24.9 Ω
V
S+
R
F
R
G
0.1 µF
R
OUT
V
OUT1
V
OUT2
R
OUT
V
S-
0.1 µF
R
F
R
G
24.9 Ω
AV
DD
AV
DD
R
PU
R
PU
0.1 µF
THS3201-EP
SGLS283B – APRIL 2005 – REVISED JANUARY 2009 .....................................................................................................................................................
www.ti.com
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:
If R is only 10 Ω , the amplifier is heavily loaded
above the pole frequency, and generates excessive
distortion.
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
Figure 61. Differential ADC Driver Circuit 2
differential current outputs. Two THS3201s can be
used as a differential drive amplifier in these
It is almost universally recommended to use a
applications as shown in Figure 62 .
resistor and capacitor between the operational
R
PU
on the DAC output is used to convert the output
amplifier output and the ADC input as shown in
current to voltage. The 24.9- Ω resistor and 47-pF
Figure 60 and Figure 61 .
capacitor between each DAC output and the
This resistor-capacitor (RC) combination has multiple
operational amplifier input is used to reduce the
functions:
images generated at multiples of the sampling rate.
The values shown form a pole a 136 MHz. R
OUT
sets
• The capacitor is a local charge reservoir for ADC.
the output impedance of each amplifier.
• 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.
Figure 62. Differential DAC Driver Circuit
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:
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