THS3110 Datasheet, PDF (19/28 Pages) Texas Instruments – LOW-NOISE, HIGH-VOLTAGE, CURRENT-FEEDBACK, OPERATIONAL AMPLIFIERS

www.ti.com
2000
1800
1600
1400
1200
1000
800
600
400 Gain = 2
200
R_F = 1 kΩ
V_S = ±15 V and ±5 V
0
100 k 1 M
10 M
100 M 1 G
f - Frequency - Hz
Figure 54. Power-down Output Impedance vs
Frequency
As with most current feedback amplifiers, the internal
architecture places some limitations on the system
when in power-down mode. Most notably is the fact
that the amplifier actually turns ON if there is a ±0.7 V
or greater difference between the two input nodes
(V+ and V-) of the amplifier. If this difference exceeds
±0.7 V, the output of the amplifier creates an output
voltage
equal
to
approximately
[(V+ - V-) -0.7 V] × Gain. This also implies that if a
voltage is applied to the output while in power-down
mode, the V- node voltage is equal to
V_O(applied)× R_G/(R_F + R_G). For low gain configurations
and a large applied voltage at the output, the ampli-
fier may actually turn ON due to the aforementioned
behavior.
The time delays associated with turning the device on
and off are specified as the time it takes for the
amplifier to reach either 10% or 90% of the final
output voltage. The time delays are in the order of
microseconds because the amplifier moves in and out
of the linear mode of operation in these transitions.
POWER-DOWN REFERENCE PIN
OPERATION
In addition to the power-down pin, the THS3110 also
features a reference pin (REF) which allows the user
to control the enable or disable power-down voltage
levels applied to the PD pin. In most split-supply
applications, the reference pin is connected to
ground. In either case, the user needs to be aware of
voltage level thresholds that apply to the power-down
pin. The usable range at the REF pin is from V_S- to
(V_S+ - 4 V)
THS3110, THS3111
SLOS422A – SEPTEMBER 2003 – REVISED NOVEMBER 2003
PRINTED-CIRCUIT BOARD LAYOUT
TECHNIQUES FOR OPTIMAL
PERFORMANCE
Achieving optimum performance with high frequency
amplifier, like the THS3110 and THS3111, requires
careful attention to board layout parasitic and external
component types.
Recommendations that optimize performance include:
• Minimize parasitic capacitance to any ac ground
for all of the signal I/O pins. Parasitic capacitance
on the output and input pins can cause instability.
To reduce unwanted capacitance, a window
around the signal I/O pins should be opened in all
of the ground and power planes around those
pins. Otherwise, ground and power planes should
be unbroken elsewhere on the board.
• Minimize the distance (< 0.25”) from the power
supply pins to high frequency 0.1-µF and 100-pF
decoupling capacitors. At the device pins, the
ground and power plane layout should not be in
close proximity to the signal I/O pins. Avoid
narrow power and ground traces to minimize
inductance between the pins and the decoupling
capacitors. The power supply connections should
always be decoupled with these capacitors.
Larger (6.8 µF or more) tantalum decoupling
capacitors, effective at lower frequency, should
also be used on the main supply pins. These may
be placed somewhat farther from the device and
may be shared among several devices in the
same area of the PC board.
• Careful selection and placement of external
components preserve the high frequency per-
formance of the THS3110 and THS3111. Re-
sistors should be a very low reactance type.
Surface-mount resistors work best and allow a
tighter overall layout. Again, keep their leads and
PC board trace length as short as possible.
Never use wirebound type resistors in a high
frequency application. Since the output pin and
inverting input pins are the most sensitive to
parasitic capacitance, always position the
feedback and series output resistors, if any, as
close as possible to the inverting input pins and
output pins. Other network components, such as
input termination resistors, should be placed
close to the gain-setting resistors. Even with a
low parasitic capacitance shunting the external
resistors, excessively high resistor values can
create significant time constants that can degrade
performance. Good axial metal-film or sur-
face-mount resistors have approximately 0.2 pF
in shunt with the resistor. For resistor values >
2.0 kΩ, this parasitic capacitance can add a pole
and/or a zero that can effect circuit operation.
Keep resistor values as low as possible, consist-
ent with load driving considerations.
19