OPA644U BURR-BROWN [Burr-Brown Corporation], OPA644U Datasheet - Page 10

• Manufacturer Part Number: OPA644U
• Description: Low Distortion Current Feedback OPERATIONAL AMPLIFIER
• Manufacturer: BURR-BROWN [Burr-Brown Corporation]

INPUT PROTECTION
Static damage has been well recognized for MOSFET de-
vices, but any semiconductor device deserves protection
from this potentially damaging source. The OPA644 incor-
porates on-chip ESD protection diodes as shown in Figure 3.
This eliminates the need for the user to add external protec-
tion diodes, which can add capacitance and degrade AC
performance.
FIGURE 3. Internal ESD Protection.
All pins on the OPA644 are internally protected from ESD
by means of a pair of back-to-back reverse-biased diodes to
either power supply as shown. These diodes will begin to
conduct when the input voltage exceeds either power supply
by about 0.7V. This situation can occur with loss of the
amplifier’s power supplies while a signal source is still
present. The diodes can typically withstand a continuous
current of 30mA without destruction. To insure long term
reliability, however, diode current should be externally lim-
ited to 10mA or so whenever possible.
The OPA644 utilizes a fine geometry high speed process
that withstands 500V using the Human Body Model and
100V using the machine model. However, static damage can
cause subtle changes in amplifier input characteristics with-
out necessarily destroying the device. In precision opera-
tional amplifiers, this may cause a noticeable degradation of
offset voltage and drift. Therefore, static protection is strongly
recommended when handling the OPA644.
OUTPUT DRIVE CAPABILITY
The OPA644 has been optimized to drive 75
resistive loads. The device can drive 2Vp-p into a 75 load.
This high-output drive capability makes the OPA644 an
ideal choice for a wide range of RF, IF, and video applica-
tions. In many cases, additional buffer amplifiers are un-
needed.
Many demanding high-speed applications such as
ADC/DAC buffers require op amps with low wideband
output impedance. For example, low output impedance is
essential when driving the signal-dependent capacitances at
the inputs of flash A/D converters. As shown in Figure 4,
the OPA644 maintains very low closed-loop output imped-
ance over frequency. Closed-loop output impedance in-
creases with frequency since loop gain is decreasing with
frequency.
External
Pin
®
OPA644
+V
–V
CC
CC
ESD Protection diodes internally
connected to all pins.
Internal
Circuitry
and 100
10
FIGURE 4. Closed-Loop Output Impedance vs Frequency.
THERMAL CONSIDERATIONS
The OPA644 does not require a heat sink for operation in
most environments. At extreme temperatures and under full
load conditions a heat sink may be necessary.
The internal power dissipation is given by the equation
P
tion and P
to the load. (For V
max). For the case where the amplifier is driving a grounded
load (R
P
= ( V
transistor, and not the load, that determines the power
dissipated in the output stage.
The short-circuit condition represents the maximum amount
of internal power dissipation that can be generated. The
variation of output current with temperature is shown in the
Typical Performance Curves.
CAPACITIVE LOADS
The OPA644’s output stage has been optimized to drive low
resistive loads. Capacitive loads, however, will decrease the
amplifier’s phase margin which may cause high frequency
peaking or oscillations. Capacitive loads greater than 5pF
should be buffered by connecting a small resistance, usually
5
This is particularly important when driving high capacitance
loads such as flash A/D converters.
FIGURE 5. Driving Capacitive Loads.
D
DL
0.001
= P
to 25 , in series with the output as shown in Figure 5.
occurs at
0.01
100
0.1
10
CC
1
DQ
L
10K
) with a DC voltage ( V
)
2
DL
+ P
/4R
is the power dissipation in the output stage due
DL
L
. Note that it is the voltage across the output
, where P
V
OUT
100K
CC
A
V
= +2V/V
= 5V, P
= V
OPA644
402
DQ
Frequency (Hz)
CC
is the quiescent power dissipa-
/ 2, and is equal to P
DQ
1M
OUT
= 10V x 26mA = 260mW,
) the maximum value of
(R
R
S
S
typically 5 to 25 )
10M
R
L
C
DL
L
100M
, max