
OPA646
9
from this potentially damaging source. The OPA646 incor-
porates on-chip ESD protection diodes as shown in Figure 2.
This eliminates the need for the user to add external protec-
tion diodes, which can add capacitance and degrade AC
performance.
All pins on the OPA646 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
limited to 10mA or so whenever possible.
The internal power dissipation is given by the equation
P
D
= P
DQ
+ P
DL
, where P
DQ
is the quiescent power dissipa-
tion and P
DL
is the power dissipation in the output stage due
to the load. (For
±
V
CC
=
±
5V, P
DQ
= 10V
x
7.5mA = 75mW,
max). For the case where the amplifier is driving a grounded
load (R
L
) with a DC voltage (
±
V
OUT
) the maximum value of
P
DL
occurs at
±
V
OUT
=
±
V
CC
/2, and is equal to P
DL
,
max = (
±
V
CC
)
2
/4R
L
. Note that it is the voltage across the
output transistor, and not the load, that determines the power
dissipated in the output stage.
The OPA646 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
without necessarily destroying the device. In precision op-
erational amplifiers, this may cause a noticeable degradation
of offset voltage and drift. Therefore, static protection is
strongly recommended when handling the OPA646.
OUTPUT DRIVE CAPABILITY
The OPA646 has been optimized to drive 75
and 100
resistive loads. The device can drive 2Vp-p into a 75
load.
This high-output drive capability makes the OPA646 an
ideal choice for a wide range of RF, IF, and video applica-
tions. In many cases, additional buffer amplifiers are
unneeded.
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 3, the
OPA646 maintains very low closed-loop output impedance
over frequency. Closed-loop output impedance increases
with frequency since loop gain is decreasing with frequency.
THERMAL CONSIDERATIONS
The OPA646 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.
FIGURE 2. Internal ESD Protection.
ESD Protection diodes internally
connected to all pins.
External
Pin
+V
CC
–V
CC
Internal
Circuitry
100
10
1
0.1
0.01
0.001
10k
Frequency (Hz)
O
)
100k
1M
10M
100M
A
V
= +1V/V
FIGURE 4. Output Current vs Temperature.
70
60
50
40
–60
Ambient Temperature (°C)
O
–40
–20
0
20
40
60
80
100
120
140
I
O
+
I
O
–
A short-circuit condition represents the maximum amount of
internal power dissipation that can be generated. The varia-
tion of output current with temperature is shown in Figure 4.
FIGURE 3. Small-Signal Output Impedance vs Frequency.