Data Sheet AD8671/AD8672/AD8674
Rev. F | Page 11 of 20
APPLICATIONS
POWER DISSIPATION CALCULATIONS
To achieve low voltage noise in a bipolar op amp, the current
must be increased. The emitter-base theoretical voltage noise is
approximately
HznV/
2
10
9
C
n
qI
kTe =
To achieve the low voltage noise of 2.8 nV/√Hz, the input stage
current is higher than most op amps with an equivalent gain
bandwidth product. The thermal noise of a 1 kΩ resistor is
4 nV/√Hz, which is higher than the voltage noise of AD8671
family. Low voltage noise requires using low values of resistors,
so low voltage noise op amps should have good drive capability,
such as a 600 Ω load. This means that the second stage and
output stage are also biased at higher currents. As a result, the
supply current of a single op amp is 3.5 mA maximum at room
temperature.
Junction temperature has a direct affect on reliability. For more
information, visit the following Analog Devices, Inc., website:
http://www.analog.com/en/quality-and-reliability/reliability-
data/content/index.html
MTTF and FIT calculations can be done based on the junction
temperature and IC process. Use the following equation to
determine the junction temperature:
T
J
= T
A
+ P
D
× θ
JA
For the AD8671 single in the 8-lead MSOP package, the thermal
resistance, θ
JA
, is 142°C/W. If the ambient temperature is 30°C
and the supply voltages are ±12 V, the power dissipation is
24 V × 3.5 mA = 84 mW
Therefore, the rise above ambient temperature is
84 mW × 142°C/W = 12°C
If the ambient temperature is 30°C, the junction temperature is
42°C. The previously mentioned website that details the effect
of the junction temperature on reliability has a calculator that
requires only the part number and the junction temperature to
determine the process technology.
For the AD8674 single in the 14-Lead TSSOP package, the thermal
resistance, θ
JA
, is 112°C/W. Although θ
JA
is lower than it is for the
8-lead package, the four op amps are powered simultaneously. If
the ambient temperature is 50°C and the supply voltages are ±15 V,
the power dissipation is
30 V × 4.2 mA × four op amps = 504 mW
Therefore, the rise above ambient temperature is
504 mW × 112°C/W = 56°C
With an ambient temperature of 50°C, the junction temperature
is 106°C. This is less than the specified absolute maximum junction
temperature, but for systems with long product lifetimes (years),
this should be considered carefully.
Note that these calculations do not include the additional
dissipation caused by the load current on each op amp. Possible
solutions to reduce junction temperature include system level
considerations such as fans, Peltier thermoelectric coolers, and
heat pipes. Board considerations include operation on lower
voltages, such as ±12 V or ±5 V, and using two dual op amps
instead of one quad op amp. If the extremely low voltage noise
and high gain bandwidth is not required, using other quad op
amps, such as ADA4091-4, OP4177, ADA4004-4, OP497, or
AD704 can be considered.
UNITY-GAIN FOLLOWER APPLICATIONS
When large transient pulses (>1 V) are applied at the positive
terminal of amplifiers (such as the OP27, LT1007, OPA227, and
AD8671) with back-to-back diodes at the input stage, the use of
a resistor in the feedback loop is recommended to avoid having
the amplifier load the signal generator. The feedback resistor,
R
F
, should be at least 500 Ω. However, if large values must be
used for R
F
, a small capacitor, C
F
, should be inserted in parallel
with R
F
to compensate for the pole introduced by the input
capacitance and R
F
.
Figure 30 shows the uncompensated output response with a
10 kΩ resistor in the feedback and the compensated response
with C
F
= 15 pF.
03718-B-032
REF1 +OVER
23.23%
CH2 +OVER
7.885%
VOLTAGE (1V/DIV)
OUTPUT UNCOMPENSATED
OUTPUT
COMPENSATED
TIME (100ns/DIV)
Figure 30. Transient Output Response
