AD797 datasheet(11/20 Pages) AD

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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

nV/

109

e =

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:

TJ = TA + PD × θJA

For the AD8671 single in the 8-lead MSOP package, the thermal

resistance, θJA, 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, θJA, is 112°C/W. Although θJA 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,

RF, should be at least 500 Ω. However, if large values must be

used for RF, a small capacitor, CF, should be inserted in parallel

with RF to compensate for the pole introduced by the input

capacitance and RF.

Figure 30 shows the uncompensated output response with a

10 kΩ resistor in the feedback and the compensated response

with CF = 15 pF.

REF1 +OVER

23.23%

CH2 +OVER

7.885%

OUTPUT UNCOMPENSATED

OUTPUT

COMPENSATED

TIME (100ns/DIV)

Figure 30. Transient Output Response

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