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Where to Connect the Exposed Pad
The exposed pad is not DC connected to any resistor
terminal. Its main purpose is to reduce the internal tem-
perature rise when the application calls for large amounts
of dissipated power in the resistors. The exposed pad can
be tied to any voltage (such as ground) as long as the
absolute maximum ratings are observed.
There is capacitive coupling between the resistors and the
exposed pad, as specified in the Electrical Characteristics
table. To avoid interference, do not tie the exposed pad to
noisy signals or noisy grounds.
Connecting the exposed pad to a quiet AC ground is
recommended as it acts as an AC shield and reduces the
amount of resistor-resistor capacitance.
Each resistor is rated for relatively high power dissipation,
as listed in the Absolute Maximum Ratings section of
this data sheet. To calculate the internal temperature rise
inside the package, add together the power dissipated in
all of the resistors, and multiply by the thermal resistance
coefficient of the package (θJA or θJC as applicable).
For example, if each resistor dissipates 250mW, for a
total of 1W, the total temperature rise inside the package
equals 40°C. All 4 resistors will be at the same temperature,
regardless of which resistor dissipates more power. The
junction temperature must be kept within the Absolute
Maximum Rating. At elevated ambient temperatures, this
places a limit on the maximum power dissipation.
In addition to limiting the maximum power dissipation,
the maximum voltage across any two pins must also be
kept less than the absolute maximum rating.
The LT5400 can withstand up to ±1kV of electrostatic
discharge (ESD, human body). To achieve the highest
precision matching, the LT5400 is designed without explicit
ESD internal protection diodes. ESD beyond this voltage
can damage or degrade the device including causing
To protect the LT5400 against large ESD strikes, external
protection can be added using diodes to the circuit supply
rails or bidirectional Zeners to ground (Figure 1).
The LT5400 specifies matching in the most conservative
possible way. In each device, the ratio error of the largest
of the four resistors to the smallest of the four resistors
meets the specified matching level. Looser definitions
would compare each resistor value to the average of the
resistor values, which would typically result in specifica-
tions that appear twice as good as they are per the LT5400’s
more conservative definition. The following two examples
illustrate this point.
In an inverting gain-of-1 amplifier, if the largest resistor
is allowed to deviate only 0.01% from the smallest resis-
tor, then the worst-case gain can be –1.00005/0.99995 =
–1.0001, which is a 0.01% error from the ideal –1.0000.
That is the LT5400 definition. In a looser definition, if each
resistor would be allowed to deviate by 0.01% from the
average, then the worst-case gain could be –1.0001/0.9999
= –1.0002, which is a 0.02% error from the ideal –1.0000.
In a divide-by-2 resistor divider network, if the largest
resistor is allowed to deviate only 0.01% from the smallest
resistor, then the worst-case ratio can be 1.00005/(1.00005
+ 0.99995) = 0.500025, which is a 0.005% error from the
ideal 0.50000. That is the LT5400 definition. In a looser
definition, if each resistor would be allowed to deviate by
0.01% from the average, then the worst-case ratio could
be 1.0001/(1.0001 + 0.9999) = 0.50005, which is a 0.01%
error from the ideal 0.50000.