lm94021-mda National Semiconductor Corporation, lm94021-mda Datasheet - Page 10

• Manufacturer Part Number: lm94021-mda
• Description: Multi-gain Analog Temperature Sensor
• Manufacturer: National Semiconductor Corporation

www.national.com
2.0 Mounting and Thermal
Conductivity
The LM94021 can be applied easily in the same way as
other integrated-circuit temperature sensors. It can be glued
or cemented to a surface.
To ensure good thermal conductivity, the backside of the
LM94021 die is directly attached to the GND pin (Pin 2). The
temperatures of the lands and traces to the other leads of the
LM94021 will also affect the temperature reading.
Alternatively, the LM94021 can be mounted inside a sealed-
end metal tube, and can then be dipped into a bath or
screwed into a threaded hole in a tank. As with any IC, the
LM94021 and accompanying wiring and circuits must be
kept insulated and dry, to avoid leakage and corrosion. This
is especially true if the circuit may operate at cold tempera-
tures where condensation can occur. If moisture creates a
short circuit from the output to ground or V
from the LM94021 will not be correct. Printed-circuit coatings
are often used to ensure that moisture cannot corrode the
leads or circuit traces.
The thermal resistance junction to ambient (θ
rameter used to calculate the rise of a device junction tem-
perature due to its power dissipation. The equation used to
calculate the rise in the LM94021’s die temperature is
where T
current, I
output voltage. For example, in an application where T
˚C, V
mV, and I
˚C, showing a self-heating error of only 0.021˚C. Since the
LM94021’s junction temperature is the actual temperature
being measured, care should be taken to minimize the load
current that the LM94021 is required to drive. Figure 1 shows
the thermal resistance of the LM94021.
3.0 Noise Considerations
The LM94021 has excellent noise rejection (the ratio of the
AC signal on V
tests, sine wave rejection of -54 dB or better was observed
over 200 Hz to 10 kHz; Also, -28 dB or better was observed
from 10 kHz to 1 MHz. A load capacitor on the output can
help filter noise; for example, a 1 nF load capacitor resulted
in -51 dB or better from 200 Hz to 1 MHz.
There is no specific requirement for the use of a bypass
capacitor close to the LM94021 because it does not draw
transient currents. For operation in very noisy environments,
some bypass capacitance may be required. The capacitance
does not need to be in close proximity to the LM94021. The
LM94021 has been bench tested successfully with a bypass
capacitor as far as 6 inches away. In fact, it can be powered
by a properly-bypassed logic gate.
Device Number
LM94021BIMG
DD
FIGURE 1. LM94021 Thermal Resistance
A
= 5 V, I
L
L
is the load current on the output, and V
is the ambient temperature, I
= 2 µA, the junction temperature would be 30.021
OUT
DD
= 9 µA, Gain Select = 11, V
to the AC signal on V
NS Package
MAA05A
Number
Q
Resistance (θ
DD
is the quiescent
). During bench
DD
Thermal
415˚C/W
JA
OUT
, the output
) is the pa-
O
= 2.231
A
is the
= 30
JA
)
10
4.0 Capacitive Loads
The LM94021 handles capacitive loading well. In an ex-
tremely noisy environment, or when driving a switched sam-
pling input on an ADC, it may be necessary to add some
filtering to minimize noise coupling. Without any precautions,
the LM94021 can drive a capacitive load less than or equal
to 1100 pF as shown in Figure 2. For capacitive loads
greater than 1100 pF, a series resistor may be required on
the output, as shown in Figure 3.
5.0 Output Voltage Shift
The LM94021 is very linear over temperature and supply
voltage range. Due to the intrinsic behavior of an NMOS/
PMOS rail-to-rail buffer, a slight shift in the output can occur
when the supply voltage is ramped over the operating range
of the device. The location of the shift is determined by the
relative levels of V
when V
This slight shift (a few millivolts) takes place over a wide
change (approximately 200 mV) in V
shift takes place over a wide temperature change of 5˚C to
20˚C, V
tions in the Electrical Characteristics table already include
this possible shift.
FIGURE 3. LM94021 with series resistor for capacitive
FIGURE 2. LM94021 No Decoupling Required for
DD
OUT
C
1.1 nF to
99 nF
100 nF to
999 nF
1 µF
Capacitive Loads Less than 1100 pF.
- V
LOAD
is always monotonic. The accuracy specifica-
OUT
Loading greater than 1100 pF.
= 1.0V.
DD
and V
OUT
. The shift typically occurs
DD
R
3 kΩ
1.5 kΩ
800 Ω
S
or V
OUT
. Since the
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