LMP7732MM National Semiconductor, LMP7732MM Datasheet - Page 16

• Manufacturer Part Number: LMP7732MM
• Description: IC, OP AMP, PREC RRIO, DUAL, 8SOIC
• Manufacturer: National Semiconductor

Specifications of LMP7732MM

• Op Amp Type: Precision
• No. Of Amplifiers: 2
• Bandwidth: 22MHz
• Slew Rate: 2.4V/µs
• Supply Voltage Range: 1.8V To 5.5V
• Amplifier Case Style: MSOP
• No. Of Pins: 8
• Operating Temperature Range: -40°C To
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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DRIVING AN ADC
Analog to Digital Converters, ADCs, usually have a sampling
capacitor on their input. When the ADC's input is directly con-
nected to the output of the amplifier a charging current flows
from the amplifier to the ADC. This charging current causes
a momentary glitch that can take some time to settle. There
are different ways to minimize this effect. One way is to slow
down the sampling rate. This method gives the amplifier suf-
ficient time to stabilize its output. Another way to minimize the
glitch, caused by the switch capacitor, is to have an external
capacitor connected to the input of the ADC. This capacitor is
chosen so that its value is much larger than the internal
switching capacitor and it will hence provide the charge need-
ed to quickly and smoothly charge the ADC's sampling ca-
pacitor. Since this large capacitor will be loading the output of
the amplifier as well, an isolation resistor is needed between
the output of the amplifier and this capacitor. The isolation
resistor, R
the output of the amplifier and will also form a low-pass filter
and can be designed to provide noise reduction as well as
anti-aliasing. The draw back of having R
signal swing since there is some voltage drop across it.
Figure 6 (a) shows the ADC directly connected to the ampli-
fier. To minimize the glitch in this setting, a slower sample rate
needs to be used. Figure 6 (b) shows R
capacitor used to minimize the glitch.
ISO
, separates the additional load capacitance from
FIGURE 6. Driving An ADC
ISO
ISO
is that it reduces
and an external
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THERMOPILE AMPLIFIER
Thermopile Sensors
Thermopiles are arrays of interconnected thermocouples
which can detect surface temperature of an object through
radiation rather than direct contact. The hot and cold junctions
of the thermocouples are thermally isolated. The hot junctions
are exposed to IR radiation emitted from the measurement
surface and the cold junctions are connected to a heat sink.
The incident IR changes the temperature of the hot junctions
of the thermopile and produces an output voltage proportional
to this change.
The hot junction of the thermopile is covered with a highly
emissive coating. The IR radiation incident to this highly emis-
sive material changes the temperature of this coating. The
temperature change is converted to a voltage by the ther-
mopile. Emissivity represents the radiation or absorption ef-
ficiency of a material relative to a black body. An ideal black
body has an emissivity of 1.0. Excluding shiny metals, most
objects have emissivities above 0.85. As a practical matter,
shiny metals are not good candidates for IR sensing because
of their low emissivity. The low emissivity means that the ma-
terial is highly reflective. Reflective materials often “reflect”
the surrounding environment's temperature rather than their
own heat radiation. This makes them not suitable for ther-
mopile applications.
The output voltage of a thermopile is related to temperature
and emissivity by the following formula:
Where:
V
K : Proportionality constant
ε
T
δ : Correction factor. This is needed since thermopile filters
do not allow all wavelengths to enter the sensor
ε
T
As mentioned above, the IR radiation generates a static volt-
age across the pyroelectric material. If the illumination is
constant, the signal level detection declines. This is why the
radiation needs to be periodically refreshed. This task is usu-
ally achieved by the means of a mechanical chopper in front
of the detector.
Thermopiles offer much faster response time compared to
other temperature measurement devices. Packaged thermis-
tors and thermocouples have response times that can range
up to a few seconds, where as packaged thermopiles can
easily achieve response times in the order of tens of millisec-
onds. Thermopiles also provide superior thermal isolation
compared to their contact temperature measurement coun-
terparts. Physical contact disturbs the systems temperature
and also creates temperature gradients.
Figure 7 shows a simplified schematic of a thermopile. The
cold junctions are connected to a heat sink, and the absorber
material covers the hot junction. The output voltage resulting
from temperature difference between the two junctions is
measured at the two ends of the array of thermocouples. As
is evident in Figure 7, increasing the number of thermocou-
ples in a thermopile increases the output voltage range. This
also increases the active area of the thermopile sensor.
OBJ
TP
OUT
OBJ
TP
: Emissivity of the thermopile
: Temperature of the thermopile
: Emissivity of object being measured
: Temperature of object being measured
: Output voltage of the thermopile