LMP7732MM National Semiconductor, LMP7732MM Datasheet - Page 17

• 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

Thermopiles have very wide temperature ranges of -100°C to
1000°C
When choosing a thermopile for a certain application, one
must pay attention to several parameters. Some of these pa-
rameters are discussed below:
Thermopiles' sensitivity, or responsivity, is determined by the
ratio of output voltage to the absorbed input signal power and
is usually specified in V/W. Typical sensitivity of thermopiles
ranges from 10s of V/W to about 100 V/W. Generally, higher
values of sensitivity are desirable. Sensitivity is dependent on
the absorber's area and number of thermocouples used in the
sensor. Sensitivity is often represented by S where:
S = V
The sensitivity of a thermopile changes with change in tem-
perature. This change is usually specified as the Temperature
Coefficient, TC, of sensitivity. Lower numbers are desired for
this parameter.
Resistance of the thermopile is usually specified in the
datasheet. This is the impedance which will be seen by the
input of the amplifier used to process the thermopile's output
signal. Typical values for thermopile resistance, R
from 10s of kilo-ohms to about 100 kΩ. This resistance is also
a function of temperature. The temperature coefficient of the
resistance is usually specified in a thermopile's datasheet. As
with any other parameter, minimum variation with tempera-
ture is desired.
The dominant noise source for a thermopile is its resistance.
Noise spectral density of a resistor is calculated by:
Where k is the Boltzman constant and T is absolute temper-
ature. Unit of noise spectral density is: V/
For the thermopile sensor, this noise is usually represented
by V
Typical values for this voltage noise are in the order of a few
tens of nV/
The Noise Equivalent Power, NEP, is often used to specify
the minimum detectable signal level per square root band-
NOISE
OUT
/P
where:
IN
.
FIGURE 7. Thermopile
TP
, range
30015007
17
width. A smaller NEP is desired, however NEP is dependent
on the thermopile active area, A
And
As it is shown above, one cannot just compare the NEP of
two thermopiles without considering the corresponding active
areas.
A better way to compare thermopiles is to look at their specific
detectivity, D
noise and its sensitivity. It is normalized with respect to the
detector's active area and also noise bandwidth. D
by:
Unit of D
range from 10
After receiving radiation, the thermopile takes some time be-
fore it comes to thermal equilibrium. The time it takes for the
sensor to achieve this equilibrium is called response time or
time constant of the sensor. Clearly, lower time constants are
very desirable.
Precision Amplifier
Since the output of thermopiles are usually very small and at
most in the order of only a few millivolts, the first part of the
signal conditioning path should involve amplification. In
choosing an amplifier for this purpose, a few different sensor
characteristics and the way they interface with the amplifier
should be considered. These are:
Sensor's Impedance and Opamp's Input Bias Current
The input bias current causes a voltage drop across the sen-
sor and the amount of this voltage is equal to the sensor's
impedance multiplied by the magnitude of bias current. The
higher the sensor's input impedance, the more accentuated
the effect of amplifier's input bias current will be. For very high
impedance sensors, it is imperative that opamps with very low
input bias currents be used. Thermopiles have input
impedances in the range of 100 kΩ, so input bias current is
not as critical as in some other applications.
Sensor's output voltage range:
The output signal of the sensor is fed into the opamp where
it will be amplified or otherwise conditioned, (e.g. level shifted,
buffered.) It is important to pay attention to different parame-
ters of this output signal.
One important aspect is the lowest expected level of the
sensor's output and compare that with different parameters
contributing to the amplifier's total input noise. If the sensor's
output level is in the same order of magnitude or smaller than
the opamp's total input noise, then signal integrity at the
opamp's output and the ADC's input will be compromised.
*
is cm
*
. Specific detectivity includes both the device
8
to 3*10
/ W. Typical values for specific detectivity
8
cm
/ W.
D
. For a thermopile:
www.national.com
*
is given