LMP7731MF NSC [National Semiconductor], LMP7731MF Datasheet - Page 18

• Manufacturer Part Number: LMP7731MF
• Description: 2.9 nV/sqrt(Hz) Low Noise, Precision, RRIO, Operational Amplifier
• Manufacturer: NSC [National Semiconductor]

www.national.com
Figure 8 shows the LMP7731 used as a thermopile amplifier.
The LMP7731 is a great choice for use with thermopile sen-
sors. The LMP7731 provides unprecedented accuracy and
precision because of its very low input voltage noise and the
very low 1/f corner frequency. The 1/f noise is one of the main
sources of error in DC operating mode. Since thermopiles and
most other sensors operate on DC signals, signal integrity at
the DC level is very important. The LMP7731 also has very
low offset voltage and offset voltage drift which greatly re-
duces the effects of input offset voltage of the amplifier on the
thermopile signal. The thermopile used in this circuit is
TPS332 from PerkinElmer Optoelectronics, PKI. This ther-
mopile has an internal resistance, R
voltage of the thermopile is represented with a DC voltage
source. The TPS332 has a thermistor integrated in the pack-
age. The thermistor is used to measure the ambient temper-
ature of the thermopile at the time of measurement. The
thermistor's resistance at room temperature is 30 kΩ. More
information about this thermopile and other sensors from PKI
can be found on http://www.perkinelmer.com/
The circuit in Figure 8 shows how the LMP7731 is connected
to the thermopile. This circuit is comprised of two LMP7731
amplifiers, the LM4140A-2.5 which is a precision voltage ref-
erence, the ADC122S021 which is a 2 channel Analog to
Digital converter, and the thermopile sensor. Note that the two
amplifiers used in this circuit are numbered for ease of refer-
TP
, of 75 kΩ. The output
FIGURE 8. Thermopile Amplifier
18
ence. The LMP7731 amplifiers are referred to as amplifier 1
and amplifier 2 per Figure 8.
In Figure 8 the LM4140A is providing a precision voltage ref-
erence of 2.5V. This reference voltage is applied to the ther-
mistor via the 30 kΩ resistor. The thermistor's resistance is
converted to a voltage using this set up. This voltage is fed
into the ADC's channel one. The ADC uses this voltage and
the thermistor's look up table to convert this voltage to tem-
perature. The 2.5V reference is also fed into amplifier 1, which
is configured as a buffer. This LMP7731 transfers the 2.5V
signal to both inputs of amplifier 2. This means the 2.5V will
show up on the output of amplifier 2. Having an output level
that is mid-supply is important since the thermopile sensor
has a bipolar output signal and this way the amplifier can ac-
curately gain the thermopile voltage, whether its polarity is
positive or negative. It is also important because the output
signal of amplifier 2 is only positive. ADCs can only handle
positive signals on their inputs. Amplifier 2 is used to gain and
filter the thermopile signal. The low pass filter ensures that AC
noise will not be gained up and as a result the output signal
will be cleaner. The output of amplifier 2 is fed into the ADC's
channel 0. The ADC uses the ambient temperature, which
was calculated using the voltage on Channel 1 and the
thermistor's look up table, along with the thermopiles' gained
output voltage available on channel 0 and the thermopile's
look up table to determine the object's temperature.
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