LM9040M/NOPB National Semiconductor, LM9040M/NOPB Datasheet - Page 5

• Manufacturer Part Number: LM9040M/NOPB
• Description: IC SENSOR DUAL INTER AMP 14-SOIC
• Manufacturer: National Semiconductor
• Type: Sensor Interfacer

Specifications of LM9040M/NOPB

• Input Type: Differential
• Output Type: Voltage
• Current - Supply: 8mA
• Mounting Type: Surface Mount
• Package / Case: 14-SOIC (7.5mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Interface: -
• Other names: *LM9040M; *LM9040M/NOPB; LM9040M

Circuit Description
The LM9040 is fabricated in CMOS technology and is de-
signed to operate from a single, well regulated, 5V supply.
The IC consists of two independent differential amplifiers
which are designed using two-phased switched capacitor net-
works (SCN). The differential inputs have a common mode
operating range of 2V above and below ground. The SCN in-
cludes the input sampling, the lowpass filter, cold sensor bias
voltage, and the gain circuitry. Each amplifier has an inde-
pendent voltage comparator to detect an open inverting input
pin. Additional support circuitry includes the oscillator, clock
generator, and V
Oscillator
The device contains an internal oscillator which is used to
drive the internal two-phase clock generator. The oscillator
requires an external resistor value of 178 kΩ from the
“OSC_RES” pin to device V
the charge rate of the internal capacitor, and thus sets the
oscillator frequency. The internal oscillator capacitor is
matched to the switched capacitor networks, so that the ab-
solute capacitance values are not as important as is the
absolute ratios of the capacitors. The oscillator frequency is
approximately 200 kHz.
The oscillator resistor should be located as close to the
OSC_RES pin as possible. Any variation of the oscillator re-
sistor value, any stray capacitance on the OSC_RES pin, or
FIGURE 3. Simplified Circuit
BB
bias generator.
CC
. This resistor value determines
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5
any changes in the supply voltage, will result in a change in
the oscillator frequency. This will directly affect the device
Differential Input Impedance, and Low Pass filter response.
Additional circuitry takes the oscillator signal and generates
two non-overlapping clock signals, and a CLK_OUT signal.
The clock signals operate at one half the oscillator frequency,
or typically 100 kHz. This results in a Nyquist frequency of
typically 50 kHz.
Clock Out/Clock In
For the input stage to work with common mode voltages be-
low Ground potential, a negative bias voltage (V
The CLK_OUT pin is used to provide the AC signal needed
to drive the internal V
coupling capacitor. A minimum coupling capacitor value of
100 pF to a maximum value of 0.1 μF is recommended. The
CLK_IN pin is the input to the V
Differential Input Circuit
The input stage can be best described as a switched Sample
and Difference circuit (see
C
plus the common mode voltage is stored on C
switched to the inverting input, C
amount equal to the common mode voltage. The remaining
charge across C
age, and a proportional charge will be transferred through the
virtual ground via the gain stage.
The differential input impedance is a function of the value of
the input capacitor array and the sampling frequency. The
capacitor C
Differential Input Impedance (Z
ilar to the Lambda Sensor output voltage at the stoichiometric
air-fuel mixture (λ = 1). The bias voltage is set by the ratio of
C
The resulting bias voltage across the Differential Input is de-
fined as:
With C
and V
FIGURE 4. Simplified Switched Capacitor Input Circuit
IN
IN
and C
is switched to the non-inverting input, the input voltage
CC
BIAS
= 5V:
BIAS
= 0.7286 pF, C
BIAS
, and the value of V
is used to generate a bias voltage across the
IN
will be equal to the differential input volt-
BB
bias generator through an external
Figure
IN
= 7.421 pF, F
DIFF
BB
4). When the input capacitor
CC
IN
bias generator circuitry.
). This bias voltage is sim-
.
will be discharged by an
CLOCK
IN
BB
. When C
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) is needed.
= 100 kHz,
1237214
IN
is