LM9040M National Semiconductor, LM9040M Datasheet - Page 5

• Manufacturer Part Number: LM9040M
• Description: Dual Lambda Sensor Interface Amplifier
• Manufacturer: National Semiconductor

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
networks (SCN). The differential inputs have a common
mode operating range of 2V above and below ground. The
SCN includes the input sampling, the lowpass filter, cold
sensor bias voltage, and the gain circuitry. Each amplifier
has an independent 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 k
“OSC_RES” pin to device V
mines 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
absolute 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
resistor value, any stray capacitance on the OSC_RES pin,
FIGURE 3. Simplified Circuit
CC
. This resistor value deter-
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bias generator.
from the
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5
or 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 fre-
quency, or typically 100 kHz. This results in a Nyquist fre-
quency of typically 50 kHz.
Clock Out/Clock In
For the input stage to work with common mode voltages
below Ground potential, a negative bias voltage (V
needed. The CLK_OUT pin is used to provide the AC signal
needed to drive the internal V
external coupling capacitor. A minimum coupling capacitor
value of 100 pF to a maximum value of 0.1 µF is recom-
mended. The CLK_IN pin is the input to the V
erator circuitry.
Differential Input Circuit
The input stage can be best described as a switched Sample
and Difference circuit (see Figure 4 ). When the input capaci-
tor C
age plus the common mode voltage is stored on C
C
by an amount equal to the common mode voltage. The
remaining charge across C
input voltage, 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
similar to the Lambda Sensor output voltage at the stoichio-
metric air-fuel mixture ( = 1). The bias voltage is set by the
ratio of C
The resulting bias voltage across the Differential Input is
defined as:
With C
and V
FIGURE 4. Simplified Switched Capacitor Input Circuit
IN
is switched to the inverting input, C
IN
CC
BIAS
is switched to the non-inverting input, the input volt-
IN
= 5V:
BIAS
= 0.7286 pF, C
and C
is used to generate a bias voltage across the
BIAS
, and the value of V
IN
IN
= 7.421 pF, F
will be equal to the differential
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DIFF
bias generator through an
). This bias voltage is
IN
will be discharged
CC
CLOCK
.
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= 100 kHz,
bias gen-
IN
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. When
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) is