LM9040M National Semiconductor, LM9040M Datasheet - Page 7

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

Cold Sensor
Typically a Lambda sensor will have an impedance of less
than 10 k
and 500 C When a Lambda sensor is not at operating tem-
perature its impedance can be more than 10 Meg
voltage signal that may be developed is seriously attenuat-
ed During this high impedance condition the LM9040 will
provide a default output voltage
Each amplifier input has a bias charge applied across the
Differential Input impedance (Z
redistribution through the switched capacitor network This
bias charge is a ratio of V
V
typically 2 025V
While the Lambda sensor is high impedance the 447 mV
across Z
da sensor is heated and the sensor impedance begins to
drop the voltage signal from the sensor will become the
dominate signal
Output Resistance
With normal operation each output has typically 2 5 k
resistance This resistance along with an external capaci-
tor form a RC low pass filter to remove any clock noise
from the output signal An external output filter capacitor
value of 0 01
resistance will provide current limiting for the output stage
should it become shorted to Ground or V
Any DC loading of the output will cause an error in the mea-
sured output voltage This error will be equal to the I R drop
across the output resistance
CC
value of 5 00V This will provide an output voltage of
FIGURE 9 V
DIFF
when operating at temperatures between 300 C
will be the dominant input signal As the Lamb-
F is recommended Additionally the output
V
ERROR
OUT
with Cold Lambda Sensor
e
CC
I
LOAD
and is typically 447 mV for a
DIFF
2 5 k
) by means of charge
CC
TL H 12372 – 19
Any
of
7
Open Input Pins Defaults
In any remote sensor application it is desirable to be able to
deal with the possibility of open connections between the
sensor and the control module The LM9040 is capable of
providing an output voltage scaled to V
both of the wires to the Lambda sensor open The two
inputs handle the open circuit condition differently The
LM9040 will provide a default V
when V
For the case of an open connection of the non-inverting
input the device would react the same as for the Cold Sen-
sor condition The internal bias voltage across Z
cause the output voltage to be at a value defined by V
and the LM9040 DC gain The inverting input would still be
connected to the Lambda sensor ground so any common
mode signals would still need to be allowed for in this condi-
tion See Figure 9
For the case of an open connection of the inverting input
the device output stage switches from the amplifier output
to a resistive voltage divider In this case the default V
is not dependent on the gain stage and any signal on the
non-inverting input will have no effect on the output Each
amplifier has a comparator to monitor the voltage on the
inverting input pin When the voltage on an inverting pin
goes above typically 2 5V the comparator will switch the
output from the amplifier output to the voltage divider stage
To fully implement this function requires external pull-up re-
sistors for each of the inverting inputs To minimize signal
errors due to DC currents through the 4 k
pull-up resistors need to be added in the application circuit
between the 4 k
Lambda sensor ground point A typical pull-up value of
51 k
effective resistance of the output stage will be 3 5 k
cally See Figure 10
FIGURE 10 V
to V
CC
is at 5V
CC
is recommended During this condition the
input resistor and the connection to the
OUT
with Open Inverting Input
OUT
that is typically 2 025V
CC
should either or
resistors the
TL H 12372– 20
IN
would
typi-
OUT
CC