lm9040 National Semiconductor Corporation, lm9040 Datasheet - Page 7

• Manufacturer Part Number: lm9040
• Description: Dual Lambda Sensor Interface Amplifier
• Manufacturer: National Semiconductor Corporation

Gain and Filter Stage
The signal gain and filter stage is designed to have a DC
gain of 4.53 V/V, with a cut-off frequency of typically 500 Hz.
The external 4 k
with the differential input impedance. Together they form a
voltage divider circuit across the input such that the net DC
gain of the application circuit is 4.50 V/V.
The internal pin is set by the ratio of C
The corner frequency (−3 dB) is set by the ratio of C
C
INT
, and by F
FIGURE 8. Equivalent Gain and Filter Circuit
FIGURE 7. Simplified Gain and Filter Circuit
CLOCK
resistors on each input pin are in series
:
IN
and C
FB
:
01237217
01237218
FB
and
7
Cold Sensor
Typically, a Lambda sensor will have an impedance of less
than 10 k when operating at temperatures between 300˚C,
and 500˚C. When a Lambda sensor is not at operating
temperature, its impedance can be more than 10 Meg . Any
voltage signal that may be developed is seriously attenu-
ated. 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
Lambda 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 capacitor,
form a RC low pass filter to remove any clock noise from the
output signal. An external output filter capacitor value of
0.01 µF is recommended. Additionally, the output 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
measured output voltage. This error will be equal to the I • R
drop across the output resistance:
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
Sensor condition. The internal bias voltage across Z
cause the output voltage to be at a value defined by V
the LM9040 DC gain. The inverting input would still be
CC
value of 5.00V. This will provide an output voltage of
FIGURE 9. V
CC
DIFF
is at 5V.
will be the dominant input signal. As the
V
ERROR
OUT
with Cold Lambda Sensor
= I
CC
LOAD
, and is typically 447 mV for a
CC
OUT
DIFF
• 2.5 k
.
) by means of charge
that is typically 2.025V
CC
should either, or
www.national.com
IN
01237219
CC
would
and
of