LM9040MX National Semiconductor, LM9040MX Datasheet - Page 7

• Manufacturer Part Number: LM9040MX
• Description: IC AMP DUAL SENSR INTERFC 14SOIC
• Manufacturer: National Semiconductor
• Type: Sensor Interfacer

Specifications of LM9040MX

• 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: *LM9040MX

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 kΩ resistors on each input pin are in series with the
differential input impedance. Together they form a voltage di-
vider 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
Cold Sensor
Typically, a Lambda sensor will have an impedance of less
than 10 kΩ when operating at temperatures between 300°C,
INT
, and by F
FIGURE 8. Equivalent Gain and Filter Circuit
FIGURE 7. Simplified Gain and Filter Circuit
CLOCK
:
12372 Version 3 Revision 2
IN
and C
FB
:
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FB
and
Print Date/Time: 2009/12/02 10:13:43
7
and 500°C. When a Lambda sensor is not at operating tem-
perature, its impedance can be more than 10 MegΩ. Any
voltage signal that may be developed is seriously attenuated.
During this high impedance condition the LM9040 will provide
a default output voltage.
Each amplifier input has a bias charge applied across the Dif-
ferential Input Impedance (Z
tribution through the switched capacitor network. This bias
charge is a ratio of V
of 5.00V. This will provide an output voltage of typically
2.025V.
While the Lambda sensor is high impedance, the 447 mV
across Z
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 kΩ of
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 be-
come 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:
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
5V.
For the case of an open connection of the non-inverting input,
the device would react the same as for the Cold Sensor con-
dition. The internal bias voltage across Z
output voltage to be at a value defined by V
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 condition. See
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
FIGURE 9. V
DIFF
will be the dominant input signal. As the Lambda
V
OUT
CC
ERROR
OUT
, and is typically 447 mV for a V
that is typically 2.025V when V
with Cold Lambda Sensor
= I
DIFF
LOAD
CC
.
) by means of charge redis-
• 2.5 kΩ
CC
IN
CC
should either, or
would cause the
and the LM9040
Figure
www.national.com
OUT
CC
1237219
CC
value
is not
9.
is at