LM9040M National Semiconductor, LM9040M Datasheet - Page 5

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

Differential Input Circuit
The differential input impedance is a function of the value of
the input capacitor array and the sampling frequency The
capacitor C
the Differential Input impedance (Z
similar to the Lambda Sensor output voltage at the stoichio-
metric air-fuel mixture (
ratio of C
The resulting bias voltage across the Differential Input is
defined as
With C
100 kHz and V
In effect the result is the same as forcing a bias current
through the Differential Input impedance
The bias current is defined as
The Differential Input impedance is defined as
This bias voltage will be developed across the Differential
Input impedance (Z
from the non-inverting input pin for I
input has a current path to ground See Figure 5 During
normal operating conditions I
fect on accuracy
BIAS
FIGURE 5 Equivalent Input Bias Circuit
IN
BIAS
V
and C
e
Z
BIAS
I
DIFF
BIAS
CC
0 7286 pF C
is used to generate a bias voltage across
V
BIAS
e
e
e
BIAS
DIFF
e
Z
(7 4213E-12
I
V
BIAS
DIFF
5V
(C
BIAS
V
and the value of V
) if there is no other path available
CC
IN
e
e
e
e
a
(C
1) The bias voltage is set by the
5 7 286E-13
e
V
C
364 3 nA
1 227 M
IN
BIAS
CC
C
IN
447 mV
BIAS
BIAS
a
e
1
a
DIFF
C
C
will have a negligible ef-
) F
BIAS
BIAS
7 286E-13)
BIAS
7 421 pF F
F
) This bias voltage is
CLOCK
CLOCK
)
(Continued)
CC
and the inverting
TL H 12372 – 15
CLOCK
e
5
Differential Input Filtering
Since each input is sampled independently an anti-aliasing
filter is required at the amplifier inputs to ensure that the
input signal does not exceed the Nyquist frequency
This external low-pass filter is implemented by adding a ca-
pacitor (C
This forms an RC network across the differential inputs in
conjunction with the required external 4 k
the differential input impedance (Z
lected should be small enough to have minimal effect on
gain accuracy in the application yet large enough to filter
out unwanted noise Given that the F
typically 500 Hz the use of a 0 01 F capacitor will general-
ly provide adequate filtering with less than
attenuation at 500 Hz and approximately
A larger value capacitor can be used if needed but a value
larger than typically 0 02 F will begin to dominate the cut-
off frequency of the application This capacitor must be a
low leakage and low ESR type so that circuit performance is
not degraded
Common Mode Filtering
The differential input sampling of the LM9040 actually re-
duces the effects of common mode input noise at low fre-
quencies The time interval between the sampling of the
inverting input and the non-inverting input is one half of a
clock period A change in the common mode voltage during
this short time interval can cause an error in the charge
stored on C
voltage For a sine-wave common mode voltage the mini-
mum common mode rejection is
Where F
and F
FIGURE 6 Differential and Common Mode Filtering
CLOCK
CMRR
CMR
DIFF
IN
is the clock frequency
is the frequency of the common mode signal
) across the differential input See Figure 6
e
This will result in an error seen on the output
2
F
CMR
(0 5 F
DIFF
CLOCK
C
) The capacitor se-
b
of the LM9040 is
b
28 dB at 50 kHz
0 4 dB of input
) 4 53
resistors and
TL H 12372– 16