Manual Calibration
You can manually program the gain of the two mic inputs on
the LMV1088 using the I
2
C interface. Table 5 shows the con-
trol bits for I
2
C Register O and P with the corresponding gains.
This can be easily done by doing the following:
1) READ contents of the I
2
C register N immediately after
powering up.
2) Set PE pin and T7 pin to Vdd.
3) WRITE to I2C register O and P to choose the calibration
settings.
Bits O<7:4> control the two mics at 300 Hz and bits
O<3:0> control the two mics at 3kHz.
Bits P<7:4> control the Mic2 gain and bits P<3:0> control
the Mic1 gain
4) WRITE a ‘0’ to I2C register Q<7> bit (storeBar) and the
bits from I2C register N<6:0> to I2C register Q<6:0>
5) When I2C register N<7> (ready) goes high, then the
EEPROM programming is complete. Now PE pin and T7 pin
should be set to GND and I2C register Q<7> (storeBar)
should be returned to ‘1’.
Low-Pass Filter At The Output
At the output of the LMV1088 there is a provision to create a
1
st
order low-pass filter (only enabled in 'Noise Cancelling'
mode). This low-pass filter can be used to compensate for the
change in frequency response that results from the noise
cancellation process.. The change in frequency response re-
sembles a first-order high-pass filter, and for many of the
applications it can be approximately compensated by a first-
order low-pass filter with cutoff frequency between 1.5kHz
and 2.5kHz.
The transfer function of the low pass filter is derived as:
H(s) = Post Amplifier / SRfCf + 1
This low-pass filter is created by connecting a capacitor be-
tween the LPF pin and the OUT pin of the LMV1088. The
value of this capacitor also depends on the selected output
gain. For different gains the feedback resistance in the Low-
pass Filter network changes as shown in Table 6.
TABLE 6. Low-pass Filter internal impedance
Post Amplifier Gain
Setting (dB) (Note 15)
Feedback resistance R
if
(kΩ)
0 20
3 29
6 40
9 57
12 80
This will result in the following values for a cutoff frequency of
2000 Hz:
TABLE 7. Low—pass Filter Capacitor for 2kHz
Post Amplifier Gain Setting (dB)
(Note 15)
R
if
(kΩ)
C
f
(nF)
0 20 3.9
3 29 2.7
6 40 2.0
9 57 1.3
12 80 1.0
Note 15: Noise Cancelling Mode
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LMV1088
