LMV1088_08 Datasheet, PDF (18/26 Pages) National Semiconductor (TI) – Dual Input, Far Field Noise Suppression Microphone Amplifier with Automatic Calibration Ability

PERFORMING A THREE TONE CALIBRATION
In a system with two microphones in an enclosure there will
always be a difference in the transfer function in both gain and
frequency response. The LMV1088 has the capability to per-
form an automatic calibration function to minimize these dif-
ferences. To perform this calibration, a test sequence of three
tones is required right after the PE and CAL inputs are brought
to a logic high level. At the end of this sequence the calibration
data is automatically stored in the internal EEPROM.
The three tones have to be applied as follows:
• A first tone with a frequency of 1kHz
• A second tone with a frequency of 300Hz
• A third tone with a frequency of 3kHz
A tone may be applied prior to the rising of CAL and PE. Sig-
nals applied to the microphone inputs before rising of CAL
and PE are ignored by the calibration system. .
Between each tone pair there is a small time, indicated by a
cross, to change the frequency. During that time the input tone
is ignored by the calibration system.
The total calibration sequence requires less then 770ms.
FIGURE 8. Three Tone Calibration Timing
TABLE 5. Automatic Calibration Timing Parameters
Symbol
Parameter
t_ST1
Calibration Start Tone 1
t_ET1
Calibration End Tone 1
t_ST2
Calibration Start Tone 2
t_ET2
Calibration End Tone 2
t_ST3
Calibration Start Tone 3
t_ET3
Calibration End Tone 3
t_CC
Calibration Complete
NOTE: Data guaranteed by design
Limits
Min
200
400
600
770
Max
10
215
415
202130r3
Unitis
ms
ms
ms
ms
ms
ms
ms
www.national.com
18