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.
202130r3
FIGURE 8. Three Tone Calibration Timing
TABLE 5. Automatic Calibration Timing Parameters
Symbol Parameter
Limits
Unitis
Min Max
t
ST1
Calibration Start Tone 1 10 ms
t
ET1
Calibration End Tone 1 200 ms
t
ST2
Calibration Start Tone 2 215 ms
t
ET2
Calibration End Tone 2 400 ms
t
ST3
Calibration Start Tone 3 415 ms
t
ET3
Calibration End Tone 3 600 ms
t
CC
Calibration Complete 770 ms
NOTE: Data guaranteed by design
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LMV1088
