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參數(shù)資料
| 型號： | FC15A140CC |
| 元件分類： | 模擬信號調(diào)理 |
| 英文描述： | SPECIALTY ANALOG CIRCUIT, CDIP20 |
| 封裝： | CERAMIC, DIP-20 |
| 文件頁數(shù)： | 13/16頁 |
| 文件大小： | 382K |
| 代理商： | FC15A140CC |
FC15A140
6/16
FUNCTIONAL DESCRIPTION
line select
AVIDEO
sensor array
column
amplifier
& mux
column
select
1
2
3
4
5
digital
analog
GAIN
OPA_REF
The circuit is divided into two main sections: digital and analog.
The digital section selects in the sensor array one particular
line (1), then each pixel of the selected line sends its electrical
information to column amplifiers (2) (one per column), then the
digital section selects one column (3) so that a particular pixel
sends its information (4) to the output AVIDEO of the circuit
through a final amplifier (5).
Sensor
Each pixel is a sensor in itself. The sensor detects a tempera-
ture differential between the beginning of acquisition and the
reading of information: this is the integration time.
The integration time begins with a reset of the pixel to a prede-
fined initial state. Note that the integration time reset has noth-
ing to do with the reset of the digital section.
Then, at a rate depending on the sensitivity of the pyroelectric
layer, on the temperature variation between the reset and the
end of the integration time, and on the duration of the integra-
tion time, electrical charges are generated at the pixel level.
The worst case occurs when the temperature differential is
null, that is at the temperature of the finger. This situation is
rather unusual as generally, worst cases happen at very high
or very low temperatures.
Analog section
Once the digital section has selected one particular pixel in the
array, the signal generated by the sensing element is amplified
by the column amplifier, followed by another amplifier that
drives the output pin.
The last amplifier has a selectable gain to enable adjustment
of the output signal level, using one pin (GAIN), so it’s possible
to divide the gain by a factor of 2 when GAIN is high. Nominal
gain is when GAIN is low.
Offset
OFFSET is set between 2.4V and 2.7V by external adjuste-
ment. Nominal value is 2.5v. A 10nF capacitance must be
added to reduce noise.This is useful to fit the signal to the Ana-
log–to–Digital Converter.
Sensitivity
TCS is currently defining a methodology to characterize the
FingerChip
T in term of sensitivity. A special measurement is
needed, as it is obviously not possible to use a living reference
finger, and as we are using thermal information, a correlation
has to be done between the quality of fingerprint (i.e. mainly the
contrast between ridges and valleys) and the measurement.
The ultimate value would be the FAR (False Acceptance Rate)
and FRR (False Rejection Rate), but these values depend on
both the sensor and authentication software, so it is not pos-
sible to perform such a test with only the sensor.
From an industrial point of view, TCS has developed a test
method to insure that every pixel is electrically tested.
Connection to an Analog–to–Digital Converter
An Analog to Digital Converter (ADC) is used to convert the
AVIDEO signal into digital data that can be used by a proces-
sor.
The U.S. Federal Bureau of Investigation requires a 256–level
grey scale image; that is, an 8 bit per pixel resolution. As the
data rate is maximum 8 MHz, user will need at least an 8 MSPS
(Mega Sample Per Second), 8 bit ADC, with a conversion
range fitted to the analog signal delivered by the present
device.
FC15A140 is recommended for use with the TS83148 ADC,
another TCS product. The rising edge of the sample clock
(SCLK) is used to trigger the TS83148 without any supplemen-
tal circuitry. The video signal delivered by the FC15A140 has
been fitted to the dynamic range of the TS83148, so that the
user may directly connect the AVIDEO pin to the input pin of
the ADC. Default values for offsets have to be used. Note that
in the first application board, default values may be adjusted
if needed.
Caution: it is important not to introduce phase delay between
the AVIDEO signal and SCLK signal, as both paths must be
equivalent. If not, sampling will not occur at the right moment
and the resulting image will not be correct.
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