LM9811 datasheet(23/36 Pages) NSC

23 / 36 page

Applications Information (Continued)

requirements of the application and is available in a TO-92

(LM4041EIZ-1.2) package as well as a SOT-23 package

(LM4041EIM3-1.2) to minimize board space.

Due to the transient currents generated by the LM9811’s

ADC, PGA, and CDS circuitry, the REF IN pins, the REF

OUT

MID pin and the REF OUTHI pin should all be bypassed

to AGND with 0.1 µF monolithic capacitors.

4.2 Clamp Capacitor Section

This section is very long because it is relatively complicated

to explain, but the answer is short and simple: A clamp ca-

pacitor value of 0.01 µF should work in almost all applica-

tions. The rest of this section describes exactly how this

value is selected.

The output signal of many CCDs rides on a large DC offset

(typically 8V to 10V) which is incompatible with the LM9811’s

5V operation. To eliminate this offset without resorting to ad-

ditional higher voltage components, the output of the CCD is

AC coupled to the LM9811 through a DC blocking capacitor,

CLAMP (the CCD’s DOS output is not used). The value of

this capacitor is determined by the leakage current of the

LM9811’s OS input and the output impedance of the CCD.

The leakage through the OS input determines how quickly

the capacitor value will drift from the clamp value of REF

OUT

MID, which then determines how many pixels can be

processed before the droop causes errors in the conversion

(±0.1V is the recommended limit). The output impedance of

the CCD determines how quickly the capacitor can be

charged to the clamp value during the black reference period

at the beginning of every line.

The minimum clamp capacitor value is determined by the

maximum droop the LM9811 can tolerate while converting

one CCD line. The following equation takes the maximum

leakage current into the OS input, the maximum allowable

droop (100 mV), the number of pixels on the CCD, and the

pixel conversion rate (f

MCLK/8) and provides the minimum

clamp capacitor value:

For example, if the OS input leakage current is 20 nA worst-

case, the CCD has 2700 active pixels, the conversion rate is

1.5MHz (f

MCLK = 12MHz), and the max droop desired is

0.1V, the minimum clamp capacitor value is:

The maximum size of the clamp capacitor is determined by

the amount of time available to charge it to the desired value

during the optical black portion of the CCD output. The inter-

nal clamp is on for each pixel from the rising edge of the S/H

ref pulse to the falling edge of the S/H signal pulse (see

Fig-

ures 7, 8). This time can be calculated using the values

stored in the Sample Signal and Sample Reference configu-

ration registers and the MCLK frequency. For normal CCDs:

And for even/odd CCDs:

Where SS is the value in the Sample Signal Position register

(0–15), SR is the value in the Sample Reference Position

MCLK is the MCLK frequency, and tDARK is

the amount of time (per pixel) that the clamp is on.

The following equation takes the number of optical black pix-

els, the amount of time (per pixel) that the clamp is closed,

the CCD’s output impedance, and the desired accuracy of

the final clamp voltage and provides the maximum clamp ca-

pacitor value that allows the clamp capacitor to settle to the

desired accuracy within a single line:

Where n = the number of optical black pixels, t

DARK is the

amount of time (per pixel) that the clamp is on, R

OUT is the

output impedance of the CCD, and accuracy is the ratio of

the worst-case initial capacitor voltage to the desired final

capacitor voltage. For example, if a CCD has 18 black refer-

ence pixels, the output impedance of the CCD is 1500

Ω, the

LM9811 is configured to clamp for 500ns, the worst case ini-

tial voltage across the capacitor is 10V, and the desired volt-

age after clamping is 0.1V (accuracy = 10/0.1 = 100), then:

DS012813-29

FIGURE 21. Voltage Reference Generation

DS012813-30

FIGURE 22. OS Clamp Capacitor and Internal Clamp

LM9811 датащи(PDF) 23 Page - National Semiconductor (TI)

LM9811 датащи(HTML) 23 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

датащи скачать

ссылки URL