LM9812 datasheet(32/37 Pages) NSC

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3.0 ANALOG INTERFACE

3.1 Voltage Reference

The VREF IN pin should be connected to a 2V±5% reference volt-

age. The LM4041-ADJ adjustable bandgap reference is recom-

mended for this application, as shown in Figure 6. The

inexpensive “D” grade meets all the requirements of the applica-

tion and is available in a TO-92 (LM4041DIZ-ADJ) package as

well as a SOT-23 package (LM4041DIM3-ADJ) to minimize board

space. To reduce noise, the VREF IN pin should be bypassed to

AGND with a 0.1µF monolithic capacitor.

The LM9812 generates three internal reference voltages for its

analog front end: VREF MID, VREF HI, and VREF LO. VREF MID is

equal to 0.49*VA, or 2.45V with a nominal 5V supply. VREF IN

drives a transconductance amplifier with two output currents, -

VREF IN/2 and +VREF IN/2. These currents go into the VREF HI and

VREF LO reference voltage buffers, which generate a nominal

3.45V (VREF HI = VREF MID + VREF IN/2), and 1.45V (VREF LO =

VREF MID - VREF IN/2). To minimize noise, the reference amplifiers

sense (IN) inputs are brought out to the chip for bypassing. Each

buffer’s output and input should be tied together and decoupled to

AGND through a 0.1µF monolithic capacitor.

3.2 Clamp Capacitor Selection

This section is very long because it is relatively complicated to

explain, but the answer is short and simple: A clamp capacitor

value of 0.01µF should work in almost all applications. The rest of

this section describes exactly how this value is selected.

The output signal of many sensors rides on a DC offset (greater

than 5V for many CCDs) which is incompatible with the LM9812’s

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

tional higher voltage components, the output of the sensor is AC

coupled to the LM9812 through a DC blocking capacitor, CCLAMP

(the CCD’s DOS output, if available, is not used). The value of

this capacitor is determined by the leakage current of the

LM9812’s OS input and the output impedance of the sensor. The

leakage through the OS input determines how quickly the capaci-

tor value will drift from the clamp value of VREF 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 sensor 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 maxi-

mum droop the LM9812 can tolerate while converting one sensor

line. The following equation takes the maximum leakage current

into the OS input, the maximum allowable droop (100mV), the

number of pixels on the sensor, and the pixel conversion rate

(fMCLK/12 for triple output sensors or fMCLK/8 for dual output sen-

sors) and provides the minimum clamp capacitor value:

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

the sensor has 2700 active pixels, the conversion rate is 2MHz

(fMCLK = 24MHz, triple output sensor), and the max droop desired

is 0.1V, the minimum clamp capacitor value is:

CDS

(Internal)

Diagram 5: x4 Preview Mode Timing

Hold Ref

Hold Signal

Ø1

(Even/Odd Mode)

Ø2

(Even/Odd Mode)

Ø1

(Standard Mode)

Ø2

(Standard Mode)

1.5 1.5

7.5

1.5

0.5

7.5

All lengths given in units of MCLK periods

pn+1

pn+2

pn+3

I=VREF IN/2R

Reference

Amplifiers

LM4041-ADJ

0.1µF

Figure 6: Voltage Reference Generation

LM9812

VREF IN

VREF LO OUT

VREF LO IN

VREF MID OUT

VREF MID IN

VREF HI OUT

VREF HI IN

0.1µF

10.1k

10.5k

4.7k

-I

DOS

SENSOR

CCLAMP

Figure 7: OS Clamp Capacitor and Internal Clamp

2.450V

LM9812

CLAMP MIN

---------dt

leakage current (A)

max droop(V)

---------------------------------------------------

number of pixels

conversion rate (Hz)

------------------------------------------------------

Equation 6: CCLAMP MIN Calculation

CLAMP MIN

20nA

0.1V

--------------

2700

2MHz

---------------

270pF

Equation 7: CCLAMP MIN Example

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