AD8385 datasheet(18/24 Pages) AD

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AD8385

Rev. 0 | Page 18 of 24

THEORY OF OPERATION

TRANSFER FUNCTION AND ANALOG OUTPUT

VOLTAGE

The DECDRIVER has two regions of operation where the video

output voltages are either above reference voltage V2 or below

reference voltage V1. The transfer function defines the video

output voltage as the function of the digital input code:

VIDx(n) = V2 + VFS × (1 – n/1023), for INV = high

VIDx(n) = V1 – VFS × (1 – n/1023), for INV = low

where:

n = input code

VFS = 2 × (VRH – VRL)

A number of internal limits define the usable range of the video

output voltages, VIDx. See Figure 15.

INPUT CODE

AGND

V1 – VFS

V2 + VFS

AVCC

1023

≤ VFS ≤ 5.5V

≤ V2 ≤ (AVCC – 4)

≤ V1

≤ (AVCC – 4)

≤ AVCC ≤ 18V

≥ 1.3V

INV = HIGH

INV = LOW

INTERNAL LIMITS AND

USABLE VOLTAGE RANGES

VOUTN(n)

VOUTP(n)

Figure 15. Transfer Function and Usable Voltage Ranges

ACCURACY

To best correlate transfer function errors to image artifacts, the

overall accuracy of the DECDRIVER is defined by two

parameters, VDE and VCME.

VDE, the differential error voltage, measures the difference

between the rms value of the output and the rms value of the

ideal. The defining expression is

VFS

VOUTP

VOUTN

VDE

⎟

⎠

⎞

⎜

⎝

⎛

1023

–

]

–

)

(

[

–

]

–

)

(

[

)

(

VCME, the common-mode error voltage, measures ½ the dc

bias of the output. The defining expression is

()

⎥⎦

⎤

⎢⎣

⎡

–

)

(

)

(

)

(

VOUTP

VOUTN

VCME

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