AD8387

Rev. 0 | Page 13 of 16

THEORY OF OPERATION

TRANSFER FUNCTION AND ANALOG OUTPUT

VOLTAGE

The DecDriver has two regions of operation where the video

output voltages are either above or below the reference voltage

VRL. The transfer function defines the video output voltage as

the function of the digital input code as:

VOUTN(n) = VIDx(n) = VRL + VFS × (1 − n/4095),

for INV = HIGH

VOUTP(n) = VIDx(n) = VRL − VFS × (1 − n/4095),

for INV = LOW

where n is the input code.

VFS = 2 × (VRH − VRL)

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

output voltages, VIDx, as shown in Figure 17.

VIDx – VOLTS

AVCC

(VRL + VFS)

VRL

(VRL – VFS)

AGND

0

VIDx vs. INPUT CODE

INPUT CODE

VOUTP

VOUTN

4095

≥1.3V

0

≤ VFS

≤ 5.25V

0

≤ VFS ≤ 5.25V

≥1.3V

5.25V

≤ VRL

≤ (AVCC – 4)

11V

≤ AVCC

≤ 18V

INTERNAL LIMITS AND

USABLE VOLTAGE RANGES

Figure 17. AD8387 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 three

parameters, VDE , VCME, and ΔVDE.

VDE, the differential error voltage, measures the difference

between the rms value of a channel and the ideal rms value of

that channel. The defining expression is

VOUTN(n) VOUTP(n)

n

VDE(n)

1

VFS

2409

⎡⎤

−

⎛⎞

⎣⎦

=−

−

⎜⎟

⎝⎠

5

×

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

bias of a channel. The defining expression is

⎥

⎦

⎤

⎢

⎣

⎡

−

+

=

VRL

n

VOUTP

n

VOUTN

n

VCME

2

)

(

)

(

2

1

)

(

ΔVDE measures the maximum VDE mismatch between

channels. The defining equation is

ΔV measures the maximum mismatch between channels. The

defining expression is

ΔV(n) = max{ΔVN(n), ΔVP(n)}

where: