REV. 0

AD8381

–11–

TRANSFER FUNCTION

The AD8381 has two regions of operation, selected by the INV

input, where the video output voltages are either above or below

a reference voltage, applied externally at the VMID input.

The transfer function defines the analog output voltage as the

function of the digital input code as follows:

VOUT

VMID VFS

n

=±

× 







1

1023

–

where:

n = input code

VFS = 2

× (VREFHI – VREFLO)

1023

INPUT CODE

AVCC

VMID

VOUT (V)

0

(VMID + VFS)

(VMID – VFS)

INV = HIGH

INV = LOW

VOUTP(n)

VOUTN(n)

AGND

Figure 5. Transfer Function

The region over which the output voltage varies with input code

is selected by the INV input. When INV is LOW, the output

voltage increases from (VMID – VFS), (where VFS = the full-

scale output voltage), to VMID as the input code increases from

0 to 1023. When INV is HIGH, the output voltage decreases

from (VMID + VFS) to VMID with increasing input code.

For each value of input code there are then two possible values

of output voltage. When INV is LOW, the output is defined as

VOUTP(n) where n is the input code and P indicates the oper-

ating region where the slope of the transfer function is positive.

When INV is HIGH, the output is defined as VOUTN(n) where n

indicates the operating region where the slope of the transfer

function is negative.

ACCURACY

To best correlate transfer function errors to image artifacts, the

overall accuracy of the AD8381 is defined by two parameters,

VDE and VCME.

VDE, the differential error voltage, measures the deviation of the

rms value of the output from the rms value of the ideal. It is depen-

dent on the difference between the output amplitudes VOUTN(n)

and VOUTP(n) at a particular code. The defining expression is:

VDE

VOUTN n

VOUTP n

VFS

n

=×

()

× 















1

2

1

1023

() –

( ) –

–

where:

1

2

×

()

VOUTN n

VOUTP n

() –

(VFS

× (1 – n/1023)) is the rms value of the ideal.

VCME, the common-mode error voltage, measures the devia-

tion of the average value of the output from the average value of

the ideal. It is dependent on the average between the output

amplitudes VOUTN(n) and VOUTP(n) at a particular code.

The defining expression is:

VCME

VOUTN n

VOUTP n

VMID

=×

×

+

()









1

2

1

2

()

() –

where:

1

2

×+

()

VOUTN n

VOUTP n

()

VMID is the average value of the ideal.

MAXIMUM FULL-SCALE OUTPUT VOLTAGE

The following conditions limit the range of usable output voltages:

•The internal DACs limit the minimum allowed voltage at the

VMID input to 5.3 V.

•The scale factor control loop limits the maximum full-scale

output voltage to 5.75 V.

•The output amplifiers settle cleanly at voltages within 1.3 V

from the supply rails.

•The common-mode range of the output amplifiers limit the

maximum value of VMID to AVCC – 3.

At any given valid value of VMID, the voltage required to reach

any one of the above limits defines the maximum usable full-

scale output voltage VFSMAX.

VFSMAX is the envelope in Figure 6. The valid range of VMID

is the shaded area.

AVCC

AVCC/2

4.3

VFS (V)

5.75

AVCC/2–1.3

2

VALID VMID

0

5.3

7

AVCC–7

AVCC–3

AVCC/2

VMID (V)

Figure 6. VFSMAX vs. VMID