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AD8381JSTZ Datasheet(PDF) 11 Page - Analog Devices |
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AD8381JSTZ Datasheet(HTML) 11 Page - Analog Devices |
11 / 16 page REV. B 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 =± ¥ Ê ËÁ ˆ ¯˜ 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 volt- age 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 =¥ () ¥ Ê ËÁ ˆ ¯˜ Ê ËÁ ˆ ¯˜ 1 2 1 1023 () – ( ) – – where: 1 2 ¥ () VOUTN n VOUTP n () – ( ) is the rms value of the output. (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 =¥ ¥ + () Ê ËÁ ˆ ¯˜ 1 2 1 2 () () – where: 1 2 ¥+ () VOUTN n VOUTP n () () is the average value of the output. 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 |
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