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AD9060 Datasheet(PDF) 7 Page - Analog Devices |
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AD9060 Datasheet(HTML) 7 Page - Analog Devices |
7 / 12 page REV. B AD9060 –7– AD9060 AIN ENCODE –FS +FS Figure 1. Imaging Application Using AD9060 The actual resolution of the converter is limited by the thermal and quantization noise of the ADC. The low frequency test for SNR or ENOB is a good measure of the noise of the AD9060. At this frequency, the static errors in the ADC determine the useful dynamic range of the ADC. Although the signal being sampled does not have a significant slew rate, this does not imply that dynamic performance is not important. The Transient Response and Overvoltage Recovery Time specifications ensure that the ADC can track full-scale changes in the analog input sufficiently fast to capture a valid sample. Transient Response is the time required for the AD9060 to achieve full accuracy when a step function is applied. Overvoltage Recovery Time is the time required for the AD9060 to recover to full accu- racy after an analog input signal 150% of full scale is reduced to the full-scale range of the converter. Professional Video Digital Signal Processing (DSP) is now common in television production. Modern studios rely on digitized video to create state- of-the-art special effects. Video instrumentation also requires high resolution ADCs for studio quality measurement and frame storage. The AD9060 provides sufficient resolution for these demanding applications. Conversion speed, dynamic performance, and analog bandwidth are suitable for digitizing both composite and RGB video sources. USING THE AD9060 Voltage References The AD9060 requires the user to provide two voltage references: +VREF and –VREF. These two voltages are applied across an internal resistor ladder (nominally 37 Ω) and set the analog input voltage range of the converter. The voltage references should be driven from a stable, low impedance source. In addition to these two references, three evenly spaced taps on the resistor ladder (1/4REF, 1/2REF, and 3/4REF) are available. Providing a reference to these quarter points on the resistor ladder will improve the integral linearity of the converter and improve ac performance (ac and dc specifications are tested while driving the quarter points at the indicated levels). The figure below is not intended to show the transfer characteristic of the ADC but illustrates how the linearity of the device is affected by the reference voltages applied to the ladder. VIN 1111111111 –VSENSE 1100000000 1000000000 0100000000 0000000000 1/4REF 1/2REF 3/4REF +VSENSE (NOT TO SCALE) TAPS DRIVEN TAPS FLOATING IDEAL LINEARITY Figure 2. Effect of Reference Taps on Linearity Resistance between the reference connections and the taps of the first and last comparators causes offset errors. These errors, called “top and bottom of the ladder offsets,” can be nulled by using the voltage sense lines, +VSENSE and –VSENSE, to adjust the refer- ence voltages. Current through the sense lines should be limited to less than 100 µA. Excessive current drawn through the voltage sense lines will affect the accuracy of the sense line voltage. Figure 4 shows a reference circuit that nulls out the offset errors using two op amps and provides appropriate voltage references to the quarter-point taps. Feedback from the sense lines causes the op amps to compensate for the offset errors. The two transistors limit the amount of current drawn directly from the op amps; resistors at the base connections stabilize their operation. The 10 k Ω resistors (R1–R4) between the voltage sense lines form an external resistor ladder; the quarter point voltages are taken off this external ladder and buffered by an op amp. The actual values of resistors R1–R4 are not critical, but they should match well and be large enough ( ≥10 kΩ) to limit the amount of current drawn from the voltage sense lines. The select resistors (RS) shown in the schematic (each pair can be a potentiometer) are chosen to adjust the quarter-point voltage references but are not necessary if R1–R4 match within 0.05%. An alternative approach for defining the quarter-point references of the resistor ladder is to evaluate the integral linearity error of an individual device and adjust the voltage at the quarter-points to minimize this error. This may improve the low frequency ac performance of the converter. |
Similar Part No. - AD9060_15 |
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Similar Description - AD9060_15 |
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