AD9020

–7–

REV. C

The Maximum Conversion Rate is defined as the encode rate at

which the SNR for the lowest analog signal test frequency tested

drops by no more than 3 dB below the guaranteed limit.

Imaging

Both visible and infrared imaging systems require similar char-

acteristics from ADCs. The signal input (from a CCD camera,

or multiplexer) is a time division multiplexed signal consisting of

a series of pulses whose amplitude varies in direct proportion to

the intensity of the radiation detected at the sensor. These vary-

ing levels are then digitized by applying encode commands at

the correct times, as shown in Figure 2.

AD9020

ENCODE

+FS

–FS

Figure 2. Imaging Application Using AD9020

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 AD9020.

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 dynamic performance is not impor-

tant. 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 AD9020 to achieve

full accuracy when a step function is applied. Overvoltage

Recovery Time is the time required for the AD9020 to recover to

full accuracy 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 AD9020 provides sufficient resolution for these demanding

applications. Conversion speed, dynamic performance and ana-

log bandwidth are suitable for digitizing both composite and

RGB video sources.

USING THE AD9020

Voltage References

The AD9020 requires that the user provide two voltage

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

to these quarter points on the resistor ladder wil 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.) Figure 3 is not intended to show the

transfer function of the ADC, but illustrates how the linearity of

the device is affected by reference voltages applied to the ladder.

0000000000

TAPS

DRIVEN

TAPS

FLOATING

IDEAL

LINEARITY

(NOT TO SCALE)

0100000000

1000000000

1100000000

1111111111

Figure 3. 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

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.