AD9020

–7–

REV. A

Digitizing Oscilloscopes

Oscilloscopes provide amplitude information about an observed

waveform with respect to time. Digitizing oscilloscopes must ac-

curately sample this signal, without distorting the information to

be displayed.

One figure of merit for the ADC in these applications is Effective

Number of Bits (ENOBs). ENOB is calculated with a sine wave

curve fit and equals:

N is the resolution (number of bits) of the ADC. The measured

error is the actual rms error calculated from the converter out-

puts with a pure sine wave input.

The Analog Bandwidth of the converter is the analog input fre-

quency at which the spectral power of the fundamental signal is

reduced 3 dB from its low frequency value. The analog band-

width is a good indicator of a converter’s stewing capabilities.

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

Visible and infrared imaging systems both 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 below.

ENCODE

AD9020

FS

+

FS

–

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 im-

portant. The Transient Response and Overvoltage Recovery Time

specifications insure 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. Overvolt-

age Recovery Time is the time required for the AD9020 to re-

cover 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 re-

quires 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.