AD9020
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.
USING THE AD9020
Voltage References
The AD9020 requires that the user 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, 3/4REF) are available. Providing a reference
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.
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.
+FS
AD9020
A
IN
–FS
1111111111
(NOT TO SCALE)
TAPS
DRIVEN
ENCODE
Figure 2. Imaging Application Using AD9020
1100000000
TAPS
FLOATING
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.
1000000000
0100000000
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.
IDEAL
LINEARITY
0000000000
–V
1/4
1/2
3/4
+V
SENSE
SENSE
REF
REF
REF
V
IN
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.
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
voltage sense lines, +VSENSE and –VSENSE, to adjust the reference
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.
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.
REV. C
–7–
ADI [ ADI ]
