AD9480BSUZ-250 Analog Devices Inc, AD9480BSUZ-250 Datasheet - Page 16

• Manufacturer Part Number: AD9480BSUZ-250
• Description: IC,A/D CONVERTER,SINGLE,8-BIT,BICMOS,TQFP,44PIN
• Manufacturer: Analog Devices Inc

Specifications of AD9480BSUZ-250

• Number Of Bits: 8
• Sampling Rate (per Second): 250M
• Data Interface: Parallel
• Number Of Converters: 1
• Power Dissipation (max): 590mW
• Voltage Supply Source: Single Supply
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 44-TQFP, 44-VQFP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

AD9480
APPLICATION NOTES
The AD9480 uses a 1.5-bit per stage architecture. The analog
inputs drive an integrated high bandwidth track-and-hold
circuit that samples the signal prior to quantization by the 8-bit
core. For ease of use, the part includes an on-board reference
and input logic that accepts TTL, CMOS, or LVPECL levels.
The digital output logic levels are LVDS (ANSI 644 compatible).
CLOCKING THE AD9480
Any high speed ADC is extremely sensitive to the quality of the
sampling clock provided by the user. A track-and-hold circuit is
essentially a mixer, and any noise, distortion, or timing jitter on
the clock is combined with the desired signal at the A/D output.
Considerable care has been taken in the design of the CLOCK
input of the AD9480, and the user is advised to give
commensurate thought to the clock source.
The AD9480 has an internal clock duty-cycle stabilization
circuit that locks to the rising edge of CLOCK and optimizes
timing internally for sample rates between 100 MSPS and
250 MSPS. This allows for a wide range of input duty cycles at
the input without degrading performance. Jitter on the rising
edge of the input is still of paramount concern and is not
reduced by the internal stabilization circuit. The duty-cycle
control loop does not function for clock rates less than 70 MHz
nominally. The loop is associated with a time constant that
needs to be considered in applications where the clock rate can
change dynamically, requiring a wait time of 5 µs after a
dynamic clock frequency increase before valid data is available.
The clock duty-cycle stabilizer can be disabled at Pin 28 (S1).
The clock inputs are internally biased to 1.5 V (nominal) and
support either differential or single-ended signals. For best
dynamic performance, a differential signal is recommended. An
MC100LVEL16 performs well in the circuit to drive the clock
inputs (ac coupling is optional). If the clock buffer is greater
than 2 inches from the ADC, a standard LVPECL termination
may be required instead of the simple pull-down termination,
as shown in Figure 30.
Table 8. S1 Voltage Levels
S1 Voltage
0.9 × AVDD −> AVDD
2/3 AVDD ± (0.1 × AVDD)
1/3 AVDD ± (0.1 × AVDD)
AGND −> (0.1 × AVDD)
PECL
GATE
510Ω
Figure 30. Clocking the AD9480
510Ω
0.1µF
0.1µF
CLK+
CLK–
AD9480
Data Format
Offset binary
Offset binary
Twos complement
Twos complement
Rev. A | Page 16 of 28
ANALOG INPUTS
The analog input to the AD9480 is a differential buffer. For best
dynamic performance, impedances at VIN+ and VIN− should
match. Optimal performance is obtained when the analog
inputs are driven differentially. SNR and SINAD performance
can degrade if the analog input is driven with a single-ended
signal; however, performance can be adequate for some
applications (see Figure 6). The analog inputs self-bias to
approximately 1.9 V; this common-mode voltage can be
externally overdriven by approximately ±300 mV if required.
A wideband transformer, such as the Mini-Circuits® ADT1-1WT,
can provide the differential analog inputs for applications that
require a single-ended-to-differential conversion. Note that the
filter and center-tap capacitor on the secondary side is optional
and dependent on application requirements. An RC filter at
the secondary side helps reduce any wideband noise aliased
by the ADC.
For dc-coupled applications, the AD8138/AD8139 or AD8351
can serve as a convenient ADC driver, depending on
requirements. Figure 32 shows an example with the AD8138.
The AD9480 PCB has an optional AD8351 on board, as shown
in Figure 41 and Figure 42. The AD8351 typically yields better
performance for frequencies greater than 30 MHz to 40 MHz.
0.1µF
1.3kΩ
2kΩ
49.9Ω
49.9Ω
Figure 31. Driving the ADC with an RF Transformer
Figure 32. Driving the ADC with the AD8138
Duty-Cycle Stabilizer
Disabled
Enabled
Enabled
Disabled
0.1µF
499Ω
523Ω
(R, C OPTIONAL)
499Ω
499Ω
AD8138
33Ω
33Ω
10pF
33Ω
33Ω
20pF
VIN+
VIN–
AD9480
AGND
AVDD
VIN+
VIN–
AD9480
AGND
AVDD