AD9049

REV. A

–9–

Overdrive of the Analog Input

Special care was taken in the design of the analog input section

of the AD9049 to prevent damage and corruption of data when

the input is overdriven. The nominal input range is +2.788 V to

3.812 V (1.024 V p-p centered at 3.3 V). Out-of-range com-

parators detect when the analog input signal is out of this range

and shut the T/H off. The digital outputs are locked at their

maximum or minimum value (i.e., all “0” or all “1”). This pre-

cludes the digital outputs from changing to an invalid value

when the analog input is out of range.

When the analog input signal returns to the nominal range, the

out-of-range comparators switch the T/H back to the active

mode and the device recovers in approximately 10 ns.

The input is protected to one volt outside the power supply

rails. For nominal power (+5 V and ground), the analog input

will not be damaged with signals from +6.0 V to –1.0 V.

Timing

The performance of the AD9049 is very insensitive to the duty

cycle of the clock. Pulse width variations of as much as

±10%

will cause no degradation in performance (see Figure 13, SNR

vs. Clock Pulse Width).

The AD9049 provides latched data outputs, with five pipeline

after the rising edge of the encode command (refer to the

AD9049 Timing Diagram). The length of the output data lines

and loads placed on them should be minimized to reduce tran-

sients within the AD9049; these transients can detract from the

converter’s dynamic performance.

The minimum guaranteed conversion rate of the AD9049 is

3 MSPS. Below a nominal of 1.5 MSPS the internal T/H

switches to a track function only. This precludes the T/H from

drooping to the rail during the conversion process and mini-

mizes saturation issues. At clock rates below 3 MSPS dynamic

performance degrades. The AD9049 will operate in burst mode

operation, but the user must flush the internal pipeline each

time the clock stops. This requires 5 clock pulses each time the

clock is restarted for the first valid data output (refer to Fig-

ure 2 Timing Diagram).

Power Dissipation

The power dissipation specification in the parameter table is

measured under the following conditions: encode is 30 MSPS,

analog input is –1 dBFS at 10.3 MHz, the digital outputs are

5 V. These conditions intend to reflect actual usage of the device.

As shown in Figure 4, the actual power dissipation varies based

on these conditions. For instance, reducing the clock rate will

reduce power as expected for CMOS type devices. Also the

loading determines the power dissipated in the output stages.

From an ac standpoint, the capacitive loading will be the key

(refer to Equivalent Output Stage).

The analog input frequency and amplitude in conjunction with

the clock rate determine the switching rate of the output data

bits. Power dissipation increases as more data bits switch at

faster rates. For instance, if the input is a dc signal that is out of

range, no output bits will switch. This minimizes power in the

output stages but is not realistic from a usage standpoint.

The dissipation in the output stages can be minimized by inter-

facing the outputs to 3 V logic (refer to USING THE AD9049,

3 V System). The lower output swings minimize consumption.

Refer to Figure 4 for performance characteristics.

Voltage Reference

A stable and accurate +2.5 V voltage reference is built into the

reference is used by strapping Pins 3 and 4 of the AD9049 to-

gether. The internal reference has 500

µA of extra drive current

that can be used for other circuits.

Some applications may require greater accuracy, improved tem-

perature performance or adjustment of the gain of the AD9049,

which cannot be obtained by using the internal reference. For

these applications, an external +2.5 V reference can be used to

drive current.

The input range can be adjusted by varying the reference volt-

age applied to the AD9049. No appreciable degradation in per-

formance occurs when the reference is adjusted

±5%. The

full-scale range of the ADC tracks reference voltage changes

linearly.