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AD9432

12-Bit, 80 MSPS/105 MSPS A/D Converter

Analog Devices 

AD9432

APPLICATION NOTES

Theory of Operation

The AD9432 is a multibit pipeline converter that uses a switched

capacitor architecture. Optimized for high speed, this converter

provides flat dynamic performance up to frequencies near Nyquist.

DNL transitional errors are calibrated at final test to a typical

accuracy of 0.25 LSB or less.

USING THE AD9432

Analog Input

The analog input to the AD9432 is a differential buffer. The input

buffer is self-biased by an on-chip resistor divider that sets the

dc common-mode voltage to a nominal 3 V (see Equivalent

Circuits section). Rated performance is achieved by driving the

input differentially. Minimum input offset voltage is obtained when

driving from a source with a low differential source impedance

such as a transformer in ac applications. Capacitive coupling at the

inputs will increase the input offset voltage by as much as ± 25 mV.

Driving the ADC single-endedly will degrade performance.

For best dynamic performance, impedances at AIN and AIN

should match.

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

of the AD9432 to prevent damage and corruption of data when

the input is overdriven. The nominal input range is 2 V p-p.

Each analog input will be 1 V p-p when driven differentially.

PECL

GATE

510⍀

0.1␮F

510⍀ 0.1␮F

AD9432

ENCODE

ENCODE

GND

Figure 8. AC Coupling to ENCODE Inputs

ENCODE Voltage Level Definition

The voltage level definitions for driving ENCODE and ENCODE

in single-ended and differential mode are shown in Figure 9.

ENCODE Inputs

Differential Signal Amplitude (VID) . . . . . . . . . . . 500 mV min

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 750 mV nom

High Differential Input Voltage (VIHD) . . . . . . . . . . 3.5 V max

Low Differential Input Voltage (VILD) . . . . . . . . . . . . . 0 V min

Common-Mode Input (VICM) . . . . . . . 1.25 V min, 1.6 V nom

High Single-Ended Voltage (VIHS) . . . . . 2 V min to 3.5 V max

Low Single-Ended Voltage (VILS) . . . . . 0 V min to 0.8 V max

ENCODE

VIHD

ENCODE

VICM

VILD

VID

4.0

AIN

3.5

3.0

AIN

2.5

2.0

Figure 7. Full-Scale Analog Input Range

ENCODE Input

Any high speed A/D converter is extremely sensitive to the qual-

ity of the sampling clock provided by the user. A track/hold

circuit is essentially a mixer, and any noise, distortion, or timing

jitter on the clock will be combined with the desired signal at the

A/D output. For that reason, considerable care has been taken

in the design of the ENCODE input of the AD9432, and the

user is advised to give commensurate thought to the clock source.

The ENCODE input supports either differential or single-ended

and is fully TTL/CMOS compatible.

Note that the ENCODE inputs cannot be driven directly from

PECL level signals (VIHD is 3.5 V max). PECL level signals can

easily be accommodated by ac coupling as shown in Figure 8.

Good performance is obtained using an MC10EL16 in the

circuit to drive the encode inputs.

ENCODE

VIHS

0.1␮F

VILS

Figure 9. Differential and Single-Ended Input Levels

Often, the cleanest clock source is a crystal oscillator producing

a pure sine wave. In this configuration, or with any roughly

symmetrical clock input, the input can be ac-coupled and biased

to a reference voltage that also provides the ENCODE. This

ensures that the reference voltage is centered on the encode signal.

Digital Outputs

The digital outputs are 3.3 V (2.7 V to 3.6 V) TTL/CMOS-

compatible for lower power consumption. The output data

format is Two’s Complement, illustrated in Table I. The out of

range (OR) output (logic LOW for normal operation) will be

HIGH during any clock cycle when the ADC output data (Dx)

reach positive or negative full scale (–2048 or +2047). The OR

is internally generated each clock cycle, has the same pipe-

line latency and propagation delay as the ADC output data, and

will remain HIGH until the output data reflect an in-range

condition. The ADC output bits (Dx) will not roll over, and

will therefore remain at positive or negative full scale (+2048 or

–2047) while the OR output is HIGH.

–10–

REV. E

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