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AD9432

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

Analog Devices 

AD9432

Table I. Output Coding (VREF = 2.5 V) (Two’s Complement)

Code

AIN–AIN (V)

Digital Output

+2047

•

•

0

–1

•

•

–2048

1.000

•

•

0

–0.00049

•

•

–1.000

0111 1111 1111

•

•

0000 0000 0000

1111 1111 1111

•

•

1000 0000 0000

Voltage Reference

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

AD9432 (VREFOUT). In normal operation the internal refer-

ence is used by strapping Pin 45 to Pin 46 and placing a 0.1 µF

decoupling capacitor at VREFIN.

The input range can be adjusted by varying the reference voltage

applied to the AD9432. No appreciable degradation in perfor-

mance occurs when the reference is adjusted ±5%. The full-scale

range of the ADC tracks reference voltage changes linearly.

Timing

The AD9432 provides latched data outputs, with 10 pipeline

delays. Data outputs are included or available one propagation

delay (tPD) after the rising edge of the encode command

(see Figure 1). The length of the output data lines and loads

placed on them should be minimized to reduce transients within

the AD9432; these transients can detract from the converter’s

dynamic performance.

The minimum guaranteed conversion rate of the AD9432 is

1 MSPS. At internal clock rates below 1 MSPS, dynamic

performance may degrade. Therefore, input clock rates below

1 MHz should be avoided.

During initial power-up, or whenever the clock to the AD9432

is interrupted, the output data will not be accurate data for 200 ns

or 10 clock cycles, whichever is longer.

Using the AD8138 to Drive the AD9432

A new differential output op amp from Analog Devices, Inc.,

the AD8138, can be used to drive the AD9432 in dc-coupled

applications. The AD8138 was specifically designed for ADC

driver applications. Superior SNR performance is maintained up

to analog frequencies of 30 MHz. The AD8138 op amp provides

single-ended-to-differential conversion, providing for a low-cost

option to transformer coupling for ac applications as well.

The circuit in Figure 10 was breadboarded and the measured

performance is shown in Figures 11 and 12. The figures shown

are for ± 5 V supplies at the AD8138—performance dropped by

about 1 dB–2 dB with a single 5 V supply at the AD8138.

Figure 11 shows SNR and SINAD for a –1 dBFS analog input

frequency varied from 2 MHz to 40 MHz with an encode rate of

105 MSPS. The measurements are for nominal conditions at

room temperature. Figure 12 shows the second and third har-

monic distortion performance under the same conditions.

The dc common-mode voltage for the AD8138 outputs can be

adjusted via input VOCM to provide the 3 V common-mode voltage

the AD9432 inputs require.

500⍀

AD9432

10pF

VIN

500⍀

50⍀

AD8138

VOCM

500⍀

50⍀

AIN

22pF

50⍀

AIN

5V

2k⍀

25⍀ 500⍀

10pF

3k⍀

0.1␮F

Figure 10. AD8138/AD9432 Schematic

66

65

SNR

64

63

SINAD

62

61

60

0

20

40

60

AIN – MHz

Figure 11. Measured SNR and SINAD (Encode = 105 MSPS)

–70

H2

–80

H3

–90

–100

0

20

40

60

AIN – MHz

Figure 12. Measured Second and Third Order Harmonic

Distortion (Encode = 105 MSPS)

REV. E

–11–

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