AD9432

Rev. F | Page 15 of 16

APPLICATIONS INFORMATION

USING THE AD8138 TO DRIVE THE AD9432

The AD8138 differential output op amp 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, which

allows for a low cost alternative to transformer coupling for ac

applications, as well.

The circuit in Figure 29 was breadboarded, and the measured

performance is shown in Figure 30 and Figure 31. These figures

are for ±5 V supplies at the AD8138; with a single 5 V supply at

the AD8138, performance dropped by about 1 dB to 2 dB.

Figure 30 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 31 shows the second-order and

third-order harmonic distortion performance under the same

conditions.

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

voltage that the AD9432 inputs require.

AD8138

AD9432

10pF

0.1µF

25Ω

500Ω

500Ω

50Ω

50Ω

50Ω

2kΩ

3kΩ

5V

500Ω

VIN

AIN

AIN

10pF

500Ω

22pF

Figure 29. AD8138/AD9432 Schematic

AIN (MHz)

020

40

62

63

64

65

66

61

60

60

SNR

SINAD

Figure 30. Measured SNR and SINAD (Encode = 105 MSPS)

AIN (MHz)

020

40

–90

–80

60

–70

–100

H2

H3

Figure 31. Measured Second-Order and Third-Order Harmonic Distortion

(Encode = 105 MSPS)