REV. E

AD8138

–13–

3 V OPERATION

The circuit in Figure 7 shows a simplified front end connection

for an AD8138 driving an AD9203, a 10-bit, 40 MSPS A/D con-

verter that is specified to work on a single 3 V supply. The ADC

works best when driven differentially to make the best use of the

signal swing available within the 3 V supply. The appropriate

outputs of the AD8138 are connected to the appropriate differen-

tial inputs of the AD9203 via a low-pass filter.

The AD8138 is configured for unity gain for a single-ended

input to differential output. The additional 23

W at the input to

–IN is to balance the impedance of the 50

W source and its 50 W

termination that drives the noninverting input.

The signal generator has ground-referenced, bipolar output,

i.e., it can drive symmetrically above and below ground. Even

though the AD8138 has ground as its negative supply, it can

still function as a level-shifter with such an input signal.

The output common mode is raised up to midsupply by the

vides dc coupling and level-shifting of a bipolar signal, without

inverting the input signal.

The low-pass filter between the AD8138 and the AD9203 pro-

vides filtering that helps to improve the signal-to-noise ratio.

Lower noise can be realized by lowering the pole frequency, but

the bandwidth of the circuit will be lowered.

49.9

0.1 F

10k

523

499

10k

20pF

49.9

20pF

499

499

0.1 F

AINN

+3V

DRVDD

AVDD

AVSS DRVSS

AINP

0.1 F

0.1 F

DIGITAL

OUTPUTS

+3V

49.9

AD8138

+

AD9203

Figure 7. AD8138 Driving an AD9203, a 10-Bit, 40 MSPS

A/D Converter

The circuit was tested with a –0.5 dBFS signal at various frequen-

cies. Figure 8 shows a plot of the total harmonic distortion (THD)

vs. frequency at signal amplitudes of 1 V and 2 V differential

drive levels.

FREQUENCY – MHz

–40

0

510

1520

25

–45

–50

–55

–60

–65

–70

–75

–80

AD8138–2V

AD8138–1V

Figure 8. AD9203 THD @ –0.5 dBFS AD8138

Figure 9 shows the signal to noise plus distortion (SINAD)

under the same conditions as above. For the smaller signal

swing, the AD8138 performance is quite good, but its performance

degrades when trying to swing too close to the supply rails.

FREQUENCY – MHz

65

0

510

1520

25

63

61

59

57

55

53

51

45

49

47

AD8138–1V

AD8138–2V

Figure 9. AD9203 SINAD @ –0.5 dBFS AD8138