AD8138ARZ-R7 datasheet(22/24 Pages) AD

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AD8138

Rev. F | Page 22 of 24

3 V OPERATION

The circuit in Figure 47 shows a simplified front-end

connection for an AD8138 driving an AD9203, a 10-bit,

40 MSPS ADC 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 differential 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 Ω at the input to

−IN is to balance the impedance of the 50 Ω source and its 50 Ω

termination that drives the noninverting input.

The signal generator has ground-referenced, bipolar output,

that is, 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

voltage divider that biases VOCM. In this way, the AD8138

provides dc coupling and level-shifting of a bipolar signal,

without inverting the input signal.

The low-pass filter between the AD8138 and the AD9203

provides filtering that helps to improve the signal-to-noise ratio

(SNR). Lower noise can be realized by lowering the pole

frequency, but the bandwidth of the circuit is lowered.

49.9Ω

0.1µF

10kΩ

523Ω

499Ω

10kΩ

20pF

49.9Ω

20pF

499Ω

0.1µF

DRVDD

AVDD

AINN

AINP

0.1µF

49.9Ω

AD8138

AD9203

AVSS DRVSS

DIGITAL

OUTPUTS

Figure 47. AD8138 Driving an AD9203, a 10-Bit, 40 MSPS A/D Converter

The circuit was tested with a −0.5 dBFS signal at various

frequencies. Figure 48 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

–45

–50

–55

–60

–65

–70

–75

–80

AD8138–2V

AD8138–1V

Figure 48. AD9203 THD @ −0.5 dBFS AD8138

Figure 49 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)

AD8138–1V

AD8138–2V

Figure 49. AD9203 SINAD @ −0.5 dBFS AD8138

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