AD8138ARMZ datasheet(21/24 Pages) AD

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AD8138

Rev. F | Page 21 of 24

HIGH PERFORMANCE ADC DRIVING

The circuit in Figure 46 shows a simplified front-end

connection for an AD8138 driving an AD9224, a 12-bit,

40 MSPS ADC. The ADC works best when driven differentially,

which minimizes its distortion. The AD8138 eliminates the

need for a transformer to drive the ADC and performs single-

ended-to-differential conversion, common-mode level-shifting,

and buffering of the driving signal.

The positive and negative outputs of the AD8138 are connected

to the respective differential inputs of the AD9224 via a pair of

49.9 Ω resistors to minimize the effects of the switched-capacitor

front end of the AD9224. For best distortion performance, it

runs from supplies of ±5 V.

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

input-to-differential output. The additional 23 Ω, 523 Ω total, at

the input to −IN is to balance the parallel impedance of the

50 Ω source and its 50 Ω termination that drives the

noninverting input.

The signal generator has a ground-referenced, bipolar output,

that is, it drives symmetrically above and below ground.

Connecting VOCM to the CML pin of the AD9224 sets the output

common-mode of the AD8138 at 2.5 V, which is the midsupply

level for the AD9224. This voltage is bypassed by a 0.1 μF

capacitor.

The full-scale analog input range of the AD9224 is set to

4 V p-p, by shorting the SENSE terminal to AVSS. This has

been determined to be the scaling to provide minimum

harmonic distortion.

For the AD8138 to swing at 4 V p-p, each output swings 2 V p-p

while providing signals that are 180° out of phase. With a

common-mode voltage at the output of 2.5 V, each AD8138

output swings between 1.5 V and 3.5 V.

A ground-referenced 4 V p-p, 5 MHz signal at DIN+ was used to

test the circuit in Figure 46. When the combined-device circuit

was run with a sampling rate of 20 MSPS, the spurious-free

dynamic range (SFDR) was measured at −85 dBc.

49.9Ω

0.1pF

523Ω

499Ω

49.9Ω

499Ω

VINB

+5V

DRVDD

AVDD

VINA

+5V

AD9224

VOCM

AD8138

–5V

SENSE

DIGITAL

OUTPUTS

0.1pF

DRVSS

CML

AVSS

49.9Ω

50Ω

SOURCE

15 26

16 25

Figure 46. AD8138 Driving an AD9224, a 12-Bit, 40 MSPS ADC

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