REV. E

–12–

AD8138

HIGH PERFORMANCE ADC DRIVING

The circuit in Figure 6 shows a simplified front-end connection

for an AD8138 driving an AD9224, a 12-bit, 40 MSPS A/D

converter. The ADC works best when driven differentially, which

minimizes its distortion as described in its data sheet. 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

W resistors to minimize the effects of the switched-capacitor

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

run from supplies of

±5 V.

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

input-to-differential output. The additional 23

W, 523 W total, at

the input to –IN is to balance the parallel impedance of the 50

W

source and its 50

W termination that drives the noninverting input.

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

i.e., it drives symmetrically above and below ground. Connecting

mode of the AD8138 at 2.5 V, which is the midsupply level for

the AD9224. This voltage is bypassed by a 0.1

mF 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 deter-

mined 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, this means that

each AD8138 output will swing between 1.5 V and 3.5 V.

to test the circuit in Figure 6. When the combined-device circuit

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

free dynamic range) was measured at –85 dBc.

49.9

0.1pF

523

499

49.9

499

499

VINB

+5V

DRVDD

AVDD

AVSS

DRVSS

VINA

0.1pF

0.1pF

DIGITAL

OUTPUTS

+5V

AD9224

AD8138

–5V

SENSE CML

50

SOURCE

+

49.9

Figure 6. AD8138 Driving an AD9224, a 12-Bit, 40 MSPS A/D Converter