AD8138
HIGH PERFORMANCE ADC DRIVING
The signal generator has a ground-referenced, bipolar output,
i.e., 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 mF capacitor.
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 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.
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.
For the AD8138 to swing at 4 V p-p, each output swings 2 V p-p
while providing signals that are 180Њ 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.
A ground-referenced 4 V p-p, 5 MHz signal at DIN+ was used
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.
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.
+5V
+5V
0.1pF
0.1pF
499⍀
49.9⍀
49.9⍀
499⍀
AVDD DRVDD
+
VINB
DIGITAL
OUTPUTS
V
OCM
AD9224
50⍀
SOURCE
49.9⍀
AD8138
523⍀
VINA
AVSS
DRVSS
SENSE CML
0.1pF
499⍀
–5V
Figure 6. AD8138 Driving an AD9224, a 12-Bit, 40 MSPS A/D Converter
–12–
REV. E
ADI [ ADI ]
