AD8138_16 Datasheet, PDF (22/25 Pages) Analog Devices – Low Distortion, Differential ADC Driver

Data Sheet
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.
AD8138
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.
50Ω
SOURCE
+5V
499Ω
0.1µF
+5V
0.1µF
499Ω
49.9Ω
0.1µF
523Ω
3
8+
5
2
V_OCM
AD8138
1
4
6
499Ω
49.9Ω
49.9Ω
15 26
28
24 VINB ^{AVDD DRVDD}
AD9224
23 VINA _{AVSS SENSE CML DRVSS}
16 25
17
22
27
DIGITAL
OUTPUTS
–5V
Figure 46. AD8138 Driving an AD9224, a 12-Bit, 40 MSPS ADC
Rev. G | Page 21 of 24