Datasheet

ADA4939-1/ADA4939-2
Rev. 0 | Page 23 of 24
HIGH PERFORMANCE ADC DRIVING
The ADA4939 is ideally suited for broadband ac-coupled and
differential-to-differential applications on a single supply.
The circuit in
Figure 54 shows a front-end connection for an
ADA4939 driving an
AD9445, 14-bit, 105 MSPS ADC, with ac
coupling on the ADA4939 input and output. (The
AD9445
achieves its optimum performance when driven differentially.)
The ADA4939 eliminates the need for a transformer to drive
the ADC and performs a single-ended-to-differential conversion
and buffering of the driving signal.
The ADA4939 is configured with a single 5 V supply and gain
of 2 for a single-ended input to differential output. The 60.4 Ω
termination resistor, in parallel with the single-ended input
impedance of approximately 300 Ω, provides a 50 Ω termination
for the source. The additional 27.4 Ω (227.4 Ω total) at the
inverting input balances the parallel impedance of the 50 Ω
source and the termination resistor driving the noninverting input.
In this example, the signal generator has a 1 V p-p symmetric,
ground-referenced bipolar output when terminated in 50 Ω.
The V
OCM
pin of the ADA4939 is bypassed for noise reduction
and left floating such that the internal divider sets the output
common-mode voltage nominally at midsupply. Because the
inputs are ac-coupled, no dc common-mode current flows in
the feedback loops, and a nominal dc level of midsupply is
present at the amplifier input terminals. Besides placing the
amplifier inputs at their optimum levels, the ac coupling technique
lightens the load on the amplifier and dissipates less power than
applications with dc-coupled inputs. With an output common-
mode voltage of nominally 2.5 V, each ADA4937 output swings
between 2.0 V and 3.0 V, providing a gain of 2 and a 2 V p-p
differential signal to the ADC input.
The output of the amplifier is ac-coupled to the ADC through a
second-order, low-pass filter with a cutoff frequency of 100 MHz.
This reduces the noise bandwidth of the amplifier and isolates
the driver outputs from the ADC inputs.
The
AD9445 is configured for a 2 V p-p full-scale input by
connecting the SENSE pin to AGND, as shown in
Figure 54.
07429-061
VIN–
VIN+
47pF
30nH
30nH
24.3Ω
24.3Ω
50Ω
SIGNAL
GENERATOR
200Ω
200Ω
V
OCM
5V
ADA4939
+
60.4Ω
412Ω
27.4Ω
412Ω
14
BUFFER T/H
ADC
CLOCK/
TIMING
REF
SENSEAGND
0.1µF0.1µF
0.1µF0.1µF
0.1µF
AD9445
3.3V (A)
AVDD1
5V (A)
AVDD2
3.3V (D)
DRVDD
Figure 54. ADA4939 Driving an AD9445 ADC with AC-Coupled Input and Output