ada4937-2 Analog Devices, Inc., ada4937-2 Datasheet - Page 23

• Manufacturer Part Number: ada4937-2
• Description: Ultralow Distortion Differential Adc Driver
• Manufacturer: Analog Devices, Inc.

HIGH PERFORMANCE ADC DRIVING
The ADA4937 is ideally suited for broadband IF applications.
The circuit in Figure 61 shows a front-end connection for an
ADA4937 driving an AD9445, 14-bit, 105 MSPS ADC. The
AD9445 achieves its optimum performance when driven
differentially. The ADA4937 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 ADA4937 is configured with a single 5 V supply and unity
gain for a single-ended input to differential output. The 61.9 Ω
termination resistor, in parallel with the single-ended input
impedance of 267 Ω, provides a 50 Ω termination for the
source. The additional 26 Ω (226 Ω total) at the inverting input
balances the parallel impedance of the 50 Ω source and the
termination resistor driving the noninverting input.
SIGNAL
GENERATOR
50Ω
61.9Ω
Figure 61. ADA4937 Driving an AD9445, a 14-Bit, 105 MSPS ADC
200Ω
226Ω
V
OCM
ADA4937
+
200Ω
200Ω
5V
Rev. A | Page 23 of 28
0.1µF
0.1µF
24.3Ω
24.3Ω
The signal generator has a symmetric, ground-referenced
bipolar output. The V
allowing the internal divider to set the output common-mode
voltage at midsupply. One half of the common-mode voltage is
fed back to the summing nodes, biasing −IN and +IN at 1.25 V. For
a common-mode voltage of 2.5 V, each ADA4937 output swings
between 2.0 V and 3.0 V, providing a 2 V p-p differential output.
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 61.
30nH
30nH
VIN–
47pF
VIN+
AVDD2
5V (A)
BUFFER T/H
CLOCK/
TIMING
3.3V (A)
AVDD1
AGND
OCM
pin of the ADA4937 is left floating,
3.3V (D)
DRVDD
ADA4937-1/ADA4937-2
REF
ADC
SENSE
AD9445
14