ADA4937-1 Analog Devices, ADA4937-1 Datasheet - Page 22

• Manufacturer Part Number: ADA4937-1
• Description: Ultra-Low Distortion Differential ADC Driver
• Manufacturer: Analog Devices

ADA4937-1
HIGH PERFORMANCE ADC DRIVING
The ADA4937-1 is ideally suited for broadband IF applications.
The circuit in Figure 51 shows a front-end connection for an
ADA4937-1 driving an AD9445, 14-bit, 105 MSPS ADC. The
AD9445
differentially. The ADA4937-1 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 ADA4937-1 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.
achieves its optimum performance when driven
50Ω
SIGNAL
GENERATOR
61.9Ω
Figure 51. ADA4937-1 Driving an AD9445, 14-Bit, 105 MSPS ADC
200Ω
226Ω
V
OCM
ADA4937-1
200Ω
+
200Ω
5V
Rev. 0 | Page 22 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 ADA4937-1 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
connecting the SENSE pin to AGND, as shown in Figure 51.
30nH
30nH
AD9445
VIN–
47pF
VIN+
is configured for a 2 V p-p full-scale input by
AVDD2
5V (A)
BUFFER T/H
CLOCK/
TIMING
3.3V (A)
AVDD1
OCM
AGND
pin of the ADA4937-1 is left floating,
3.3V (D)
DRVDD
REF
ADC
SENSE
AD9445
14