Datasheet

Data Sheet ADA4937-1/ADA4937-2
Rev. D | Page 23 of 28
HIGH PERFORMANCE ADC DRIVING
The ADA4937-x is ideally suited for broadband IF applications.
The circuit in Figure 64 shows a front-end connection for an
ADA4937-1 driving an AD9445, 14-bit, 105 MSPS ADC. The
AD9445 achieves its optimum performance when driven dif-
ferentially. The ADA4937-x 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-x 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 imped-
ance 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.
The signal generator has a symmetric, ground-referenced bipolar
output. The V
OCM
pin of the ADA4937-x remains unconnected
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-x 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 64.
VIN–
VIN+
47pF
30nH
30nH
24.3Ω
24.3Ω
50Ω
SIGNAL
GENERATOR
226Ω
200Ω
V
OCM
5V
ADA4937-1
+
61.9Ω
200Ω
200
Ω
14
BUFFER T/H
ADC
CLOCK/
TIMING
REF
SENSEAGND
0.1µF
0.1µF
AD9445
3.3V (A)
AVDD1
5V (A)
AVDD2
3.3V (D)
DRVDD
06591-054
Figure 64. Driving an AD9445, 14-Bit, 105 MSPS ADC