ada4950-1 Analog Devices, Inc., ada4950-1 Datasheet - Page 24

• Manufacturer Part Number: ada4950-1
• Description: Low Power, Selectable Gain Differential Adc Driver, G = 1, 2, 3
• Manufacturer: Analog Devices, Inc.

ADA4950-1/ADA4950-2
HIGH PERFORMANCE ADC DRIVING
The ADA4950-x is ideally suited for broadband dc-coupled
applications. The circuit in Figure 63 shows a front-end
connection for an ADA4950-1 driving an
with dc coupling on the ADA4950-1 input and output. (The
AD9245 achieves its optimum performance when driven
differentially.) The ADA4950-1 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 ADA4950-1 is configured with a single 3.3 V supply and a
gain of 2 for a single-ended input to differential output. The
57.6 Ω termination resistor, in parallel with the single-ended
input impedance of 375 Ω, provides a 50 Ω termination for the
source. The additional 26.7 Ω Thevenin resistance added to the
inverting input balances the parallel impedance of the 50 Ω
source and the termination resistor driving the noninverting
input. The required Thevenin bias voltage of 0.27 VDC applied
to the lower loop is obtained by scaling the VREF output of the
AD9245 and buffering it with the AD8031.
In this example, the 50 Ω signal generator has a 1 V p-p unipolar
open-circuit output voltage, and 0.5 V p-p output voltage when
terminated in 50 Ω. The V
tion and set externally with 1% resistors to maximize output
dynamic range on the tight 3.3 V supply.
1.0V
0.5V
0V
OCM
Figure 63. ADA4950-1 Driving an AD9245 ADC with Unipolar DC-Coupled Input and Output, Gain = 2
input is bypassed for noise reduc-
UNIPOLAR
SIGNAL
SOURCE
50Ω
1.0V p-p
0.1µF
10kΩ
10kΩ
AD9245
26.7Ω
0.1µF
57.6Ω
ADC,
NC
NC
10µF
500Ω
500Ω
250Ω
250Ω
+
V
Rev. 0 | Page 24 of 28
OCM
500Ω
ADA4950-1
500Ω
+3.3V
Because the inputs are dc-coupled, dc common-mode current
flows in the feedback loops, and a nominal dc level of 0.76 V is
present at the amplifier input terminals. A fraction of the output
signal is also present at the input terminals as a common-mode
signal; its level is equal to the ac output swing at the noninverting
output, divided down by the feedback factor of the lower loop. In
this example, this ripple is 0.5 V p-p × [276.7/(276.7 + 500)] =
0.18 V p-p. This ac signal is riding on the 0.76 V dc level, producing
a voltage swing between 0.67 V and 0.85 V at the input terminals.
This is well within the specified limits of 0.2 V to 1.5 V.
With an output common-mode voltage of 1.65 V, each ADA4950-1
output swings between 1.4 V and 1.9 V, opposite in phase, provid-
ing a gain of 2 and a 1 V p-p differential signal to the ADC input.
The differential RC section between the ADA4950-1 output and
the ADC provides single-pole low-pass filtering and extra buffering
for the current spikes that are output from the ADC input when
its SHA capacitors are discharged.
The AD9245 is configured for a 1 V p-p full-scale input by
connecting its SENSE pin to VREF, as shown in Figure 63.
0.1µF
0.1µF
AD8031
0.1µF
20pF
33Ω
33Ω
0.1µF
10µF
VIN+
+
VIN–
VREF SENSE
V
V
OUT, dm
OUT, cm
1.0kΩ
866Ω
AD9245
= +1.65V
= 1V p-p
AGND
AVDD
0.1µF