ADA4950-1/ADA4950-2

Data Sheet

Rev. B | Page 24 of 26

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 AD9245 ADC,

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

tion and set externally with 1% resistors to maximize output

dynamic range on the tight 3.3 V supply.

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.

33Ω

33Ω

50Ω

57.6Ω

0.1µF

20pF

VIN–

VIN+

AGND

AVDD

AD9245

VREF SENSE

10µF

+

0.1µF

1.0V p-p

UNIPOLAR

SIGNAL

SOURCE

ADA4950-1

+3.3V

500Ω

500Ω

500Ω

500Ω

250Ω

NC

250Ω

NC

26.7Ω

0.1µF

0V

1.0V

0.5V

10kΩ

10kΩ

0.1µF

0.1µF

866Ω

1.0kΩ

0.1µF

AD8031

0.1µF

10µF

+

Figure 63. ADA4950-1 Driving an AD9245 ADC with Unipolar DC-Coupled Input and Output, Gain = 2