Data Sheet

ADA4939-1/ADA4939-2

Rev. A | Page 23 of 24

HIGH PERFORMANCE ADC DRIVING

The ADA4939-1/ADA4939-2 are ideally suited for broadband

ac-coupled and differential-to-differential applications on a

single supply.

The circuit in Figure 54 shows a front-end connection for an

ADA4939-1 driving an AD9445, 14-bit, 105 MSPS ADC, with

ac coupling on the ADA4939-1 input and output. (The AD9445

achieves its optimum performance when driven differentially.)

The ADA4939-1 eliminates the need for a transformer to drive

the ADC and perform a single-ended-to-differential conversion

and buffering of the driving signal.

The ADA4939-1 is configured with a single 5 V supply and gain

of 2 for a single-ended input to differential output. The 60.4 Ω

termination resistor, in parallel with the single-ended input

impedance of approximately 300 Ω, provides a 50 Ω termination

for the source. The additional 27.4 Ω (227.4 Ω total) at the

inverting input balances the parallel impedance of the 50 Ω

source and the termination resistor driving the noninverting input.

In this example, the signal generator has a 1 V p-p symmetric,

ground-referenced bipolar output when terminated in 50 Ω.

The VOCM pin of the ADA4939-1 is bypassed for noise reduction

and left floating such that the internal divider sets the output

common-mode voltage nominally at midsupply. Because the

inputs are ac-coupled, no dc common-mode current flows in

the feedback loops, and a nominal dc level of midsupply is

present at the amplifier input terminals. Besides placing the

amplifier inputs at their optimum levels, the ac coupling technique

lightens the load on the amplifier and dissipates less power than

applications with dc-coupled inputs. With an output common-

mode voltage of nominally 2.5 V, each ADA4939-1 output

swings between 2.0 V and 3.0 V, providing a gain of 2 and a 2 V p-p

differential signal to the ADC input.

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 54.

VIN–

VIN+

47pF

30nH

30nH

24.3Ω

24.3Ω

50Ω

SIGNAL

GENERATOR

200Ω

200Ω

VOCM

5V

+

60.4Ω

412Ω

27.4Ω

412Ω

14

BUFFER T/H

ADC

CLOCK/

TIMING

REF

SENSE

AGND

0.1µF

0.1µF

0.1µF

0.1µF

0.1µF

AD9445

3.3V (A)

AVDD1

5V (A)

AVDD2

3.3V (D)

DRVDD

ADA4939-1

Figure 54. ADA4939-1 Driving an AD9445 ADC with AC-Coupled Input and Output