ADA4927-1/ADA4927-2

Rev. 0 | Page 23 of 24

HIGH PERFORMANCE ADC DRIVING

The ADA4927 is ideally suited for high gain, broadband ac-

coupled and differential-to-differential applications on a single

supply, though other applications are possible. Compared with

voltage feedback amplifiers, the current feedback architecture

provides superior distortion and bandwidth performance at

high gains. This is because the ideal current feedback amplifier

loop gain depends only on the feedback value and open-loop

transimpedance, T(s).

The circuit in Figure 58 shows a front-end connection for an

ADA4927 driving an AD9445, 14-bit, 105 MSPS ADC, with ac

coupling on the ADA4927 input and output. (The AD9445

achieves its optimum performance when driven differentially.)

The ADA4927 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 ADA4927 is configured with a single 5 V supply and gain

of 10 for a single-ended input to differential output. The 158 Ω

termination resistor, in parallel with the single-ended input

impedance of approximately 73.2 Ω, provides a 50 Ω termination

for the source. The additional 38.3 Ω at the inverting input closely

matches the parallel impedance of the 50 Ω source and the

termination resistor driving the noninverting input. Because of the

high gain, a few iterations of the termination technique described

in the Terminating a Single-Ended Input section are required.

obtain resistor values that are close to standard 1% values.

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

ground-referenced bipolar output when terminated in 50 Ω.

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.

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 58.

VIN–

VIN+

47pF

30nH

30nH

24.3Ω

24.3Ω

50Ω

SIGNAL

GENERATOR

39.2Ω

39.2Ω

5V

ADA4927

+

158Ω

511Ω

38.3Ω

511Ω

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

Figure 58. ADA4927 Driving an AD9445 ADC with AC-Coupled Input and Output