ADA4932-1/ADA4932-2

Rev. 0 | Page 25 of 28

HIGH PERFORMANCE ADC DRIVING

The ADA4932-x is ideally suited for broadband dc-coupled

applications. The circuit in Figure 67 shows a front-end

connection for an ADA4932-1 driving an AD9245, a 14-bit,

20 MSPS/40 MSPS/65 MSPS/80 MSPS ADC, with dc coupling

on the ADA4932-1 input and output. (The AD9245 achieves

its optimum performance when driven differentially.) The

ADA4932-1 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 ADA4932-1 is configured with a single 3.3 V supply and a

gain of 1 for a single-ended input to differential output. The

53.6 Ω termination resistor, in parallel with the single-ended

input impedance of approximately 665 Ω, provides a 50 Ω

termination for the source. The additional 25.5 Ω (524.5 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.

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 0.84 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 × [524.5/(524.5 + 511)] =

0.25 V p-p. This ac signal is riding on the 0.84 V dc level, produc-

ing a voltage swing between 0.72 V and 0.97 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 ADA4932-1

output swings between 1.4 V and 1.9 V, opposite in phase, provid-

ing a gain of 1 and a 1 V p-p differential signal to the ADC input.

The differential RC section between the ADA4932-1 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 67.

ADA4932-1

3.3V

50

Ω

499

Ω

499

Ω

25.5

Ω

10k

Ω

1%

511

Ω

511

Ω

33

Ω

33

Ω

2V p-p

SIGNAL

GENERATOR

1V p-p CENTERED

AT GROUND

53.6

Ω

0.1µF

0.1µF

10µF

+

0.1µF

0.1µF

10k

Ω

1%

20pF

VIN–

AVDD

VIN+ VREF SENSE AGND

AD9245

Figure 67. ADA4932-1 Driving an AD9245 ADC with DC-Coupled Input and Output