Data Sheet

ADA4932-1/ADA4932-2

HIGH PERFORMANCE ADC DRIVING

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Rev. F | 25 of 27

The ADA4932-1/ADA4932-2 are ideally suited for broadband dc-

coupled applications. The circuit in Figure 65 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. The

VOCM input is bypassed for noise reduction, 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 0.84 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, producing 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 connect-

ing its SENSE pin to VREF, as shown in Figure 65.

Figure 65. ADA4932-1 Driving an AD9245 ADC with DC-Coupled Input and Output