ADS2806

SBAS178B

11

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Figure 2 shows an example of such an interface circuit

specifically designed to maximize the dynamic performance.

The voltage feedback amplifier, OPA642, maintains an

excellent distortion performance for input frequencies of up

to 15MHz. The two amplifiers (A1, A2) are configured as an

inverting and noninverting gain stage to convert the input

signal from single-ended to differential. The nominal gain for

this stage is set to +2V/V. The outputs of the OPA642s are

AC-coupled to the converter’s differential inputs. This will

keep the distortion performance at its best since the signal

range stays within the linear region of the op amp and

sufficient headroom to the supply rails can be maintained.

Four resistors located between the top (REFT) and bottom

(REFB) reference shift the input signal to a common-mode

voltage of approximately +2.5V.

The interface circuit of Figure 2 can be modified to extend

the bandwidth to approximately 25MHz, by replacing the

OPA642 with its decompensated version, the OPA643. The

OPA643 provides the necessary slew rate for a low distor-

FIGURE 2. AC-Coupled Differential Driver Interface with OPA642.

FIGURE 3. AC-Coupled, Differential Interface for Single-Supply Operation.

V

IN

1/2

ADS2806Y

100pF

IN

IN

402

Ω

1.82k

Ω

1.82k

Ω

16.5

Ω

200

Ω

402

Ω

402

Ω

REFB

100pF

0.1

µF

0.1

µF

A1

OPA642

A2

OPA642

REFT

1.82k

Ω

1.82k

Ω

16.5

Ω

One Channel of Two

V

IN

1/2

ADS2806Y

68pF

IN

CM

IN

+5V

68pF

0.1

µF

0.1

µF

0.1

µF

1/2

OPA2681

1/2

OPA2681

R

G

249

Ω

R

F

499

Ω

499

Ω

R

S

24.9

Ω

V

R

P

499

Ω

R

P

499

Ω

R

F

499

Ω

R

IN

249

Ω

R

S

24.9

Ω

One Channel of Two

tion front end to the ADS2806. With a minimum gain stability

of +3, the gain resistors have to be modified, as well as

optimizing the series resistor and shunt capacitance at each

of the converter inputs.

AC-Coupled, Single-Ended-to-Differential Interface

for Single-Supply Operation

The previously discussed interface circuit can be modified if

the system only allows for a single-supply operation, e.g.,

fier to be biased as well in order to process a bipolar input

signal. Typically, single-supply amplifiers do not achieve

distortion performance as well as dual-supply op amps. The

driver amplifier’s output swing must exceed the full-scale

input range of the converter. In addition, dual op amps, such

as the current-feedback OPA2681, should be considered

since they provide the closest open-loop gain and phase

matching between the two channels. Shown in Figure 3 is

a single-supply interface circuit for an AC-coupled input

signal. With the ADS2806 set to the 2Vp-p input range, the