10

®

ADS828

FIGURE 4. DC-Coupled Interface Circuit with Level Shifting Using Dual Current-Feedback Amplifier OPA2681.

FIGURE 5. Transformer Coupled Input.

FIGURE 6. Equivalent Reference Circuit with Recommended

Reference Bypassing.

SINGLE-ENDED-TO-DIFFERENTIAL CONFIGURATION

(Transformer Coupled)

If the application requires a signal conversion from a single-

ended source to feed the ADS828 differentially, a RF trans-

former might be a good solution. The selected transformer

must have a center tap in order to apply the common-mode

DC voltage necessary to bias the converter inputs.

AC grounding the center tap will generate the differential

signal swing across the secondary winding. Consider a step-

up transformer to take advantage of a signal amplification

without the introduction of another noise source. Further-

more, the reduced signal swing from the source may lead to

an improved distortion performance.

The differential input configuration may provide a notice-

able advantage of achieving good SFDR performance over a

wide range of input frequencies. In this mode, both inputs of

the ADS828 see matched impedances, and the differential

signal swing can be reduced to half of the swing required for

single-ended drive. Figure 5 shows the schematic for the

suggested transformer-coupled interface circuit. The compo-

nent values of the R-C low-pass may be optimized depend-

ing on the desired roll-off frequency. The resistor across the

power transfer and Voltage Standing Wave Ratio (VSWR).

REFERENCE OPERATION

Figure 6 depicts the simplified model of the internal refer-

ence circuit. The internal blocks are the bandgap voltage

reference, the drivers for the top and bottom reference, and

the resistive reference ladder. The bandgap reference circuit

2Vp-p

NOTE: R

V

IN

R

2

200

Ω

R

1

1k

Ω

ADS828

R

S

50

Ω

10pF

0.1

µF

IN

IN

CM (+2.5V)

INT/EXT

R

F

499

Ω

R

IN

499

Ω

V

REFB

(+1.5V)

REFT

(+3.5V)

1/2

OPA2681

1/2

OPA2681

R

F

1k

Ω

50

Ω

0.1

µF

0.1

µF

RSEL

+V

S

+5V

+5V

V

IN

IN

IN

CM

22

Ω

22

Ω

47pF

R

T

47pF

+

10

µF

0.1

µF

INT/EXT

RSEL

+5V

ADS828

1:n

0.1

µF

R

G

ADS828

REFT

ByT

CM

ByB

REFB

Bypass Capacitors: 0.1

µF || 2.2µF each

Bandgap Reference and Logic

V

REF

400

Ω

400

Ω

400

Ω

400

Ω

+1

+1

+V

S

50k

Ω

50k

Ω

INT/EXT

RSEL