ADS822, ADS825

10

SBAS069B

FIGURE 4. DC-Coupled Interface Circuit with 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 ADS822 and ADS825 differentially,

a RF transformer 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 differ-

ential signal swing across the secondary winding. Consider

a step-up transformer to take advantage of a signal amplifi-

cation without the introduction of another noise source.

Furthermore, the reduced signal swing from the source may

lead to an improved distortion performance.

The differential input configuration may provide a noticeable

advantage of achieving good SFDR performance over a wide

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

ADS822 and ADS825 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 sche-

matic for the suggested transformer-coupled interface circuit.

The component values of the R-C low-pass may be opti-

mized depending on the desired roll-off frequency. The

resistor across the secondary side (RT) should be calculated

using the equation RT = n2 • RG to match the source impedance

(RG) for good power transfer and Voltage Standing Wave Ratio

(VSWR).

REFERENCE OPERATION

Figure 6 depicts the simplified model of the internal reference

circuit. The internal blocks are the bandgap voltage refer-

ence, the drivers for the top and bottom references, and the

2Vp-p

NOTE: R

F = RIN, G = –1

V

IN

R

2

200

Ω

R

1

1k

Ω

ADS822

ADS825

R

S

50

Ω

10pF

0.1

µF

IN

IN

CM (+2.5)

INT/EXT

R

F

499

Ω

R

IN

499

Ω

V

CM = +1.25V

REFB

(+1.5V)

REFT

(+3.5V)

1/2

OPA2691

1/2

OPA2691

R

F

1k

Ω

50

Ω

0.1

µF

0.1

µF

RSEL

+V

S

+5V

+5V

A1

A2

V

IN

IN

IN

CM

22

Ω

22

Ω

47pF

R

T

47pF

+

10

µF

0.1

µF

INT/EXT

RSEL

+5V

ADS822

ADS825

1:n

0.1

µF

R

G

ADS822

REFT

ByT

CM

ByB

REFB

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

Bypass Capacitors: 0.1

µF || 2.2µF each (optionally, 2.2µF tantalum

capacitors maybe added to ByT and ByB pins for the best results).