ADS830

9

SBAS086A

FIGURE 5. Transformer Coupled Input.

FIGURE 6. Equivalent Reference Circuit with Recommended

Reference Bypassing.

V

IN

IN

IN

CM

22

Ω

22

Ω

47pF

R

T

47pF

+

10µF

0.1µF

INT/EXT

RSEL

+5V

ADS830

1:n

0.1µF

R

G

ADS830

REFT

CM

REFB

Bypass Capacitors: 0.1µF || 2.2µF each

Bandgap Reference and Logic

V

REF

400

Ω

400

Ω

+1

+1

+V

S

50k

Ω

50k

Ω

INT/EXT

RSEL

SINGLE-ENDED-TO-DIFFERENTIAL CONFIGURATION

(Transformer Coupled)

If the application requires a signal conversion from a single-

ended source to feed the ADS830 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 ADS830 see closely matched impedances, and the

differential signal swing is reduced to half of the swing

required for single-ended drive. Figure 5 shows the sche-

matic for the suggested transformer coupled interface cir-

cuit. The component values of the R-C lowpass may be

optimized depending on the desired roll-off frequency. The

resistor across the secondary side (RT) should be calculated

using the equation RT = n2 x RG to match the source

impedance (RG) for good power transfer and 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

2Vp-p

NOTE: R

F = RIN, G = –1

V

IN

R

2

301

Ω

R

1

499

Ω

ADS830

R

S

39

Ω

47pF

0.1µF

IN

IN

CM (+2.5V)

INT/EXT

R

F

499

Ω

R

IN

499

Ω

V

CM = +1.25V

REFB

(+2.0V)

REFT

(+3.0V)

1/2

OPA2681

1/2

OPA2681

R

F

1k

Ω

50

Ω

0.1µF

0.1µF

RSEL

+V

S

+5V

+5V

FIGURE 4. DC-Coupled Interface Circuit with Dual Current-Feedback Amplifier OPA2681. The OPA2680 can be used in place

of the OPA2681 if a voltage feedback amplifier is preferred.