ADS5421

11

SBAS237D

www.ti.com

FIGURE 4. Driving the ADS5421 with a Low-Distortion IF Amplifier and a Transformer Suited for IF Sampling Applications.

ADS5421

R

IN

R

S

R

IN

C

IN

0.1

µF

2.2

µF

R

T

1:n

IF

Amplifier

V

IN

IN

CM

Optional

Bandpass

Filter

+

+5V

NOTE: Supply bypassing not shown.

XFR

TRANSFORMER-COUPLED, SINGLE-ENDED-TO-

DIFFERENTIAL CONFIGURATION

For applications in which the input frequency is limited to

approximately 10MHz (e.g., baseband), a high-speed opera-

tional amplifier may be used. The OPA847 is configured for the

noninverting mode; this amplifies the single-ended input signal

and drives the primary of a RF transformer (see Figure 5). To

maintain the very low distortion performance of the OPA847, it

may be advantageous to set the full-scale input range of the

ADS5421 to 3Vp-p or 2Vp-p (refer to the Reference section for

details on selecting the converter’s full-scale range).

The circuit also shows the use of an additional RC low-pass

filter placed in series with each converter input. This optional

filter can be used to set a defined corner frequency and

attenuate some of the wideband noise. The actual compo-

nent values would need to be tuned for individual application

requirements. As a guideline, resistor values are typically in

the range of 10

Ω to 50Ω, and capacitors in the range of 10pF

a low tolerance. This will ensure that the ADS5421 sees

closely matched source impedances.

FIGURE 5. Converting a Single-Ended Input Signal into a Differential Signal Using a RF Transformer.

R

IN

R

IN

C

IN

2.2

µF

0.1

µF

R

T

0.1

µF

1:n

R

S

R

G

OPA847

R

1

R

2

V

IN

+

ADS5421

IN

IN

CM

+5V

–5V

+5V

V