ADS5121

12

SBAS281

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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

(AIN+ and AIN–) of the ADS5121 see matched impedances.

Figure 1 shows the schematic for the suggested transformer-

coupled interface circuit. The component values of the R-C

low-pass may be optimized depending on the desired roll-off

frequency.

Single-Ended, AC-Coupled Driver

The circuit of Figure 2 shows an example for driving the

inputs of the ADS5121 in a single-ended configuration. The

signal is AC-coupled between the driver amplifier and the

converter input (AIN+). This allows for setting the required

common-mode voltages for the ADC and op amp separately.

The single-supply op amp is biased at mid-supply by two

resistors connected at its noninverting input. Connecting

each input to the CML pin provides the required common-

mode voltage for the inputs of the ADS5121. Here, two

resistors of equal value ensure that the inputs see closely

matched source impedances. If the op amp features a

disable function, it could be easily tied together with the

circuit example depicted in Figure 2, the OPA355’s EN pin is

mode of the entire circuit. Other suitable op amps for single-

supply driver applications include the OPA634, OPA635, or

OPA690, for example.

DC-Coupled Interface with Differential Amplifier

Differential input/output amplifiers can simplify the driver

circuit for applications requiring input DC-coupling. Flexible in

their configurations, such amplifiers can be used for single-

ended to differential conversion, allow for signal amplifica-

tion, and also for filtering prior to the ADC. See Figure 3 for

one possible circuit implementation using the THS4130 am-

plifier. Here, the amplifier operates with a gain of +1. The

common- mode voltage available at the CML pin can be

conveniently connected to the amplifier’s VOCM pin to set

the required input bias for the ADS5121.

FIGURE 1. Converting a Single-Ended Input Signal into a Differential Signal Using an RF-Transformer.

FIGURE 2. Single-Ended, AC-Coupled Driver Configuration for a Single Supply.

R

IN

R

IN

C

IN

0.1

µF

R

T

0.1

µF

1:n

R

S

OPA690

R

1

R

2

V

IN

ADS5121

AIN+

AIN–

CML

–5V

+5V

OPA355

V

IN

R

F

604

Ω

R

G

604

Ω

1.82k

Ω

1.82k

Ω

ADS5121

R

S

24

Ω

EN

33pF

0.1

µF

0.1

µF

R

1

AIN+

STBY

CML

AIN–

0.1

µF

0.1

µF

R

2

+3V/+5V