ADS2807

10

SBAS169B

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increasing the size of the series resistor and/or capacitor will

improve the SNR performance, but depending on the signal

source, large resistor values may be detrimental to achieving

good harmonic distortion. In any case, optimizing the R-C

values for the specific application is encouraged.

Transformer Coupled, Single-Ended to Differential

Configuration

If the application requires a signal conversion from a single-

ended source to drive the ADS2807 differentially, an RF

transformer might be a good solution. The selected trans-

former 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 with-

out the introduction of another noise source. Furthermore,

the reduced signal swing from the source may lead to

improved distortion performance.

The differential input configuration provides the noticeable

advantage of achieving high SFDR over a wide range of

input frequencies. In this mode, both inputs of the ADS2807

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. The resistor

The circuit example of Figure 1 shows voltage feedback

amplifier OPA680 driving the RF transformer, which converts

the single-ended signal into a differential. The OPA680 can

be employed for either single- or dual-supply operation. For

details on how to optimize its frequency response, refer to

the OPA680 data sheet. With the 49.9

Ω series output resis-

tor, the amplifier emulates a 50

Ω source (R

content of the signal can be easily blocked by a capacitor

(0.1

µF) to avoid DC loading of the op amp’s output stage.

AC-Coupled, Single-Ended to Differential Interface

with Dual-Supply Op Amps

Some applications demand a very high dynamic range and

low levels of intermodulation distortion, but usually allow the

input signal to be AC-coupled into the ADC. Appropriate

driver amplifiers need to be selected to maintain the excellent

distortion performance of the ADS2807. Often, these op

amps deliver the lowest distortion with a small, ground-

centered signal swing that requires dual power supplies.

Because of the AC-coupling, this requirement can be easily

accomplished, and the needed level shifting of the input

signal can be implemented without affecting the driver circuit.

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

V

IN

IN

IN

CM

+2.5V

24.9

Ω

24.9

Ω

10

µF

0.1

µF

47pF

R

T

47pF

1:n

0.1

µF

R

G

R

2

R

1

OPA680

49.9

Ω

1/2

ADS2807Y

+

One Channel of Two