ADS807

10

SBAS072A

www.ti.com

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 ADS807 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 a noticeable

advantage of achieving good SFDR over a wide range of

input frequencies. In this mode, both inputs of the ADS807

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 the voltage-feedback

amplifier OPA680 driving the RF transformer, which converts

the single-ended signal into a differential one. 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 (SBOS083), available at

www.ti.com. With the 49.9

Ω series output resistor, the ampli-

fier emulates a 50

Ω source (R

signal can be easily blocked by a capacitor (0.1

µF) and to

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

AC-Coupled, Single-Ended-to-Differential Interface

with Dual-Supply Op Amps

Communications applications, in particular, demand a very

high dynamic range and low levels of intermodulation distor-

tion, but usually allow the input signal to be AC-coupled into

the A/D converter. Appropriate driver amplifiers need to be

selected to maintain the excellent distortion performance of

the ADS807. Often, these op amps deliver the lowest distor-

tion with a small, ground-centered signal swing that requires

dual power supplies. Because of the AC-coupling, this re-

quirement can be easily accomplished and the needed level

shifting of the input signal can be implemented without

affecting the driver circuit.

See Figure 2 for an example of such an interface circuit

specifically designed to maximize the dynamic performance.

The voltage feedback amplifier, OPA642, maintains an ex-

cellent distortion performance for input frequencies of up to

15MHz. The two amplifiers (A1, A2) are configured as an

inverting and noninverting gain stage to convert the input

signal from single-ended to differential. The nominal gain for

this stage is set to +2V/V. The outputs of the OPA642s are

AC-coupled to the converter’s differential inputs. This will

keep the distortion performance at its best since the signal

range stays within the linear region of the op amp and

sufficient headroom to the supply rails can be maintained.

Four resistors located between the top (REFT) and bottom

(REFB) reference shift the input signal to a common-mode

voltage of approximately +2.5V.

The interface circuit of Figure 2 can be modified to extend

the bandwidth to approximately 25MHz by replacing the

OPA642 with its decompensated version, the OPA643. The

OPA643 provides the necessary slew rate for a low distortion

front end to the ADS807. With a minimum gain stability of +3,

the gain resistors have to be modified, as well as optimizing

the series resistor and shunt capacitance at each of the

converter inputs.

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

V

IN

IN

IN

CM

+2.5V

24.9

Ω

24.9

Ω

47pF

R

T

47pF

10µF

0.1µF

1:n

0.1µF

R

G

R

2

R

1

OPA680

49.9

Ω

ADS807E

+