ADS808

11

SBAS179C

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attenuate some of the wideband noise. The actual compo-

nent values would need to be tuned for the individual appli-

cation requirements. As a guideline, resistor values are

typically in the range of 10

Ω to 100Ω, capacitors in the range

have a low tolerance. This will ensure that the ADS808 sees

closely matched source impedances.

AC-Coupled, Differential Interface with Gain

The interface circuit example presented in Figure 6 employs

two OPA685s, (current-feedback op amps), optimized for

gains of 8V/V or higher. The input transformer (T1) converts

the single-ended input signal to a differential signal required

at the amplifier’s inverting inputs, that are tuned to provide a

50

Ω impedance match to an assumed 50Ω source. To

achieve the 50

Ω input match at the primary of the 1:2

transformer, the secondary input must see a 200

Ω load

impedance. Both amplifiers are configured for the inverting

mode resulting in close gain and phase matching of the

differential signal. This technique, along with a highly sym-

metrical layout, is instrumental in achieving a substantial

reduction of the 2nd-harmonic, while retaining excellent 3rd-

plied to the noninverting inputs of the OPA685. Additional

series of 43.2

Ω resistors isolate the output of the op amps

from the capacitive load presented by the 22pF capacitors

and the input capacitance of the ADS808. This 43.2

Ω/22pF

combination sets a pole at approximately 167MHz and rolls

off some of the wideband noise.

REFERENCE

REFERENCE OPERATION

Integrated into the ADS808 is a bandgap reference circuit

including some logic that provides a +0.5V, +0.75V, or +1V

reference output by selecting the corresponding pin-strap

configuration. Table I gives a complete overview of the

possible reference options and pin configurations.

+5V

–5V

OPA685

22pF

22pF

+5V

–5V

600

Ω

OPA685

T1

1:2

50

Ω Source

Noise

Figure

11.8dB

V

CM

V

CM

600

Ω

100

Ω

100

Ω

43.2

Ω

Power-supply decoupling

not shown.

43.2

Ω

V

I

V

O

A/D Converter Input

V

O

V

I

= 12V/V (21.6dB)

DIS

DIS

FIGURE 6. Wideband Differential A/D Converter Driver.

DESIRED FULL-SCALE RANGE,

CONNECT SEL1

CONNECT SEL2

VOLTAGE AT REFT

VOLTAGE AT REFB

FSR (Differential)

(Pin 33)

(Pin 32)

(Pin 34)

(Pin 41)

(Pin 39)

2Vp-p (+10dBm)

GND

GND

+1.0V

+3V

+2V

1.5Vp-p (+7.5dBm)

GND

+0.75V

+2.875V

+2.125V

1Vp-p

GND

+0.5V

+2.75V

+2.25V

External Reference

——

> +3.5V

+2.75V to +4.5V

+0.5V to +2.25V

TABLE I. Reference Pin Configurations and Corresponding Voltage on the Reference Pins.