ADS805

9

SBAS073B

www.ti.com

FIGURE 2. DC-Coupled, Single-Ended Input Configuration with DC-level Shift.

INPUT

FULL-SCALE

REQUIRED

MODE

RANGE

CONNECT

TO

Internal

2Vp-p

+1V

SEL

Internal

5Vp-p

+2.5V

SEL

GND

Internal

2V

≤ FSR < 5V

SEL and Gnd

External

1V < FSR < 5V

SEL

TABLE I. Selected Reference Configuration Examples.

V

IN

IN

IN

CM

22

Ω

22

Ω

100pF

R

T

100pF

+

4.7µF

0.1µF

ADS805

1:n

0.1µF

R

G

FIGURE 3. Transformer-Coupled Input.

REFERENCE OPERATION

Integrated into the ADS805 is a bandgap reference circuit

including logic that provides either a +1V or +2.5V reference

output, by simply selecting the corresponding pin-strap con-

figuration. Different reference voltages can be generated by

the use of two external resistors, which will set a different

gain for the internal reference buffer. For more design flexibil-

ity, the internal reference can be shut off and an external

reference voltage used. Table I provides an overview of the

possible reference options and pin configurations.

offset. The selection criteria for an appropriate op amp should

include the input bias current, output voltage swing, distortion,

and noise specification. Note that in this example the overall

signal phase is inverted. To reestablish the original signal

connections.

SINGLE-ENDED-TO-DIFFERENTIAL

CONFIGURATION (TRANSFORMER-COUPLED)

In order to select the best suited interface circuit for the

ADS805, the performance requirements must be known. If

an ac-coupled input is needed for a particular application, the

next step is to determine the method of applying the signal;

either single-ended or differentially. The differential input

configuration may provide a noticeable advantage of achiev-

ing good SFDR performance based on the fact that, in the

differential mode, the signal swing can be reduced to half of

the swing required for single-ended drive. Secondly, by

driving the ADS805 differentially, the even-order harmonics

will be reduced. Figure 3 shows the schematic for the

suggested transformer-coupled interface circuit. The resistor

One application example that will benefit from the differential

input configuration is the digitization of IF signals. The wide

track-and-hold input bandwidth makes the ADS805 well

suited for IF down conversion in both narrow and wideband

applications. The ADS805 maintains excellent dynamic per-

formance in multiple Nyquist regions covering a variety of IF

frequencies (see the Typical Characteristics). Using the

ADS805 for direct IF conversion eliminates the need of an

analog mixer along with subsequent functions like amplifiers

and filters, thus reducing system cost and complexity.

V

IN

2Vp-p

0

+1V

–1V

R

F

R

IN

NOTE: R

+V

S

+V

S

OPA691

ADS805

R

S

50

Ω

R

3

2k

Ω

22pF

10

µF

+

+2.5V

R

2

R

1

R

4

2k

Ω

IN

IN

REFB

(+1V)

V

REF

SEL

REFT

0.1

µF

0.1

µF