ADS804

9

SBAS068B

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DC voltage differences between the IN and IN inputs of the

ADS804 effectively will produce an offset, which can be

The bias current of the op amp may also result in an

undesired offset. The selection criteria of the 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 re-establish

the original signal polarity it is always possible to interchange

the IN and IN connections.

SINGLE-ENDED-TO-DIFFERENTIAL

CONFIGURATION (TRANSFORMER COUPLED)

In order to select the best suited interface circuit for the

ADS804, 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 ADS804 differentially, the even-order harmonics

will be reduced. Figure 3 shows the schematic for the

suggested transformer-coupled interface circuit. The resistor

REFERENCE OPERATION

Integrated into the ADS804 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

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

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.

FIGURE 3. Transformer-Coupled Input.

V

IN

IN

IN

CM

22

Ω

22

Ω

100pF

R

T

100pF

+

4.7µF

0.1µF

ADS804

1:n

0.1µF

R

G

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

2Vp-p

0

+1V

–1V

R

F

R

IN

NOTE: R

+V

S

ADS804

R

S

24.9

Ω

2k

Ω

100pF

10

µF

+

+2.5V

R

2

R

1

2k

Ω

IN

IN

REFB

(+1V)

V

REF

SEL

REFT

0.1

µF

0.1

µF

OPA691

+V

S