9

ADS902

®

a high-pass filter. At the same time, the input impedance is

or LM6152. Depending on the selected amplifier, the use of

output, may considerably improve its distortion performance.

load to avoid gain peaking or even oscillation. It can also be

used to establish a defined bandwidth to reduce the wideband

noise. Its value is usually between 10

Ω and 100Ω.

DC-COUPLED INTERFACE CIRCUIT

Shown in Figure 5 is a single-supply, DC-coupled circuit

which can be set in a gain of –1V/V or higher. Depending on

voltage for the ADS902. With a +5V supply, the nominal

signal input range of the ADS902 is 1Vp-p, typically centered

around the common-mode voltage of +2.75V.

EXTERNAL REFERENCE

The ADS902 requires an external top and bottom reference

on pin 22 (REFT) and 24 (REFB). Internally those pins are

connected through a resistor ladder, which has a nominal

resistance of 4k

Ω (±15%). In order to establish a correct

voltage drop across the ladder the external reference circuit

must be able to supply typically 250

µA of current. With this

current the full-scale input range of the ADS902 is set

between +2.25V and +3.25V. In general, the voltage drop

across REFT and REFB determines the input full-scale

range (FSR) of the ADS902. Equation (4) can be used to

calculate the span.

FSR = REFT - REFB

(4)

Depending on the application several options exist how to

supply the external reference voltages to the ADS902 with-

out degrading the typical performance.

LOW-COST REFERENCE SOLUTION

The easiest way to achieve the required reference voltages is

to place the reference ladder of the ADS902 between the

to set the correct current through the ladder (see Figure 6).

However, depending on the desired full-scale swing and

supply voltage, different resistor values might be selected.

A

1

V

IN

R

F

+V

S

R

1

0.1µF

2k

Ω

2k

Ω

IN

CM

REFB

External

Bottom Reference

REFT

R

S

R

IN

R

2

22pF

C

2

ADS902

External

Top Reference

FIGURE 5. DC-Coupled, Single-Supply Interface Circuit.

1k

Ω

1k

Ω

LpBy

REFT

REFB

LnBy

+V

S

IN

CM

R

T

7k

Ω

1k

Ω

+2.25V

R

B

9k

Ω

1k

Ω

0.1µF

0.1µF

0.1µF

0.1µF

0.1µF

+5V

V

IN

ADS902

0.1µF

+2.75V

1k

Ω

0.1µF

L1

10µF

+3.25V

FIGURE 6. Low Cost Solution to Supply External Reference Voltages.