AD8627/AD8626/AD8625

Rev. B | Page 16 of 20

AD5551/AD5552

AD8627

DGND

*AD5552 ONLY

OUT

SCLK

DIN

CS

AGND

5V

2.5V

UNIPOLAR

OUTPUT

LDAC*

0.1

10

0.1

SERIAL

INTERFACE

5V

Figure 46. Unipolar Output

In applications with full 4-quadrant multiplying capability or a

bipolar output swing, the circuit in Figure 47 can be used. In

this circuit, the first and second amplifiers provide a total gain

of 2, which increases the output voltage span to 20 V. Biasing the

external amplifier with a 10 V offset from the reference voltage

results in a full 4-quadrant multiplying circuit.

ONE CHANNEL

AD5544

1/2

AD8626

DIGITAL INTERFACE CONNECTIONS

OMITTED FOR CLARITY

ADR01

VREF

10V

1/2

AD8626

–13V

+13V

10k

Ω

5k

Ω

10k

Ω

Figure 47. 4-Quadrant Multiplying Application Circuit

EIGHT-POLE SALLEN KEY LOW-PASS FILTER

The AD862x’s high input impedance and dc precision make it a

great selection for active filters. Due to the very low bias current

of the AD862x, high value resistors can be used to construct low

frequency filters. The AD862x’s picoamp-level input currents

contribute minimal dc errors. Figure 49 shows an example, a

10 Hz, 8-pole Sallen Key filter constructed using the AD862x.

Different numbers of the AD862x can be used depending on

the desired response, which is shown in Figure 48. The high

value used for R1 minimizes interaction with signal source

resistance. Pole placement in this version of the filter minimizes

the Q associated with the lower pole section of the filter. This

eliminates any peaking of the noise contribution of resistors in

the preceding sections, minimizing the inherent output voltage

noise of the filter.

V2

V4

V3

V1

FREQUENCY (Hz)

0.1

0

0.4

0.8

1.2

1

10

100

1k

Figure 48. Frequency Response Output at Different Stages

of the Low-Pass Filter