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11/20/01

SPT7866

Figure 1 – Typical Interface Circuit

TYPICAL INTERFACE CIRCUIT

REFERENCES

The SPT7866 has a differential analog input. The input

around 2.5 V. The input range can be defined between

±0.6 V and ±1.5 V. An internal reference exists, providing

references should be bypassed as close to the converter

pins as possible using 100 nF capacitors in parallel with

smaller capacitors (e.g. 220 pF) to ground.

ANALOG INPUT

The input of the SPT7866 can be configured in various

ways, dependent upon whether a single-ended or differen-

tial, AC- or DC-coupled input is wanted.

AC-coupled input is most conveniently implemented using

a transformer with a center-tapped secondary winding.

figure 1. In order to obtain low distortion, it is important that

the selected transformer does not exhibit core saturation

at full scale. Excellent results are obtained with the Mini-

Circuits T1-6T or T1-1T. Proper termination of the input is

important for input signal purity. A small capacitor (typically

68 pF) across the inputs attenuates kickback noise from

the sample-and-hold. A small capacitor (1 nF) between

geous.

If a DC-coupled, single-ended input is wanted, a solution

based on operational amplifiers, as shown in figure 2, is

usually preferred. The AD8138 is suggested for low distor-

tion and video bandwidth. Lower cost operational amplifi-

ers may be used if the demands are less strict.

Figure 2 – DC-Coupled, Single-Ended to Differential

Conversion (power supplies and bypassing

not shown)

Analog

In

Input

Offset

51 W

AD8138

470 W

470 W

100 W

51 W

100 W

15 pF

ADC

51 W

51 W

470 W

470 W

470 W

AD8138

AD8138

Mini-Circuits

T1–6T or

T1–1T

50 W

OR

D9

D8

D7

D6

D5

D4

D3

D2

D1

D0

(MSB)

(LSB)

Sleep

Sleep

0.1

10

+

EXT/INT

T1

Ext

+A5

0.1

10

+

0.1

10

+

50 W

T2

+A5

1 kW

1 kW

50 W

+A5

AGND

0.1

10

+

+A3/5

AGND

0.1

10

+

DGND

+D3/5

+D3/5

Buffer

Ferrite Bead

SPT7866