SPT7866 datasheet(5/8 Pages) CADEKA

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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

range is determined by the voltages VIN and VIN applied to

reference pins REFH and REFL respectively, and is equal

to ±(VIN–VIN). Externally generated reference voltages

connected to REFH and REFL should be symmetric

around 2.5 V. The input range can be defined between

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

reference voltages at pins REFH and REFL equal to +3.0 V

(VREFH) and +2.0 V (VREFL). These can be connected to

REFH and REFL by connecting pin EXT/INT to GND. The

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.

The center tap is connected to the VCM node, as shown in

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

VCM and ground has also been proven to be advanta-

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

Input

Offset

51 W

AD8138

470 W

100 W

51 W

100 W

15 pF

ADC

VIN

51 W

470 W

AD8138

AIN

Mini-Circuits

T16T or

T11T

RT1

50 W

(MSB)

(LSB)

VIN

VCM

Sleep

0.1

VIN

EXT/INT

Ext

+A5

0.1

REFL

REFH

RT2

50 W

+A5

1 kW

CLKIN

50 W

+A5

AGND

0.1

+A3/5

AGND

0.1

DGND

+D3/5

Buffer

Ferrite Bead

SPT7866

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