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11/8/01

SPT7721

TYPICAL INTERFACE CIRCUIT

Very few external components are required to achieve the

stated device performance. Figure 3 shows the typical

interface requirements when using the SPT7721 in normal

circuit operation. The following sections provide descrip-

tions of the major functions and outline performance

criteria to consider for achieving the optimal device

performance.

ANALOG INPUT

The input of the SPT7721 can be configured in various

ways depending on whether a single-ended or differential

input is desired.

The AC-coupled input is most conveniently implemented

using a transformer with a center-tapped secondary wind-

figure 3. To obtain low distortion, it is important that the

selected transformer does not exhibit core saturation at

the full-scale voltage. Proper termination of the input is im-

portant for input signal purity. A small capacitor across the

input attenuates kickback noise from the internal track-

and-hold.

Figure 4 illustrates a solution (based on operational ampli-

fiers) that can be used if a DC-coupled single-ended input

is desired. It is very important to select op amps with a high

open-loop gain, a bandwidth high enough so as not to im-

pair the performance of the ADC, low THD, and high SNR.

INPUT PROTECTION

All I/O pads are protected with an on-chip protection

circuit. This circuit provides ESD robustness and prevents

latchup under severe discharge conditions without

degrading analog transmission times.

POWER SUPPLIES AND GROUNDING

The SPT7721 is operated from a single power supply in

the range of 4.75 to 5.25 volts. Normal operation is sug-

gested to be 5.0 volts. All power supply pins should be by-

passed as close to the package as possible. The analog

and digital grounds should be connected together with a

ferrite bead as shown in the typical interface circuit and as

close to the ADC as possible.

R3

(R3)/2

+

–

Input

Voltage

(±0.5 V)

R3

R

R2

R2

+

–

–

+

R

RR

51 W

51 W

R

ADC

15 pF

51 W

Figure 4 – DC-Coupled Single-Ended to Differential

Conversion (power supplies and bypassing

are not shown)

+A5

10

+

+D3/5

10

T1

Mini-Circuit

T1-6T

50

Interfacing

Logics

+D3/5

Mode

Select

Reset

Diff In

Clock

Diff In

FB

.01(2x)

.01(3x)

.01(3x)

SPT7721

.01

Notes:

1) FB = Ferrite bead. It must placed as close to the ADC as possible.

2) All inputs are internally biased:

a) DMode1 to GND through 100K

c) CLK, PD and Rest pins to GND through 100K

d) /CLK and /Reset pins to 1.5 V through 5K

3) All 0.01microfarad capacitors are surface mount caps. They must be

placed as close to the respective pin as possible

channel output

Figure 3 – Typical Interface Circuit