SPT7936 datasheet(6/8 Pages) CADEKA

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SPT7936

ANALOG INPUT

The SPT7936 has a differential input that should have a

common mode voltage of +1.5 V. The input voltage range is

determined by the reference voltages which may be gener-

ated internally or applied externally.

The input of the SPT7936 can be configured in various ways

depending on if a single-ended or differential, AC- or DC-

coupled input is desired.

The AC coupled input is most conveniently implemented

using a transformer with a center tapped secondary winding.

The center tap is connected to the VCM pin as shown in

figure 2. To obtain low distortion, it is important that the

selected transformer does not exhibit core saturation at the

full-scale voltage. 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 across the

inputs attenuates kickback noise from the internal sample

and hold.

Figure 3 illustrates a solution (based on operational amplifi-

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

desired. The selection criteria of the buffer op-amps is as

follows:

- Open loop gain >75 dB

- Gain bandwidth product >50 MHz

- Total harmonic distortion

≤-75 dB

- Signal to Noise ratio >75 dB

REFERENCES

The SPT7936 can use either an internal or external voltage

reference. When the digital input EXTREF is high, the exter-

nal reference is used. When EXTREF is low, the internal

reference is used.

INTERNAL REFERENCE

The internal references are set at +1.0 V and +2.0 V. When

the internal reference is used, the full-scale range of the

analog input is set at

±1.0 V differential. Do not connect

external references when the internal reference is used.

EXTERNAL REFERENCE

When external references are used, the voltages applied to

the VREF+ and VREF- pins determine the input voltage range

which is equal to

±(VREF+ - VREF-). Externally generated

reference voltages must be connected to these pins and

should be symmetric about the common mode voltage. (See

figure 2, Typical Interface Circuit.)

COMMON MODE OUTPUT VOLTAGE

REFERENCE CIRCUIT

The SPT7936 has an on-board common mode voltage refer-

ence circuit (VCM). It is set at +1.5 V and can drive loads of up

to 20

µA. This circuit is commonly used to drive the center tap

of the RF transformer in fully differential applications. For

single-ended applications, this output can be used to provide

the level shifting required for the single-to-differential con-

verter conversion circuit.

BIAS CIRCUITS

The best AC performance is achieved when the bias currents

are optimized for the selected sample rate. Two digital input

pins are provided to control the optimum internal bias cur-

rents. Table I shows the settings for Bias 0 and Bias 1 at

selected frequencies.

Table I - Frequencies for Biases 0 and 1

Typical Power

Dissipation

Bias 1 Bias 0 Description

Ext. Ref.

Int. Ref.

Sleep mode (power save)*

25 mW

36 mW

≤5 MHz sampling

61 mW

73 mW

≤20 MHz sampling

172 mW

184 mW

≤28 MHz sampling

248 mW

260 mW

*Clock = 28 MHz

Figure 3 - DC-Coupled Single-Ended-to-Differential

Conversion (Power Supplies are Not

Shown)

VCM

(R3)/2

Input

Voltage

(

±0.5 V)

Ω

ADC

VIN+

VIN–

15 pF

Ω

POWER SUPPLIES AND GROUNDING

The SPT7936 is operated from a single power supply in the

range of 3.0 to 3.6 volts. Nominal operation is suggested to

be 3.3 volts. All power supply pins should be bypassed 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.

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