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SPT7935

COMMON MODE VOLTAGE

REFERENCE CIRCUIT

The SPT7935 has an on-board common mode voltage refer-

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 converter conversion circuit.

BIAS CURRENT CIRCUITS

The bias currents suggested (Bias 1 and Bias 2 in figure 2)

optimize device performance for the stated sample rate of

20 MSPS. To achieve the best dynamic performance when

operating the device at sample rates other than 20 MSPS, the

bias current levels should be adjusted. Table I shows the

settings for Bias 1 and Bias 2 for selected sample rates. The

“Bias Voltage vs Bias Current” graphs on page 4 show the

relationship between the bias current and the bias voltage.

Table I – Sample Rate Settings

Sample Rate (MHz)

Bias 1 (

Bias 2 (

1

20

3.5

5

50

6.5

10

80

8.0

20

90

9.5

The AC coupled input is most conveniently implemented

using a transformer with a center tapped secondary winding.

figure 2. 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 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 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

POWER SUPPLIES AND GROUNDING

The SPT7935 is operated from a single power supply in the

range of 2.8 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.

REFERENCES

The SPT7935 has a differential analog input. The voltages

voltage range and are equal to

voltage range will be symmetric about the common mode

voltage. Externally generated reference voltages must be

connected to these pins. (See figure 2, Typical Interface

Circuit.) For best performance, these voltages should be

symmetrical about the midpoint of the supply voltage.

VCM

R3

(R3)/2

+

–

Input

Voltage

(

±0.5 V)

R3

R

R2

R2

+

–

–

+

R

RR

51

Ω

51

Ω

R

ADC

VIN+

VIN–

15 pF

51

Ω

Figure 3 – DC-Coupled Single Ended to Differential Conversion (Power Supplies and Bypassing are Not Shown)