6

REV. 1B October 2003

DATA SHEET

SPT7851

Analog Input

The input of the SPT7851 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.

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 impor-

tant 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 amplifiers)

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 > 75dB

• Gain bandwidth product > 50MHz

• Total Harmonic Distortion ≤ –75dB

• Signal-to-Noise Ratio > 75dB

Power Supplies and Grounding

The SPT7851 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 SPT7851 has a differential analog input. The voltages

applied to the VREF+ and VREF– pins determine the input

voltage range and are equal to ±(VREF+ – VREF–). This

voltage range will be symmetrical about the common mode

voltage. Externally generated reference voltages must be

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

cuit.) For best performance, these voltages should be sym-

metrical about the midpoint of the supply voltage.

Common Mode Voltage

Reference Circuit

The SPT7851 has an on-board common mode voltage refer-

ence circuit (VCM). It is typically one-half of the supply

voltage 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 applica-

tions, 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 oper-

ating 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 relation-

ship between the bias current and the bias voltage. Please

refer to the application note for more information.

Table I – Sample Rate Settings

Sample Rate (MHz)

Bias1 (µA)

Bias2 (µA)

130

3.0

550

6.0

10

70

7.5

20

90

9.5

R3

+

–

Input

Voltage

(±0.5V)

R2

+

–

–

+

51Ω

ADC

15pF

51Ω

R3

(R3)/2

R

R2

R

RR

R

51Ω

Figure 3: DC-coupled single-ended to differential

conversion (power supplies/bypassing not shown)