CLC5957PCASM datasheet(7/12 Pages) NSC

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Analog Inputs and Bias

Figure 1 depicts the analog input and bias scheme. Each

of the differential analog inputs are internally biased to a

nominal voltage of 2.40 volts DC through a 500

Ω resistor

to a low impedance buffer. This enables a simple

interface to a broadband RF transformer with a center-

tapped output winding that is decoupled to the analog

ground. If the application requires the inputs to be DC

coupled, the Vcm output can be used to establish the

proper common -mode input voltage for the ADC. The

Vcm voltage reference is generated from an internal

bandgap source that is very accurate and stable.

Figure 1: CLC5957 Bias Scheme

The Vcm output may also be used to power down the

ADC. When the Vcm pin is pulled above 3.5V, the internal

bias mirror is disabled and the total current is reduced to

less than 10mA. Figure 2 depicts how this function can

be used. The diode is necessary to prevent the logic gate

from altering the ADC bias value.

Figure 2: Power Shutdown Scheme

ENCODE Clock Inputs

The CLC5957’s differential input clock scheme is

compatible with all commonly used clock sources.

Although small differential and single-ended signals are

adequate, for best aperture jitter performance a low noise

differential clock with a high slew rate is preferred. As

depicted in Figure 3, both ENCODE clock inputs are

internally biased to VCC/2 though a pair of 5KΩ resistors.

The clock input buffer operates with any common-mode

voltage between the supply and ground.

Figure 3: CLC5957 ENCODE Clock Inputs

The internal bias resistors simplify the clock interface to

another center-tapped transformer as depicted in Figure

4. A low phase noise, RF synthesizer of moderate ampli-

tude (1 - 4Vpp) can drive the ADC through this interface.

Figure 4: Transfer Coupled Clock Scheme

Vcm

1.23V

Bandgap

Reference

ADC

Bias Mirror

Ω

To T/H

and ADC

Ain

2.4V

–

500

Ω

500

Ω

Vref

5V CMOS

"1" = on

"0" = off

CLC5957

BJT Current Mirror

VCCA

GNDA

ENC

Ω

ENC

CLC5957

CLC5957 Applications

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