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CLC5957MTD Datasheet(PDF) 7 Page - National Semiconductor (TI) |
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CLC5957MTD Datasheet(HTML) 7 Page - National Semiconductor (TI) |
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7 / 12 page  7 http://www.national.com 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 2K Ω To T/H and ADC Ain Ain 2.4V – + - 500 Ω 500 Ω Vref 5V CMOS "1" = on "0" = off CLC5957 BJT Current Mirror VCCA GNDA ENC ENC 5k Ω 5k Ω 5k Ω 5k Ω ENC ENC CLC5957 ~ CLC5957 Applications |
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