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LTC6754 Datasheet(PDF) 23 Page - Analog Devices |
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LTC6754 Datasheet(HTML) 23 Page - Analog Devices |
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23 / 34 page  LTC6563 23 Rev. 0 For more information www.analog.com APPLICATIONS INFORMATION In applications where the LTC6563 is to drive an ADC, this is useful in matching the TIA output(s) to the ADC analog input CM. This effectively eliminates the need for a separate ADC-driver which saves cost, power, as well as board complexity and real estate. The CM servo loop operates by varying the CM CTRL vari- able resistor internal to the LTC6563 such that VCMO, aver- age of VOUT and VOUTBAR voltages or (VOUT + VOUTBAR)/2, matches the voltage applied to the CM. The loop will NOT function unless two conditions are met: 1. PWRMD must be set HI to activate the CM circuit. 2. OUT and OUT must have a DC connection to TERM and TERM respectively. The internal 50Ω resistor(s) on the TERM and TERM pins provide a current path for CM servo functionality and serve as the load for the LTC6563’s current mode output. Driving 50Ω terminated transmission lines or other impedance-matched loads is easily done using these internal resistors. When using the output MUXing capability with multiple LTC6563’s, such as the schematic shown in Figure 6, only one pair of TERM and TERMBAR pins should be connected to the OUT and OUT. The device with the connected TERM and TERM will then set the CM for the MUX’ed devices. Determining the CM Range CM servo loop’s voltage range is easily calculated using Ohm’s law, as the output circuit can be described as a pair of current sources and a resistor network (see Figure 8). The amount of current supplied by the OUT and OUT cur- rent sources is determined by the ADJ setting, and the impedance at the output pins is determined by the internal and external resistor networks. These factors will deter- mine the range of possible DC voltages at the outputs. Figure 7. Typical Connection for CM Control/ADC Driving 4.99 4.99 4.99 4.99 4.99 4.99 4pF 4pF 4pF 100 100 0.1µF 50 TRANSMISSION LINE 200 DIFF INPUT 6563 F07 R R OMUX OUT OUT TERM TERM PWRMD OUTPUT STAGE TILT ADJ0 ADJ1 BIAS CTRL 50 50 IO CTRL IO CTRL VCC LTC6563 OUTPUT SECTION CM 0.1µF CM ×2 ×1 OUTPUT CLAMPS LO HI CM CONROL LOOP HI CTRL + – + – 50 TRANSMISSION LINE AD9094 VCM I1 I2 50 50 RCM RADC 200 R1 100 R2 100 OUT OUT TERM TERM LTC6563 6563 F08 VCC VCC 2 TO OPEN Figure 8. Equivalent Circuit for Driving an ADC with no GND (CM Referenced) External Load |
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