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LTC6754 Datasheet(PDF) 25 Page - Analog Devices |
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LTC6754 Datasheet(HTML) 25 Page - Analog Devices |
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25 / 34 page  LTC6563 25 Rev. 0 For more information www.analog.com APPLICATIONS INFORMATION Figure 10. TEE Resistor Loading (Left) and Its Equivalent CM Half-Circuit (Right) d. ADC FS diff voltage calculation = 90µA • 5.56kΩ = 500mVP-P e. IO CM/Average Output Current (from Table 1): 7mA f. VCMO (minimum) = IO • (50Ω + 2 • 2Ω) = 380mV where IO = 7mA, 50Ω is the TERM internal resistance, and the 2Ω is the internal RCM minimum value The data in Table 5 can be used to examine the real-world problem of driving the AD9094 ADC, the input require- ments of which are shown below: Differential Input Voltage Range: 1.44VP-P to 2.16VP-P (programmable) Common Mode Voltage: 1.43V As can be seen from Table 5, operating the LTC6563 with ADJ 10 falls in the range of the required AD9094 FS swing voltage (1.44VP-P to 2.16VP-P), and also complies with the required AD9094 CM voltage (1.43V) by allowing an output CM voltage as low as 1.13V. Therefore, ADJ 10 is an appropriate setting for driving the AD9094. Detailed Example 2: Lowering the CM Voltage with a Tee Resistor or External DC Termination to Ground Adding a path to ground from the center of the load resis- tors (R1, and R2) can lower the CM voltage. This can be in the form of grounding the middle of R1, and R2, or alternatively adding a TEE resistor to ground to avoid dropping the CM voltage too low. This technique (shown in Figure 10) allows for more flexibility in aligning the TIA to an ADC or a given differential load and ADJ setting. The TEE resistor will appear as double its value in the half-circuit. The equivalent CM load and CM voltage can be written as: VCMO = IO • [(50Ω + 2RCM)||(R1 + 2RTEE)] (3) Note that adding RTEE to the schematic only affects the CM parameters of the circuit and has no bearing on the differential voltage swing and the resulting transimped- ance gain of the circuit. A shunt capacitor to ground across RTEE can be added to ensure no spurious AC signal appears at this node. For the particular case of Figure 7 schematic modified to include RTEE = 20Ω with ADJ = 11 (IO = 28mA from Table 1), the AD9094 CM voltage requirement (1.43V) would be easily satisfied since the CM servo loop would be able to adjust the output CM from 1.1V minimum (with RCM = 2Ω, previously 1.51V minimum from Table 5 with- out RTEE) to >2.3V (device output compliance volage) maximum using Equation 3. Operation without Internal CM Servo Loop There may be applications where the CM servo loop func- tion built into the LTC6563 is not needed, as would be the case if the TIA output is not used to drive an ADC. RTEE R1 100 2 • RTEE VCC I1 50 2 • RCM 4 TO OPEN OUT TERM LTC6563 6563 F10 I1 I2 50 50 RCM RADC 200 R1 100 R2 100 OUT OUT TERM TERM LTC6563 VCC VCC 2 TO OPEN |
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Similar Description - LTC6754 |
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