ADCMP604 datasheet(25/34 Pages) AD

25 / 34 page

LTC6563

Rev. 0

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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

100

2 • RTEE

VCC

2 • RCM

4 TO

OPEN

OUT

TERM

LTC6563

6563 F10

RCM

RADC

200

100

OUT

TERM

LTC6563

VCC

2 TO

OPEN

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