ADCMP604 datasheet(23/34 Pages) AD

23 / 34 page

LTC6563

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

4pF

100

0.1µF

50 TRANSMISSION LINE

200 DIFF INPUT

6563 F07

OMUX

OUT

TERM

PWRMD

OUTPUT

STAGE

TILT

ADJ0

ADJ1

BIAS

CTRL

IO CTRL

VCC

LTC6563

OUTPUT SECTION

0.1µF

×2

×1

OUTPUT

CLAMPS

CM CONROL

LOOP

HI CTRL

–

50 TRANSMISSION LINE

AD9094

VCM

RCM

RADC

200

100

OUT

TERM

LTC6563

6563 F08

VCC

2 TO

OPEN

Figure 8. Equivalent Circuit for Driving an ADC with

no GND (CM Referenced) External Load

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