LTC5585_15 datasheet(25/38 Pages) LINER

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LTC5585

5585fb

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APPLICATIONS INFORMATION

Care should be taken when the demodulator’s outputs

are DC coupled to the external load to make sure that the

I/Q mixers are biased properly. If the current drain from

the outputs exceeds about 6mA, there can be significant

degradationofthelinearityperformance.Keepingthecom-

monmodeoutputvoltageofthedemodulatorabove3.15V,

with a 5V supply, will ensure optimum performance. Each

output can sink no more than 30mA when the outputs are

connected to an external load with a DC voltage higher

than VCC – 1.5V.

In order to achieve the best IIP2 performance, it is im-

portant to minimize high frequency coupling among the

baseband outputs, RF port, and LO port. Although it may

increase layout complexity, routing the baseband output

tracesonthebacksideofthePCBcanimproveuncalibrated

IIP2 performance. Figure 19 shows the alternate layout

having the baseband outputs on the backside of the PCB.

Analog Control Voltage Pins

Figure 20 shows the equivalent circuit for the DCOI, DCOQ,

IP2I, and IP2Q pins. Internal temperature compensated

62.5μAcurrentsourceskeepthesepinsbiasedatanominal

500mV through 8k resistors. A low impedance voltage

source with a source resistance of less than 200Ω is

recommended to drive these pins.

Figure 19. Alternate Layout of PCB with

Baseband Outputs on the Backside

Figure 20. Simplified Schematic of the Interface for the

DCOI, DCOQ, IP2I and IP2Q Pins

Figure 21. Simplified Schematic of the REF Pin Interface

GND

5585 F20

VCC

DCOI, DCOQ,

IP2I, IP2Q

62.5µA

LTC5585

GND

5585 F21

VCC

REF

250µA

LTC5585

As shown in Figure 21, the REF pin is similar to the DCOI

pin, but the bias current source is 250µA, and the inter-

nal resistance is 2k. If this pin is left disconnected, it will

self-bias to 500mV. A low impedance voltage source with

a source resistance of less than 200Ω is recommended

to drive this pin. The control voltage range of the DCOI,

DCOQ, IP2I and IP2Q pins is set by the REF pin. This range

is equal to 0V to twice the voltage on the REF pin, whether

internally or externally applied.

It is recommended to decouple any AC noise present on

the signal lines that connect to the analog control-voltage

inputs. A shunt capacitor to ground placed close to these

pins can provide adequate filtering. For instance, a value

of 1000pF on the DCOI, DCOQ, IP2I and IP2Q pins will

provide a corner frequency of around 6 to 7MHz. A similar

corner frequency can be obtained on the REF pin with a

value of 3900pF. Using larger capacitance values such as

0.1µFisrecommendedonthesepinsunlessafastercontrol

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