LT5575

14

5575f

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 6mA, there can be significant degrada-

tion of the linearity performance. Each output can sink no

more than 16.8mA when the outputs are connected to an

The I/Q output equivalent circuit is shown in Figure 8.

In order to achieve best IIP2 performance, it is important

to minimize high frequency coupling among the baseband

outputs, RF port and LO port. For a multilayer PCB layout

design, the metal lines of the baseband outputs should be

placed on the backside of the PCB as shown in Figures 2

and 3. Typically, output shunt capacitors C1-C4 are not

required for the application near 1900MHz. However, for

other frequency bands, these capacitors can be optimized

for best IIP2 performance. For example, when the oper-

ating frequency is 900MHz, the IIP2 can be improved to

54dBm or better when 10pF shunt capacitors are placed

at each output.

Enable Interface

A simplified schematic of the EN pin is shown in Fig-

ure 9. The enable voltage necessary to turn on the LT5575

is 2V. To disable or turn off the chip, this voltage should

be below 1V. If the EN pin is not connected, the chip is

disabled. However, it is not recommended that the pin be

left floating for normal operation.

It is important that the voltage applied to the EN pin

the supply current may be sourced through the upper

ESD protection diode connected at the EN pin. Under no

circumstances should voltage be applied to the EN pin

occurs, damage to the IC may result.

Figure 9. Enable Pin Simplified Circuit

5

EN

5575 F09

60k

60k

LT5575