Layout Considerations

The MAX9986 evaluation board can be a guide for your

board layout. Pay close attention to thermal design and

close placement of components to the IC. The MAX9986

package exposed paddle (EP) conducts heat from the

device and provides a low-impedance electrical connec-

tion to the ground plane. The EP MUST be attached to

the PC board ground plane with a low-thermal and elec-

trical impedance contact. Ideally, this is achieved by sol-

dering the backside of the package directly to a top

metal ground plane on the PC board. Alternatively, the

EP can be connected to an internal or bottom-side

ground plane using an array of plated vias directly below

the EP. The MAX9986 EV kit uses nine evenly spaced,

0.016in-diameter, plated through holes to connect the

EP to the lower ground planes.

Depending on the ground-plane spacing, large sur-

face-mount pads in the IF path may need to have the

ground plane relieved under them to reduce parasitic

shunt capacitance. The layout should be such to min-

mize the coupling from L1, L2, and L3.

Modifying the EV Kit

The RF and LO inputs are broadband matched, so

there is no need to modify the circuit for use anywhere

in the 815MHz to 995MHz RF range (960MHz to

1180MHz LO range).

Retuning for a different IF is as simple as scaling the

values of the IF pullup inductors up or down with fre-

quency. The IF output looks like 200

Ω differential in

parallel with a capacitor. The capacitance is due to the

combination of the IC, PC board, and external IF com-

ponents. The capacitance from the IC is approximately

1pF to ground (0.5pF differential), while that from the

PC board and external components is approximately

1pF to ground. The total 2pF of capacitance is resonat-

ed out at the frequency of interest by bias inductors L1

and L2. To determine the inductor value use the follow-

ing equation:

The IF output is tuned for operation at approximately

200MHz, so a 330nH inductor is used. For lower IF fre-

quency (i.e., larger component values), maintain the

component’s Q value at the cost of larger case size,

unless it is unavoidable.

The DC current of the device can be reduced but the

performance will be degraded. Reducing the current is

accomplished by increasing the values of R1 and R2.

Resistor R1 sets the current in the IF amplifier, whereas

R2 sets the current in the LO buffer that drives the

mixer core.

Setting R1 to 953

Ω and R2 to 619Ω results in IF and LO

currents of 130mA and 71mA, respectively. Approxi-

mately 21mA of additional current is used in other

circuits and cannot be reduced. To reduce these adjust-

able currents in half, double the values of R1 and R2.

Doing so would reduce the current to approximately

130mA but the gain and IP3 would drop approximately

0.3dB and 2.5dB, respectively. Some of the other per-

formance values will degrade or improve due to

reduced current.

Since the linearity of the device is a result of the cascad-

ed performance of the IF amplifier and the mixer, care-

fully choose the correct combination of R1 and R2 to

produce the highest IP3 at the lowest desired current.

f

LC

1

2

π

MAX9986 Evaluation Kit

4

_______________________________________________________________________________________

Component Suppliers

SUPPLIER

PHONE

WEBSITE

Coilcraft

800-322-2645

www.coilcraft.com

Digi-Key

800-344-4539

www.digikey.com

Johnson

507-833-8822

www.johnsoncomponents.com

Mini-Circuits

718-934-4500

www.minicircuits.com

Murata

770-436-1300

www.murata.com

Note: Indicate that you are using the MAX9986 when contacting these component suppliers.