LTC6431-20

11

643120f

APPLICATIONS INFORMATION

useful bandwidth of the bypass capacitor. The suggested

bypass capacitor network consists of two capacitors: a

low value 1000pF capacitor to handle high frequencies

in parallel with a larger 0.1µF capacitor to handle lower

frequencies. Use ceramic capacitors of an appropriate

physical size for each capacitance value (e.g., 0402 for the

1000pF and 0805 for the 0.1µF) to minimize the equivalent

series resistance (ESR) of the capacitor.

Low Frequency Stability

Most RF gain blocks suffer from low frequency instability.

Toavoidanystabilityissues,theLTC6431-20hasaninternal

feedback network that lowers the gain and matches the

inputandoutputimpedancesatfrequenciesabove20MHz.

This feedback network contains a series capacitor so if at

some low frequency the feedback fails, the gain increases

andgrossimpedancemismatchesoccur—indeedarecipe

for instability. Luckily this situation is easily resolved with

a parallel capacitor and resistor network on the input as

seen in Test Circuit A. This network provides resistive loss

at low frequencies and is bypassed by the parallel capaci-

tor within the desired band of operation. However, if the

LTC6431-20 is preceded by a low frequency termination,

such as a choke, the stability network is NOT required.

Test Circuit

The test circuit shown in Figure 2 is designed to allow

evaluation of the LTC6431-20 with standard single-ended

50Ω test equipment. The circuit requires a minimum of

externalcomponents.SincetheLTC6431-20isawideband

part, the evaluation test circuit is optimized for wideband

operation. Obviously, for narrowband applications the

circuit can be further optimized. As mentioned earlier,

input and output DC blocking capacitors are required as

this device is internally biased for optimal operation. A

frequency appropriate choke and decoupling capacitors

are required to provide DC bias to the RF OUT node. A 5V

the device. A suggested parallel 60pF, 350Ω network has

been added to the input to ensure low frequency stabil-

ity. The 60pF capacitance can be increased to improve

low frequency (<150MHz) performance. However, the

designer needs to be sure that the impedance presented

at low frequency will not create instability

Please note that a number of DNC pins are connected on

the demo board. These connections are not necessary for

normal circuit operation.

Exposed Pad and Ground Plane Considerations

As with any RF device, minimizing ground inductance

is critical. Care should be taken with board layout using

these exposed pad packages. The maximum allowable

number of minimum diameter via holes should be placed

underneath the exposed pad and connected to as many

ground plane layers as possible. This will provide good

RF ground and low thermal impedance. Maximizing the

copper ground plane will also improve heat spreading

and lower inductance. It is a good idea to cover the via

holes with a solder mask on the back side of the PCB to

prevent solder from wicking away from the critical PCB

to exposed pad interface.

The LTC6431-20 is a wide bandwidth part but it is not

intended for operation down to DC. The lower frequency

cutoff (20MHz) is limited by on-chip matching elements.