Datasheet

LTC6431-15
12
643115f
applicaTions inForMaTion
supply should also be applied to both of the V
CC
pins on
the device. A suggested parallel 60pF, 350Ω network has
been added to the input to ensure low frequency stability.
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 layouts using
these exposed pad packages. The maximum allowable
number of minimum diameter via holes should be placed
underneath the exposed pad and connect 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 backside of the PCB to prevent
the solder from wicking away from the critical PCB to the
exposed pad interface.
The LTC6431-15 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.
Low Frequency Stability
Most RF gain blocks suffer from low frequency instability.
To avoid any stability issues, the LTC6431-15 has an internal
feedback network that lowers the gain and matches the
input and output impedances at frequencies above 20MHz.
This feedback network contains a series capacitor, so if at
some low frequency the feedback fails, the gain increases
and gross impedance mismatches occur—indeed a recipe
for instability. Luckily, this situation is easily resolved with
a parallel capacitor and resistor network on the input, as
seen in Figure 1. 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
LTC6431-15 is preceeded by a low frequency termination,
such as a choke, the input stability network is NOT required.
Test Circuit
The test circuit shown in Figure 2 is designed to allow
evaluation of the LTC6431-15 with standard single-ended
50Ω test equipment. The circuit requires a minimum of
external components. Since the LTC6431-15 is a wideband
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