LTC6605-10
11
660510f
APPLICATIONS INFORMATION
Setting the passband gain (GAIN = R2/R1) only requires
choosing a value for R1, since R2 is a fi xed internal 400Ω.
Therefore, the following three gains can be easily confi gured
without external components:
Table 1. Confi guring the Passband Gain Without External
Components
GAIN
(V/V)
GAIN (dB) R1 () INPUT PINS TO USE
1 0 400 Drive the 400Ω Resistors. Tie
the 100Ω Resisters Together.
4 12 100 Drive the 100Ω Resistors.
5 14 80 Drive the 400Ω and 100Ω
Resistors in Parallel.
The resonant frequency, f
O
, is independent of R1, and
therefore independent of the gain. For any LTC6605-10
fi lter confi guration that conforms to Figure 3, the f
O
is
fi xed at 11.36MHz. The f
–3dB
frequency depends on the
combination of f
O
and Q. For any specifi c gain, Q is adjusted
by the selection of R4.
Setting the f
–3dB
Frequency
Using an external resistor (R
EXT
), the f
–3dB
frequency is ad-
justable in the range of 9.7MHz to 14.0MHz (see Figure 3).
The minimum f
–3dB
is set for R
EXT
equal to 0Ω and the
maximum f
–3dB
is arbitrarily set for a maximum passband
gain peak less than 1dB.
Table 2. R
EXT
Selection GAIN = 1,
R1 = 400Ω, R4A = R4B = 100Ω
f
–3dB
(MHz) R
EXT
9.7 0
10 5.11
10.5 13.3
11 22.1
11.5 31.6
12 41.2
12.5 52.3
13 64.9
13.5 80.6
14 97.6
Figure 4 shows three filter configurations with an
f
–3dB
= 9.7MHz, without any external components. These
fi lters have a Q = 0.61, which is an almost ideal Bessel
characteristic with linear phase.
Figure 5 shows three fi lter confi gurations that use some
external resistors, and are tailored for a very fl at ±0.7dB
11.2MHz passband.
Many other confi gurations are possible by using the equa-
tions in Figure 3. For example, external resistors can be
added to modify the value of R1 to confi gure GAIN ≠ 1. For
an even more fl exible fi lter IC with similar performance,
consider the LTC6601.
BIAS Pin
Each channel of the LTC6605-10 has a BIAS pin whose
function is to tailor both performance and power. The BIAS
pin can be modeled as a voltage source whose potential
is 1.15V above the V
–
supply and that has a Thevenin
equivalent resistance of 150k. This three-state pin has fi xed
logic levels relative to V
–
(see the Electrical Characteristics
table), and can be driven by any external source that can
drive the BIAS pin’s equivalent input impedance.
If the BIAS pin is tied to the positive supply, the part is
in a fully active state confi gured for highest performance
(lowest noise and lowest distortion).
If the BIAS pin is fl oated (left unconnected), the part is in a
fully active state, but with amplifi er currents reduced and
p e r f o r m a n c e s c a l e d b a c k t o p r e s e r v e p o w e r c o n s u m p t i o n .
Care should be taken to limit external leakage currents
to this pin to under 1μA to avoid putting the part in an
unexpected state.
If the BIAS pin is tied to the most negative supply (V
–
),
the part is in a low power shutdown mode with amplifi er
outputs disabled. In shutdown, all internal biasing current
sources are shut off, and the output pins each appear as
open collectors with a non-linear capacitor in parallel and
steering diodes to either supply. Because of the non-linear
capacitance, the outputs can still sink and source small
amounts of transient current if exposed to signifi cant
voltage transients. Using this function to wire-OR outputs
together is not recommended.
