LTC6605CDJC-14-TRPBF datasheet(11/20 Pages) LINER

Part #	LTC6605CDJC-14-TRPBF

Description	Dual Matched 14MHz Filter with Low Noise, Low Distortion Differential Amplifi er

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Manufacturer	LINER [Linear Technology]

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LTC6605-14

11

660514f

APPLICATIONS INFORMATION

Setting the passband gain (GAIN = R2/R1) only requires

choosing a value for R1, since R2 is a ﬁxed internal 200Ω.

Therefore, the following three gains can be easily conﬁgured

without external components:

Table 1. Conﬁguring the Passband Gain Without External

Components

GAIN

(V/V)

GAIN (dB)

R1 (Ω)

INPUT PINS TO USE

1

0

200

Drive the 200Ω Resistors. Tie

the 100Ω Resisters Together.

2

6

100

Drive the 100Ω Resistors.

3

9.5

66.7

Drive the 200Ω and 100Ω

Resistors in Parallel.

The resonant frequency, fO, is independent of R1, and

therefore independent of the gain. For any LTC6605-14 ﬁlter

conﬁguration that conforms to Figure 3, the fO is ﬁxed at

16.1MHz. The f–3dB frequency depends on the combina-

tion of fO and Q. For any speciﬁc gain, Q is adjusted by

the selection of R4.

Setting the f–3dB Frequency

Using an external resistor (REXT), the f–3dB frequency is ad-

justable in the range of 12.4MHz to 20.0MHz (see Figure 3).

The minimum f–3dB is set for REXT equal to 0Ω and the

maximum f–3dB is arbitrarily set for a maximum passband

gain peak less than 1dB.

Table 2. REXT Selection GAIN = 1,

R1 = 200Ω, R4A = R4B = 100Ω

f–3dB (MHz)

REXT Ω

12.4

0

14

30.9

14.5

41.2

15

52.3

15.5

64.9

16

78.7

16.5

80.6

17

110

17.5

127

18

150

18.5

174

19

205

19.5

241

20

287

Figure 4 shows three filter configurations with an

f–3dB = 12.3MHz, without any external components. These

ﬁlters have a Q = 0.57, which is an almost ideal Bessel

characteristic with linear phase.

Figure 5 shows ﬁlter conﬁgurations that use some external

resistors, and are tailored for a very ﬂat passband.

Many other conﬁgurations are possible by using the equa-

tions in Figure 3. For example, external resistors can be

added to modify the value of R1 to conﬁgure GAIN ≠ 1. For

an even more ﬂexible ﬁlter IC with similar performance,

consider the LTC6601.

BIAS Pin

Each channel of the LTC6605-14 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 ﬁxed

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 conﬁgured for highest performance

(lowest noise and lowest distortion).

If the BIAS pin is ﬂoated (left unconnected), the part is in

a fully active state, but with ampliﬁer currents reduced and

performance scaled back to preserve power consumption.

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 ampliﬁer

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 signiﬁcant

voltage transients. Using this function to wire-OR outputs

together is not recommended.

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