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LTC6603

6603f

APPLICATIONS INFORMATION

than 100k. If the value of RBIAS is too large, the ﬁlters will

not receive a large enough bias current, possibly causing

errors due to insufﬁcient settling. Be sure to obey the

absolute maximum speciﬁcations when driving a clock

into CLKIO (Pin 15).

Input Common Mode and Differential Voltage Range

The input signal range extends from zero to the V+IN

supply voltage. This input supply can be tied to V+A and

V+D, or driven up to 5.5V for increased input signal range.

Figure 9 shows the distortion of the ﬁlter versus common

mode input voltage with a 2VP-P differential input signal

(V+IN = 5V).

control bits LPF1 and LPF0. The differential input imped-

ance is a function of the clock frequency and the control

bits LPF1, LPF0, GAIN1 and GAIN0. Table 5 shows the

typical input impedances for a clock frequency of 80MHz.

These input impedances are all proportional to 1/fCLK, so

if the clock frequency were reduced by half to 40MHz,

the impedances would be doubled. The typical variation

in dynamic input impedance for a given clock frequency

is –20% to +35%.

Table 5. Differential, Common Mode Input Impedances,

fCLK = 80MHz

GAIN1

GAIN0

LPF1

LPF0

Differential

Input Impedance

(kΩ)

Common Mode

Input Impedance

(kΩ)

2.5

9.5

2.5

5.4

1.9

5.2

2.8

1.6

Output Common Mode and Differential Voltage Range

The output voltage is a fully differential signal with a

common mode level equal to the voltage at VOCM. Any of

the ﬁlter outputs may be used as single-ended outputs,

although this will degrade the performance. The output

voltage range is typically 0.5V to V+A – 0.5V (V+A = 2.7V

to 3.6V).

The common mode output voltage can be adjusted by

overdriving the voltage present on VOCM. To maximize

the undistorted peak-to-peak signal swing of the ﬁlter,

the VOCM voltage should be set to V+A/2. Note that the

output common mode voltages of the two channels are

Figure 9. Distortion vs Common Mode Input Voltage (5V)

COMMON MODE INPUT VOLTAGE (V)

1.0

–60

–70

–80

–90

3.0

2.0

4.0

6603 F09

5.0

2.5

4.5

1.5

3.5

RBIAS = 30.9k, VS = 3V, V+IN = 5.5V

LPF1 = 1, BW = 2.5MHz, GAIN = 24dB

VOUT = VP-P, TA = 25°C

HD3, f = 1MHz

HD3, f = 200kHz

For best performance, the inputs should be driven dif-

ferentially. For single ended signals, connect the unused

input to VOCM (Pin 3) or to a quiet DC reference voltage.

To achieve the best distortion performance, the input

signal should be centered around the DC voltage of the

unused input.

Refer to the Typical Performance Characteristics section

to estimate the distortion for a given input level.

Dynamic Input Impedance

The unique input sampling structure of the LTC6603

has a dynamic input impedance which depends on the

conﬁguration and the clock frequency. This dynamic

input impedance has both a differential component and

a common mode component. The common mode input

impedance is a function of the clock frequency and the

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