MAX2101CMQ Ver la hoja de datos (PDF) - Maxim Integrated

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MAX2101CMQ

6-Bit Quadrature Digitizer

Maxim Integrated 

6-Bit Quadrature Digitizer

FILTER CUTOFF FREQUENCY

vs. FTUNE

30

TA = +25°C

25

20

15

10

5

1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8

FTUNE (V)

Figure 6. Typical Filter Cutoff Frequency vs. FTUNE Input

Voltage

The MAX2101 provides temperature-compensated bias

voltages that, when scaled and summed with the user-

supplied filter-control signal, provide the necessary

compensation for the filters. The filter-control signal can

originate in one of two forms: an analog current, or an

analog voltage. The temperature compensation signal

will be added to the control signal as discussed below.

Voltage Drive

A suggested technique of filter drive uses a voltage

source, such as a voltage output DAC. The tempera-

ture compensation signals, VPTAT and VREF, are shift-

ed and scaled, then summed with the control voltage,

and the sum is applied to the FTUNE inputs. See Figure

8 for a possible implementation.

The transfer function for Figure 8’s voltage drive config-

uration can be evaluated as follows:

VTC

=

VREF

+

RF

R TC

(VREF

−

VPTAT )

VFTUNE

=

VSET

+

RF

R TC

(VREF

−

VPTAT )

Thus, the user-supplied signal VSET, which is character-

ized by a very small (ideally 0) temperature coefficient,

will be summed with a small signal (|VREF - | VPTAT ≤

200mV) whose temperature dependence compensates

for the filter’s TC.

FILTER CUTOFF FREQUENCY

TEMPERATURE DEPENDENCE

2.00

fC = 15MHz

1.95

1.90

1.85

1.80

1.75

1.70

0 20 40 60 80 100 120 140

TEMPERATURE (°C)

Figure 7. Typical Filter Cutoff Frequency Temperature

Dependence

Current Drive

An alternate form of filter drive uses a current source,

such as a current-output DAC. The current is trans-

formed to the appropriate voltage via a transresistance

network, which will drive the FTUNE input(s). The tem-

perature compensation signals, VPTAT and VREF, are

shifted and scaled, transformed to current, added to

the user-supplied current, and the sum is transformed

back into the temperature compensated control voltage

(Figure 9).

Amplifier U1A generates a shifted reference signal,

VTC. VTC is transformed into a current through the

resistor RTC. RTC also scales this signal such that,

when compared to the feedback resistor RF, the proper

temperature dependence is added to the user-supplied

filter control current ISET to compensate for the TC of

the filter.

The expression for the final filter tune signal is

expressed as:

VFTUNE

= ISET(RF) +

RF

R TC

(VREF

−

VPTAT )

______________________________________________________________________________________ 13

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