參數(shù)資料
型號(hào)： MAX038CWP
廠商： MAXIM INTEGRATED PRODUCTS INC
元件分類(lèi)： 運(yùn)動(dòng)控制電子
英文描述： High-Frequency Waveform Generator
中文描述： SQUARE; TRIANGULAR; SINE; SAWTOOTH; PULSE, 20 MHz, WAVEFORM GENERATION, PDSO20
封裝： SOP-20
文件頁(yè)數(shù)： 10/16頁(yè)
文件大?。?/td> 203K
代理商： MAX038CWP
M
When the MAX038’s frequency is controlled by a volt-
age source (V
IN
) in series with a fixed resistor (R
IN
), the
output frequency is a direct function of V
IN
as shown in
the above equations. Varying V
IN
modulates the oscilla-
tor frequency. For example, using a 10k
resistor for
R
IN
and sweeping V
IN
from 20mV to 7.5V produces
large frequency deviations (up to 375:1). Select R
IN
so
that I
IN
stays within the 2μA to 750μA range. The band-
width of the IIN control amplifier, which limits the modu-
lating signal’s highest frequency, is typically 2MHz.
IIN can be used as a summing point to add or subtract
currents from several sources. This allows the output
frequency to be a function of the sum of several vari-
ables. As V
IN
approaches 0V, the I
IN
error increases
due to the offset voltage of IIN.
Output frequency will be offset 1% from its final value
for 10 seconds after power-up.
FADJ Input
The output frequency can be modulated by FADJ ,
which is intended principally for fine frequency control,
usually inside phase-locked loops. Once the funda-
mental, or center frequency (F
o
) is set by I
IN
, it may be
changed further by setting FADJ to a voltage other than
0V. This voltage can vary from -2.4V to +2.4V, causing
the output frequency to vary from 1.7 to 0.30 times the
value when FADJ is 0V (F
o
±70%). Voltages beyond
±2.4V can cause instability or cause the frequency
change to reverse slope.
The voltage on FADJ required to cause the output to
deviate from F
o
by D
x
(expressed in %) is given by the
formula:
VFADJ = -0.0343 x D
x
where V
FADJ
, the voltage on FADJ , is between
-2.4V and +2.4V.
Note:
While I
IN
is directly proportional to the fundamen-
tal, or center frequency (F
o
), V
FADJ
is linearly related to
% deviation from F
o
. V
FADJ
goes to either side of 0V,
corresponding to plus and minus deviation.
The voltage on FADJ for any frequency is given by the
formula:
V
FADJ
= (F
o
- F
x
) ÷ (0.2915 x F
o
)
where:
F
x
= output frequency
F
o
= frequency when V
FADJ
= 0V.
Likewise, for period calculations:
V
FADJ
= 3.43 x (t
x
- t
o
) ÷ t
x
where:
t
x
= output period
[5]
[6]
[7]
t
o
= period when V
FADJ
= 0V.
Conversely, if V
FADJ
is known, the frequency is given
by:
F
x
= F
o
x (1 - [0.2915 x V
FADJ
])
and the period (t
x
) is:
t
x
= t
o
÷ (1 - [0.2915 x V
FADJ
])
[8]
[9]
Programming FADJ
FADJ has a 250μA constant current sink to V- that must
be furnished by the voltage source. The source is usu-
ally an op-amp output, and the temperature coefficient
of the current sink becomes unimportant. For manual
adjustment of the deviation, a variable resistor can be
used to set V
FADJ
, but then the 250μA current sink’s
temperature coefficient becomes significant. Since
external resistors cannot match the internal tempera-
ture-coefficient curve, using external resistors to pro-
gram V
FADJ
is intended only for manual operation,
when the operator can correct for any errors. This
restriction does not apply when V
FADJ
is a true voltage
source.
A variable resistor, R
F
, connected between REF (+2.5V)
and FADJ provides a convenient means of manually
setting the frequency deviation. The resistance value
(R
F
) is:
R
F
= (V
REF
- V
FADJ
) ÷ 250μA
V
REF
and V
FADJ
are signed numbers, so use correct
algebraic convention. For example, if V
FADJ
is -2.0V
(+58.3% deviation), the formula becomes:
R
F
= (+2.5V - (-2.0V)) ÷ 250μA
= (4.5V) ÷ 250μA
= 18k
[10]
Disabling FADJ
The FADJ circuit adds a small temperature coefficient
to the output frequency. For critical open-loop applica-
tions, it can be turned off by connecting FADJ to GND
(not REF) through a 12k
resistor (R1 in Figure 2). The
-250μA current sink at FADJ causes -3V to be devel-
oped across this resistor, producing two results. First,
the FADJ circuit remains in its linear region, but discon-
nects itself from the main oscillator, improving tempera-
ture stability. Second, the oscillator frequency doubles.
If FADJ is turned off in this manner, be sure to correct
equations 1-4 and 6-9 above, and 12 and 14 below by
doubling F
o
or halving t
o
. Although this method doubles
the normal output frequency, it does not double the
upper frequency limit. Do not operate FADJ open cir-
cuit or with voltages more negative than -3.5V. Doing
so may cause transistor saturation inside the IC, lead-
ing to unwanted changes in frequency and duty cycle.
High-Frequenc y Waveform Generator
10
______________________________________________________________________________________
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