LTC6990CS6-PBF(V2) View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC6990CS6-PBF Datasheet PDF : 28 Pages
| |||
LTC6990
APPLICATIONS INFORMATION
Depending on the application’s requirements, the non-
inverting VCO circuit in Figure 13 may be preferred for
this wide of a frequency variation as its maximum inac-
curacy due to VSET error is only ±9% and can be reduced
to only ±3% with a small change to the voltage tuning
range specification.
Reducing VSET Error Effects in VCO Applications
Figure 13 shows a VCO that reduces the effect of ΔVSET
by adding an op-amp to make VCTRL dependent on VSET.
This circuit also has a positive transfer function (the out-
put frequency increases as VIN increases). Furthermore,
for positive VIN voltages, this circuit places the greatest
absolute frequency error at the highest output frequency.
Compared to the simple VCO circuit of Figure 11, the
absolute frequency error is unchanged. However, with
the maximum absolute frequency error (in Hertz) now
occurring at the highest output frequency, the relative
frequency error (in percent) is greatly improved.
Additionally, by choosing the VCO’s specifications shrewdly,
the frequency error (in percent) due to VSET variation is
reduced to ΔVSET/VSET = ±3%. To realize this improve-
ment, the design must abide by three conditions. First,
the VIN voltage must be positive throughout the range.
Second, choose VMAX/VMIN ≥ fMAX / fMIN. Last, choose
RVCO/RSET ≥ R4/R3.
Figure 13 shows a design similar to the previous design
example where the VMIN voltage is now specified to be
0.4V. This satisfies the VMAX/VMIN ≥ fMAX /fMIN condition
and the design assures that the output frequency error
due to VSET variation is only ±3%.
R3
0.4V TO 4V 100k
VIN
10kHz TO 100kHz
3V
fOUT
OE
OUT
LTC6990
3V
3V
1/2
LTC6078
VSET
RVCO
VCTRL 75k
R4
30.1k
C4
33pF
GND
V+
SET
DIV
6990 F13
C1
0.1μF
R1
1M
DIVCODE = 3
(NDIV = 8, Hi-Z = 0)
R2
280k
RSET
249k
fOUT
1MHz
NDIV
• 50k7
• RVCO
•
¨
©
ª©
RVCO
RSET
¥
§¦
VIN
VSET
1´¶µ
•
R4
R3
·
¸
¹¸
IF
R4
R3
RVCO
RSET
,
THE EQUATION REDUCES TO:
fOUT
=
1MHz • 50k7 • VIN
NDIV • RSET VSET
VIN
• 25kHz/V
Figure 13. VCO with Reduced ΔVSET Sensitivity
6990f
18
Share Link: 