Datasheet

LTC6990
18
6990fc
For more information www.linear.com/LTC6990
APPLICATIONS INFORMATION
Step 2: Calculate K
VCO
and f
(0V)
Next, calculate the intermediate values K
VCO
and f
(0V)
using
Equations (3b) and (3c).
K
VCO
=
100kHz −10kHz
4V −1V
= 30kHz/V
f
(0V)
= 100kHz + 30kHz/V • 1V = 130kHz
Step 3: Calculate and Select R
VCO
The next step is to use Equation (3d) to calculate the cor-
rect value for R
VCO
.
R
VCO
=
1MHz • 50k
8 • 1V • 30kHz/V
= 208.333k
Select R
VCO
= 210k.
Step 4: Calculate and Select R
SET
The final step is to calculate the correct value for R
SET
using Equation (3e).
R
SET
=
1MHz • 50k
8 • 130kHz −1V • 30kHz/V
(
)
= 62.5k
Select R
SET
= 61.9k
In this design example, with its wide 10:1 frequency
range, the potential output frequency error due to V
SET
error alone ranges from less than ±1% when V
CTRL
is at
its minimum up to ±36% when V
CTRL
is at its maximum.
This error must be accounted for in the system design.
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 V
SET
error is only ±9% and can be reduced
to only ±3% with a small change to the voltage tuning
range specification.
Reducing V
SET
Error Effects in VCO Applications
Figure 13 shows a VCO that reduces the effect of ∆V
SET
by adding an op-amp to make V
CTRL
dependent on V
SET
.
This circuit also has a positive transfer function (the out-
put frequency
increases as V
IN
increases). Furthermore,
for positive V
IN
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 per
cent) is greatly improved.
Figure 13. VCO with Reduced ∆V
SET
Sensitivity
6990 F13
LTC6990
OE
GND
SET
OUT
V
+
DIV
C1
0.1µF
R1
1M
R2
280k
DIVCODE = 3
(N
DIV
= 8, Hi-Z = 0)
R
SET
249k
3V
R
VCO
75k
–
+
3V
3V
10kHz TO 100kHz
f
OUT
V
CTRL
V
SET
1/2
LTC6078
R4
30.1k
C4
33pF
R3
100k
0.4V TO 4V
V
IN
f
OUT
=
1MHz • 50kΩ
N
DIV
•R
VCO
•
R
VCO
R
SET
+
V
IN
V
SET
−1






•
R4
R3








IF
R4
R3
=
R
VCO
R
SET
, THE EQUATION REDUCES TO:
f
OUT
=
1MHz • 50kΩ
N
DIV
•R
SET
•
V
IN
V
SET
= V
IN
• 25kHz/V